Table of Contents

Volume 6, Issue 9

  September 2011

  Pages 1253 - 1422

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 Issue Catalogue (MARC XML)

SHORT COMMUNICATIONS

Open Access Article

V-ATPase dysfunction under excess zinc inhibits Arabidopsis cell expansion

Yoichiro Fukao and Ali Ferjani
Pages 1253 - 1255
http://dx.doi.org/10.4161/psb.6.9.16529
Abstract | PDF

Although zinc is an essential micronutrient for all living organisms, zinc is harmful to cells at high levels. In the presence of excess zinc, plants exhibit several major symptoms, including root growth inhibition, abnormal root hair morphology, and chlorosis. To dissect the molecular mechanisms underlying the effects of excess zinc on plant cells, we used an iTRAQ-based quantitative proteomics approach to analyze the microsomal protein profiles of Arabidopsis roots from wild-type (WT) plants and de-etiolated 3-1 (det3-1), a vacuolar H⁺-ATPase (V-ATPase) subunit C-defective mutant. A comparative analysis of the iTRAQ data from WT and det3-1 plants exposed to excess zinc suggests that the reduction in V-ATPase subunit levels and its activity are the cause of the symptoms of zinc toxicity, including the inhibition of cell expansion. Provided that reduced V-ATPase activity in the trans-Golgi network (TGN) alone can inhibit cell expansion, it is possible that the det3-1 mutant phenotype is caused mainly by a defect in TGN acidification, leading to reduced cell wall component trafficking and cell expansion in the presence of excess zinc. To evaluate the contribution of V-ATPase activity to vacuolar acidification under excess zinc, the vacuolar pH was measured. Our results indicate clear alkalinization of Deep cell vacuoles treated with 300 μM ZnSO₄.

Open Access Article

Grasshopper oral secretions increase salicylic acid and abscic acid levels in wounded leaves of Arabidopsis thaliana

Martin Schäfer, Christine Fischer, Ian T. Baldwin and Stefan Meldau
Pages 1256 - 1258
http://dx.doi.org/10.4161/psb.6.9.16552
Abstract | PDF

Recent investigations showed that the model plant Arabidopsis thaliana specifically responds to herbivory-associated molecular patterns by activating a sophisticated signaling network. The lipase activity of insect oral secretions was shown to elevate oxylipin levels when applied to puncture wounds in leaves. The results also demonstrated that the oral secretions of the generalist Schistocerca gregaria contained other, probably non-proteinous, elicitors of plant defense responses which induced mitogen-activated protein kinases, calcium signaling and ethylene levels.¹ This addendum presents data on the levels of additional phytohormones that are elevated after application of S. gregaria oral secretion to wounded leaves. Abscisic acid and salicylic acid levels are significantly elevated after elicitation with S. gregaria oral secretions, adding another layer of complexity to the herbivory-induced response of A. thaliana.

Open Access Article

Subcellular distribution of glutathione in the gametophyte

Bernd Zechmann and Scott D. Russell
Pages 1259 - 1262
http://dx.doi.org/10.4161/psb.6.9.16722
Abstract | PDF

Glutathione is an important antioxidant and redox buffer in plants. Despite its crucial roles in plant metabolism and defense in the sporophyte, its roles in the gametophyte are largely unexplored. Recently, we demonstrated that glutathione synthesis is essential for pollen germination in vitro. In this study, we extend these results and focus on the subcellular distribution of glutathione in pollen grains and compare it to the situation in the sporophyte. Glutathione was equally distributed within mitochondria, plastids, nuclei and the cytosol in the gametophyte -- in contrast to youngest fully developed leaves and root tips of the sporophyte, where glutathione was highest in the mitochondria, followed by nuclei, cytosol, peroxisomes and plastids in decreasing concentration. Glutathione was not detected in vacuoles. We can conclude that glutathione synthesis is essential for pollen germination in vitro and that the subcellular distribution of glutathione in the gametophyte differs significantly from the sporophyte.

Open Access Article

Sublethal concentrations of salicylic acid decrease the formation of reactive oxygen species but maintain an increased nitric oxide production in the root apex of the ethylene-insensitive Never ripe tomato mutants

Irma Tari, Péter Poór and Katalin Gémes
Pages 1263 - 1266
http://dx.doi.org/10.4161/psb.6.9.16502
Abstract | PDF

The pattern of salicylic acid (SA)-induced production of reactive oxygen species (ROS) and nitric oxide (NO) were different in the apex of adventitious roots in wild-type and in the ethylene-insensitive Never ripe (Nr) mutants of tomato (Solanum lycopersicum L. cv. Ailsa Craig). ROS were up-regulated, while NO remained at the control level in apical root tissues of wild-type plants exposed to sublethal concentrations of SA. In contrast, Nr plants expressing a defective ethylene receptor displayed a reduced level of ROS and a higher NO content in the apical root cells. In wild-type plants NO production seems to be ROS(H₂O₂)-dependent at cell death-inducing concentrations of SA, indicating that ROS and NO may interact to trigger oxidative cell death. In the absence of significant ROS accumulation, the increased NO production caused moderate reduction in cell viability in root apex of Nr plants exposed to 10^-3 M SA. This suggests that a functional ethylene signalling pathway is necessary for the control of ROS and NO production induced by SA.

Open Access Article

Beyond flowering time: Pleiotropic function of the maize flowering hormone florigen

Olga N. Danilevskaya, Xin Meng, Brian McGonigle and Michael G. Muszynski
Pages 1267 - 1270
http://dx.doi.org/10.4161/psb.6.9.16423
Abstract | PDF

The transition from vegetative to reproductive development is a critical turning point in a plant's life cycle. It is now widely accepted that a leaf-borne signal, florigen, moves via the phloem from leaves to the shoot apical meristem to trigger its reprogramming to produce flowers.¹ In part, the florigenic signal comprises a protein that belongs to the phosphatidylethanolamine-binding protein (PEBP) family that is present in all living organisms but displays diverse functions. The founding floral-promoting PEBP gene in Arabidopsis is FLOWERING LOCUS T (FT) whose functional homologs have been indentified in many flowering plants. We recently accumulated sufficient evidence to demonstrate the maize FT homolog ZCN8 has florigenic function. This task was particularly challenging due to the large number of FT-homologous genes in the maize genome. Here we show that ZCN8 function is more complex than simply regulating the floral transition. ZCN8 appears to play a pleiotropic role in the regulation of generalized growth of vegetative and reproductive tissues.

Open Access Article

Overexpression of a pea DNA helicase 45 in bacteria confers salinity stress tolerance

Marjan M. Tajrishi, Neha Vaid, Renu Tuteja and Narendra Tuteja
Pages 1271 - 1275
http://dx.doi.org/10.4161/psb.6.9.16726
Abstract | PDF

Salinity stress is one of the major factors negatively affecting growth and productivity in living organisms including plants and bacteria resulting in significant losses worldwide. Therefore, it would be fruitful to develop salinity stress tolerant useful species and also to understand the mechanism of stress tolerance. The pea DNA helicase 45 (PDH45) is a DNA and RNA helicase, homologous to eukaryotic translation initiation factor 4A (eIF-4A) and is involved in various processes including protein synthesis, maintaining the basic activities of the cell, up-regulation of topoisomerase I activity and salinity stress tolerance in plant, but its role in salinity stress tolerance in bacteria has not heretofore studied so far. This study provides an evidence for a novel function of the PDH45 gene in high salinity (NaCl) stress tolerance in bacteria (Eschericia coli, BL21 cells) also. Furthermore, it has been shown that the functionally active PDH45 gene is required to show the stress tolerance in bacteria because the single mutant (E183G or R363Q) and the double mutant (E183G + R363Q) of the gene could not confer the same function. The response was specific to Na+ ions as the bacteria could not grow in presence of LiCl. This study suggests that the cellular response to high salinity stress across prokaryotes and plant kingdom is conserved and also helps in our better understanding of mechanism of stress tolerance in bacteria and plants. It could also be very useful in developing high salinity stress tolerant useful bacteria of agronomic importance. Overall, this study provides an evidence for a novel function of the PDH45 gene in high salinity stress tolerance in bacteria.

Open Access Article

Molecular mechanism of crown root initiation and the different mechanisms between crown root and radicle in rice

Yuka Kitomi , Kitano Hidemi and Yoshiaki Inukai
Pages 1276 - 1278
http://dx.doi.org/10.4161/psb.6.9.16787
Abstract | PDF

Monocot plants produce numerous adventitious (crown) roots. The plant hormone auxin has positive effects on crown root formation, while cytokinin suppresses it. We have demonstrated that auxin-induced CROWN ROOTLESS5 (CRL5) regulates crown root initiation in rice through the induction of OsRR1, a negative regulator of cytokinin signaling. CRL5 over-expressing calli formed adventitious roots, although CRL5 over-expressing plants did not induce ectopic roots, suggesting that CRL5, which promotes de novo root initiation, might function only in de-differentiated cells. A radicle initiated normally in a crl5 mutant, in spite of the defect in crown root initiation, whereas crown roots, but not a radicle, were produced in a radicleless1 (ral1) mutant. A crl5 ral1 double mutant displayed an additive phenotype, showing that the formation of each root is regulated by different genetic mechanisms in rice.

Open Access Article

β-expansins are divergently abundant in maize cultivars that contrast in their degree of salt resistance

Christoph-Martin Geilfus, Christina Neuhaus, Karl H. Mühling and Christian Zörb
Pages 1279 - 1281
http://dx.doi.org/10.4161/psb.6.9.16760
Abstract | PDF

Zea mays L. exhibits a strong growth reduction in response to NaCl-induced stress that is attributable to a decline of cell division and elongation. Wall-loosening expansins are of major impact for cell wall extensibility and growth. This study provides an analysis of the impact of an 8-day 100 mM NaCl stress treatment on the mRNA abundance of the alpha- and beta-expansin sub-families using real-time quantitative RT-PCR. Moreover, we provide a comparative study of plants that contrast in their degree of salt resistance in order to reveal contrasting features of physiological functions that may bear a causal relation to the differential response of plants to salt. In result, the transcript abundance of wall-loosening β-expansins was impaired in size-reduced leaves of the salt-sensitive hybrid but not in leaves of the salt-resistant hybrid that maintained growth. This indicates a role for the β-expansins in processes related to salt resistance.

Open Access Article

Secondary wall NAC binding element (SNBE), a key cis-acting element required for target gene activation by secondary wall NAC master switches

Ryan L. McCarthy, Ruiqin Zhong and Zheng-Hua Ye
Pages 1282 - 1285
http://dx.doi.org/10.4161/psb.6.9.16402
Abstract | PDF

The biosynthesis of secondary walls in vascular plants requires the coordinated regulation of a suite of biosynthetic genes, and this coordination has recently been shown to be executed by the secondary wall NAC (SWN)-mediated transcriptional network. In Arabidopsis, five SWNs, including SND1, NST1/2 and VND6/7, function as master transcriptional switches to activate their common targets and consequently the secondary wall biosynthetic program. A recent report by Zhong et al.¹ revealed that SWNs bind to a common cis-acting element, namely secondary wall NAC binding element (SNBE), which is composed of an imperfect palindromic 19-bp consensus sequence, (T/A)NN(C/T)(T/C/G)TNNNNNNNA(A/C)GN(A/C/T)(A/T). Genome-wide analysis of direct targets of SWNs showed that SWNs directly activate the expression of not only many transcription factors but also a battery of genes involved in secondary wall biosynthesis, cell wall modification and programmed cell death, the promoters of which all contain multiple SNBE sites. The functional significance of the SNBE sites is further substantiated by our current in planta expression study demonstrating that representative SNBE sequences from several SWN direct target promoters are sufficient to drive the expression of the GUS reporter gene in secondary wall-forming cells. The identification of the SWN DNA binding element (SNBE) and the SWN direct targets marks an important step forward toward the dissection of the transcriptional network regulating the biosynthesis of secondary walls, the most abundant biomass produced by land plants.

Open Access Article

Multiple roles of nitrate transport accessory protein NAR2 in plants

Huimin Feng, Xiaorong Fan, Ming Yan, Xiaoqin Liu, Anthony J. Miller and Guohua Xu
Pages 1286 - 1289
http://dx.doi.org/10.4161/psb.6.9.16377
Abstract | PDF

Two component high affinity nitrate transport system, NAR2/NRT2, has been defined in several plant species. In Arabidopsis, AtNAR2.1 has a role in the targeting of AtNRT2.1 to the plasma membrane. The gene knock out mutant atnar2.1 lacks inducible high-affinity transport system (IHATS) activity, it also shows the same inhibition of lateral root (LR) initiation on the newly developed primary roots as the atnrt2.1 mutant in response to low nitrate supply. In rice, OsNAR2.1 interacts with OsNRT2.1, OsNRT2.2 and OsNRT2.3a to provide nitrate uptake over high and low concentration ranges. In rice roots OsNAR2.1 and its partner NRT2s show some expression differences in both tissue specificity and abundance. Knockdown of OsNAR2.1 suppressed expression of OsMADS (AK241644), a homologous gene of ANR1 involved in the nitrate signaling pathway of lateral root development and resulted in decreased lateral root length despite of nitrate status in the roots. It can be predicted that NAR2 plays multiple roles in addition to being an IHATS component in plants.

Open Access Article

The universal bundling activity of AtVLN4 in diffusely growing cells

Fei Du, Yi Zhang and Haiyun Ren
Pages 1290 - 1293
http://dx.doi.org/10.4161/psb.6.9.16501
Abstract | PDF

We recently reported that AtVLN4, a member of villin/gelsolin/fragmin superfamily in Arabidopsis thaliana, participated in root hair growth through its actin bundling activity. To further understand the functions of AtVLN4, we investigated its in vivo expression pattern and roles in diffusely growing cells. Transcription analysis of AtVLN4 and detection of AtVLN4 promoter-GUS activity consistently indicated that AtVLN4 had a universal expression pattern and was preferentially expressed in vegetative tissues. Observation of actin structures labeled by GFP-fABD2 revealed that there were less actin bundles in many diffusely growing cell types in atvln4-1 seedlings than in wild-type seedlings. Pharmacological studies by treatment with Latrunculin B showed that the actin filaments were much easier to be disrupted in diffusely growing cells of atvln4-1 seedlings. Collectively, these results demonstrate that AtVLN4 has a universal actin bundling activity in diffusely growing cells just like that in the tip growing cell, root hairs.

Open Access Article

A friend in need is a friend indeed: Understanding stress-associated transcriptional networks of plant metabolism using cliques of coordinately expressed genes

Tamar Avin-Wittenberg, Vered Tzin, Hadar Less, Ruthie Angelovici and Gad Galili
Pages 1294 - 1296
http://dx.doi.org/10.4161/psb.6.9.16567
Abstract | PDF

The response of plants to environmental cues, particularly stresses, involves the coordinated induction or repression of gene expression. In a previous study, we developed a bioinformatics approach to analyze the mutual expression pattern of genes encoding transcription factors and metabolic enzymes upon exposure of Arabidopsis plants to abiotic and biotic stresses. The analysis resulted in three gene clusters, each displaying a unique expression pattern. In the present addendum, we address the composition of each of these three clusters in regard to the functional identity of their encoded proteins as enzymes or transcription factors.

Open Access Article

A functional cutin matrix is required for plant protection against water loss

Guoxiong Chen, Takao Komatsuda, Jian Feng Ma, Chao Li, Naoki Yamaji and Eviatar Nevo
Pages 1297 - 1299
http://dx.doi.org/10.4161/psb.6.9.17507
Abstract | PDF

The plant cuticle, a cutin matrix embedded with and covered by wax, seals the aerial organ’s surface to protect the plant against uncontrolled water loss. The cutin matrix is essential for the cuticle to function as a barrier to water loss. Recently, we identified from wild barley a drought supersensitive mutant, eibi1, which is caused by a defective cutin matrix as the result of the loss of function of HvABCG31, an ABCG full transporter. Here, we report that eibi1 epidermal cells contain lipid-like droplets, which are supposed to consist of cutin monomers that have not been transported out of the cells. The eibi1 cuticle is fragile due to a defective cutin matrix. The rice ortholog of the EIBI1 gene has a similar pattern of expression, young shoot but not flag leaf blade, as the barley gene. The model of the function of Eibi1 is discussed. The HvABCG31 full transporter functions in the export of cutin components and contributed to land plant colonization, hence also to terrestrial life evolution.




MINI-REVIEW

Open Access Article

How does phosphate status influence the development of the arbuscular mycorrhizal symbiosis?

Mian Gu, Aiqun Chen, Xiaoli Dai, Wei Liu and Guohua Xu
Pages 1300 - 1304
http://dx.doi.org/10.4161/psb.6.9.16365
Abstract | PDF

Most terrestrial plant roots form mutualistic symbiosis with soil-borne arbuscular mycorrhizal fungi (AMF), a characteristic feature of which is nutrient exchange between the two symbiotic partners. Phosphate (Pi) is the main benefit the host plants acquired from the AMF. It has long been a common realization that high Pi supply could suppress the AMF development. However, the direct molecular regulatory mechanisms underlying this plant directed suppression are lacking. Here, we reviewed the recent work providing the evidences that high Pi supply induces transcriptional alteration, leading to the inhibition of AMF development at different stages of AM symbiosis, and gave our view on potential cross-talk among Pi starvation, AM as well as phytohormone signaling.




REVIEWS

Open Access Article

Control of stomatal aperture: A renaissance of the old guard

Wagner L. Araújo, Alisdair R. Fernie and Adriano Nunes-Nesi
Pages 1305 - 1311
http://dx.doi.org/10.4161/psb.6.9.16425
Abstract | PDF

Stomata, functionally specialized small pores on the surfaces of leaves, regulate the flow of gases in and out of plants. The pore is opened by an increase in osmotic pressure in the guard cells, resulting in the uptake of water. The subsequent increase in cell volume inflates the guard cell and culminates with the opening of the pore. Although guard cells can be regarded as one of the most thoroughly investigated cell types, our knowledge of the signalling pathways which regulate guard cell function remains fragmented. Recent research in guard cells has led to several new hypotheses, however, it is still a matter of debate as to whether guard cells function autonomously or are subject to regulation by their neighbouring mesophyll cells. This review synthesizes what is known about the mechanisms and genes critical for modulating stomatal movement. Recent progress on the regulation of guard cell function is reviewed here including the involvement of environmental signals such as light, the concentration of atmospheric CO₂ and endogenous plant hormones. In addition we re-evaluate the important role of organic acids such as malate and fumarate play in guard cell metabolism in this process.

Open Access Article

Using transgenic modulation of protein synthesis and accumulation to probe protein signaling networks in Arabidopsis thaliana

Sankalpi N. Warnasooriya and Beronda L. Montgomery
Pages 1312 - 1321
http://dx.doi.org/10.4161/psb.6.9.16437
Abstract | PDF

 Deployment of new model species in the plant biology community requires the development and /or improvement of numerous genetic tools. Sequencing of the Arabidopsis thaliana genome opened up a new challenge of assigning biological function to each gene. As many genes exhibit spatiotemporal or other conditional regulation of biological processes, probing for gene function necessitates applications that can be geared toward temporal, spatial, and quantitative functional analysis in vivo. The continuing quest to establish new platforms to examine plant gene function has resulted in the availability of numerous of genomic and proteomic tools. Classical and more recent genome-wide experimental approaches include conventional mutagenesis, tagged DNA insertional mutagenesis, ectopic expression of transgenes, activation tagging, RNA interference and two-component transactivation systems. The utilization of these molecular tools has resulted in conclusive evidence for the existence of many genes, and expanded knowledge on gene structure and function. This review covers several molecular tools that have become increasingly useful in basic plant research. We discuss their advantages and limitations for probing cellular protein function while emphasizing the contributions made to lay the fundamental groundwork for genetic manipulation of crops using plant biotechnology.

Open Access Article

The multifaceted function of BAK1/SERK3: Plant immunity to pathogens and responses 1 to insect herbivores

Da-Hai Yang, Christian Hettenhausen, Ian T. Baldwin and Jianqiang Wu
Pages 1322 - 1324
http://dx.doi.org/10.4161/psb.6.9.16438
Abstract | PDF

Almost a decade ago BRI1-associated kinase 1 (BAK1) was identified as a co-receptor of brassinosteroid (BR) insensitive 1 (BRI1), the receptor for BRs, which plays an essential role in transducing BR signaling to regulate plant development. BAK1 is also critical in resistance to various pathogens. BAK1 rapidly binds to certain receptors for pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) after the perception of pathogen elicitors and is required for the full elicitation of pathogen-induced defense responses, such as the activation of the mitogen-activated protein kinase 6 (MPK6) and production of reactive oxygen species. Thus, BAK1 functions in both BR signaling and PAMP-triggered immunity (PTI). Recently BAK1 was also found to play an important role in mediating defense responses against an insect herbivore (Manduca sexta) of Nicotiana attenuata. In this interaction, BAK1 positively modulates wound- or herbivore feeding-induced accumulation of jasmonic acid (JA) and JA-isoleucine (JA-Ile). This mini-review summarizes recent advances in our understanding the functions of BAK1 in resistance to pathogens and herbivores.

Open Access Article

Antimicrobial peptides: Modes of mechanism, modulation of defense responses

Mohammad Rahnamaeian
Pages 1325 - 1332
http://dx.doi.org/10.4161/psb.6.9.16319
Abstract | PDF

Complicated schemes of classical breeding and their drawbacks, environmental risks imposed by agrochemicals, decrease of arable land, and escalating damages of pests and pathogens have accentuated the necessity for highly efficient measures to improve crop protection. During co-evolution of host-microbe interactions, antimicrobial peptides (AMPs) have exhibited a brilliant history in protecting host organisms against devastation by invading pathogens. Since 1980s, a plethora of AMPs has been isolated from and characterized in different organisms. Nevertheless the AMPs expressed in plants render them more resistant to diverse pathogens, a more orchestrated approach based on knowledge of their mechanisms of action and cellular targets, structural toxic principle, and possible impact on immune system of corresponding transgenic plants will considerably improve crop protection strategies against harmful plant diseases. This review outlines the current knowledge on different modes of action of AMPs and then argues the waves of AMPs' ectopic expression on transgenic plants immune system.

Open Access Article

PIN polarity regulation by AGC-3 kinases and ARF-GEF: A recurrent theme with context dependent modifications for plant development and response

Pankaj Dhonukshe
Pages 1333 - 1337
http://dx.doi.org/10.4161/psb.6.9.16611
Abstract | PDF

The analysis of cell polarity in plants is fueled by the discovery and analysis of auxin efflux carrier PIN proteins that show polar localizations in various plant cell types in line with their roles in directional cell to cell auxin transport. As this asymmetry in cellular PIN localization drives directional auxin fluxes, abnormalities in PIN localizations modify auxin transport culminating into range of auxin distribution defective phenotypes. Because of this influence of PIN localization on plant development via changes in auxin distribution, mechanisms establishing, maintaining and altering PIN polarity are of intense interest in the plant field during the recent years. Recent findings suggest that two categories of molecules, namely AGC-3 kinase family members PINOID, WAG1, WAG2 and ARF-GEF family member GNOM predominantly influence the polar localization of PINs. The emerging mechanism for AGC-3 kinases and ARF-GEF action suggest that AGC-3 kinases predominantly phosphorylate PINs at the plasma membrane for eventual PIN internalization and PIN sorting into ARF-GEF GNOM independent polar recycling pathways. In case of mutant for AGC-3 kinases or mutations in AGC-3 kinase-targeted PIN residues, much less phosphorylated PINs are recruited into ARF-GEF GNOM-dependent polar recycling pathway.  When ARF-GEF GNOM is inactive, the bias is shifted for rerouting less efficiently phosphorylated PINs into GNOM-independent polar recycling pathways that generally prefer efficiently phosphorylated PINs. Thus, balance shifts between the extent of AGC-3 kinase mediated PIN phosphorylation and the functioning of ARF-GEF instruct PIN polarity establishment and/or PIN polarity alterations. Recent studies report utilization of this AGC-3 kinase and ARF-GEF PIN polarity regulation module during diverse developmental and response programs including shoot patterning, root growth, phototropism, gravitropism, organogenesis, leaf epidermal cell indentations and fruit valve margin formation. Based on these findings the same theme of phosphorylated PIN sorting into differential polar recycling pathways for PIN polarity establishment and alteration seems to be employed in a context-dependent manner.

Open Access Article

Algae as promising organisms for environment and health

Emad Shalaby
Pages 1338 - 1350
http://dx.doi.org/10.4161/psb.6.9.16779
Abstract | PDF

Algae like other plants produce a variety of remarkable compounds collectivity refereed to as secondary metabolites they are synthesized by these organisms at the end of the growth phase and/or due to metabolic alterations induced by environmental stress conditions. Carotenoids, phenolic compounds, phycobiliprotein pigments, polysaccharides and unsaturated fatty acids are same of the algal natural products which were reported to have variable biological activities, including antioxidant activity, anticancer activity, antimicroabial activity against bacteria-virus-algae-fungi, organic fertilizer and bioremediation potentials.




RESEARCH PAPERS

Open Access Article

Tryptophan auxotroph mutants suppress the superroot2 phenotypes, modulating IAA biosynthesis in Arabidopsis.

Shunsuke Kiyohara, Hiroaki Honda, Noriko Shimizu, Chika Ejima, Ryota Hamasaki and Shinichiro Sawa
Pages 1351 - 1355
http://dx.doi.org/10.4161/psb.6.9.16321
Abstract | PDF

Plant phytohormone, Indole-3-acetic acid (IAA), is synthesized by tryptophan (trp) dependent and independent pathway. Here we report that tryptophan auxotroph mutants completely suppressed the abnormalities of auxin over production mutant, superroot2. SUR2 is considered to modulate Trp dependent pathway, resulting IAA accumulation in Arabidopsis. Tryptophan auxotroph mutants showed hyper-sensitivity to the auxin polar transport inhibitor, NPA, on the phenotype of reduced gravitropism. These results together with the results of histochemical analyses, tryptophan auxotroph mutants seem to have a complete defect in Trp dependent IAA biosynthesis pathway, and it is also suggested that the Trp dependent pathway is responsible for the normal root gravitropism.

Open Access Article

The effect of steepness of temporal resource gradients on spatial root allocation

Hagai Shemesh, Ran Rosen, Gil Eshel, Ariel Novoplansky and Ofer Ovadia
Pages 1356 - 1360
http://dx.doi.org/10.4161/psb.6.9.16444
Abstract | PDF

Plants are able to discriminately allocate greater biomass to organs that grow under higher resource levels. Recent evidence demonstrates that split-root plants also discriminately allocate more resources to roots that grow under dynamically improving nutrient levels, even when their other roots grow in richer patches. Here, we further tested whether, besides their responsiveness to the direction of resource gradients, plants are also sensitive to the steepness of environmental trajectories. Split-root Pisum sativum plants were grown so that one of their roots developed under constantly-high nutrient levels and the other root was subjected to dynamically improving nutrient levels of variable steepness. As expected, plants usually allocated a greater proportion of their biomass to roots that developed under constantly high resource availability; however, when given a choice, they allocated greater biomass to roots that initially experienced relatively low but steeply improving nutrient availabilities than to roots that developed under continuously-high nutrient availability. Such discrimination was not observed when the roots in the poor patch experienced only gentler improvements in nutrient availability. The results are compatible with the notion that responsiveness to the direction and steepness of environmental gradients could assist annual plants to increase their performance by anticipating resource availabilities foreseeable before the end of their growing season. The results exemplify the ability of plants to integrate and utilize environmental information and execute adaptive behaviours which, until recently, were attributed only to animals with central nervous systems.

Open Access Article

Boolean function applied to Mimosa pudica movements

Thiago Paes de Barros De luccia and Pedro Friedman
Pages 1361 - 1364
http://dx.doi.org/10.4161/psb.6.9.16445
Abstract | PDF

Seismonastic or thigmonastic movements of Mimosa pudica L result most from the fast loss of water from swollen motor cells, generating temporarily collapse of cells and inducing quick curvature in the parts where they are located. Because this simple reason, this plant has been much studied since the 18th century, leading us to think about the classical binomial stimulus-response (action-reaction) compared with animals. Mechanic and electrical stimulus were used to investigate an analogy of the mimosa's branch with animal's neuron models. Mechanic and electric stimuli were tested to observe the action potential propagation through the mimosa's branch. Boolean function applied to the mimosa´s branch in analogy with an artificial neuron model is one of the peculiarities of this interesting plant in our hypothesis.

Open Access Article

Effects of mechanostimulation on gravitropism and signal persistence in flax roots

Susan P. John and Karl H. Hasenstein
Pages 1365 - 1370
http://dx.doi.org/10.4161/psb.6.9.16601
Abstract | PDF

Gravitropism describes curvature of plants in response to gravity or differential acceleration and clinorotation is commonly used to compensate unilateral effect of gravity. We report on experiments that examine the persistence of the gravity signal and separate mechanostimulation from gravistimulation. Flax roots were reoriented (placed horizontally for 5, 10, or 15 min) and clinorotated at a rate of 0.5 to 5 rpm either vertically (parallel to the gravity vector and root axis) or horizontally (perpendicular to the gravity vector and parallel to the root axis). Image sequences showed that horizontal clinorotation did not affect root growth rate (0.81±0.03 mm h^-1) but vertical clinorotation reduced root growth by about 7%. The angular velocity did not affect growth for either direction. However, maximal curvature for vertical clinorotation decreased with increasing rate of rotation and produced straight roots at 5 rpm. In contrast, horizontal clinorotation increased curvature with increasing angular velocity. The point of maximal curvature was used to determine the longevity (memory) of the gravity signal, which lasted about 120 min. The data indicate that mechanostimulation modifies the magnitude of the graviresponse but does not affect memory persistence.

Open Access Article

Role of chloroplastidial proteases in leaf senescence

Prashanth B. Goud and Manvendra S. Kachole
Pages 1371 - 1376
http://dx.doi.org/10.4161/psb.6.9.16316
Abstract | PDF

In this report effect of hydrogen peroxide (H₂O₂) on peroxidase (POD) activity during leaf senescence was studied with and without phenylmethylsulfonyl fluoride (PMSF) pre-treatment in detached neem (Azadirachta indica A. juss) leaf chloroplasts. Increased POD activity was detected in natural and H₂O₂-promoted senescent leaves compared to untreated control mature green leaf chloroplasts. However, under H₂O₂ POD activity markedly increased at 1 day, and then significantly decreased until 4 days. In the presence of H₂O₂, PMSF, the induction of POD activity was alleviated at 1 day, whereas reduced after 4 days. In contrast in the presence of H₂O₂, cycloheximide (CX), the induction of POD activity was reduced at 1 day, whereas alleviated after 4 days. Partial reduction in induction of POD activity under H₂O₂+PMSF and H₂O₂+CX after 4 and 1 days respectively was indicating the presence of pre-existing inactive PODs. The results also suggesting that PODs might be activated by proteases during leaf senescence. Key words: Chloroplast, leaf senescence, peroxidase, protease.




ARTICLE ADDENDA

Open Access Article

Early signaling in actinorhizal symbioses.

Valérie Hocher, Nicole Alloisio, Didier Bogusz and Philippe Normand
Pages 1377 - 1379
http://dx.doi.org/10.4161/psb.6.9.16761
Abstract | PDF

Nitrogen-fixing root nodulation, confined to four plant orders, encompasses more than 14,000 Leguminosae species, and approximately 200 actinorhizal species forming symbioses with rhizobia (Rhizobium, Bradyrhizobium, etc) and Frankia bacterial species, respectively. While several genetic components of the host-symbiont interaction have been identified in legumes, little is known about the genetic bases of actinorhizal symbiosis. However, we recently demonstrated the existence of common symbiotic signaling pathways in actinorhizals and legumes. Moreover, important data on the identification of flavonoids as plant signaling compounds and the role for auxins during Frankia infection process and nodule organogenesis have been acquired. All together these results lead us to propose a unified model for symbiotic exchange and genetic control of actinorhizal symbiosis.

Open Access Article

Recovering plant biodiversity: Mount Pinatubo lessons to learn

Thomas E. Marler
Pages 1380 - 1382
http://dx.doi.org/10.4161/psb.6.9.16962
Abstract | PDF

Studying recovering plant biodiversity on Mount Pinatubo may provide valuable insights that improve our understanding of recovery of other ecosystems following disturbances of all types. Ongoing sheet and rill erosion coupled with mass waste events in the unstable pyroclastic flow deposits persist, effectively re-setting primary succession at micro-landscape scale without affecting habitat level diversity. Spatial factors and micro-habitat diversity may exert more control over continued succession as the riparian systems become more deeply dissected and complex. The number of taxa within functional groups and conservation concerns are botanical issues that deserve further research.

Open Access Article

Arabidopsis TSPO and porphyrins metabolism: A transient signaling connection?

Celine Vanhee and Henri Batoko
Pages 1383 - 1385
http://dx.doi.org/10.4161/psb.6.9.16477
Abstract | PDF

What goes up should come down and vice versa. Cellular homeostasis requires that every signaling process involving up- or down-regulation of a given pathway should only be transient, and returning to steady state after a signaling process is as vital to living cells as being able to perceive and transduce changes of their environment. One of the best studied responses of plant cells subjected to water-related stress is the transient increase of the phytohormone abscisic acid (ABA). The increase in active ABA regulates the expression of ABA-responsive genes, some of which are strictly ABA-dependent in that their expression is almost undetectable in absence of elevated levels of cellular ABA. Since the function of these proteins may only be required transiently, a regulatory mechanism for transcriptionally and/or post-translationally regulate their expression should exist. In general during stress, molecular mechanisms aimed at shutting down the ABA-dependent signaling, as required at some point for the homeostasis of the plant cell, are poorly understood. The Arabidopsis TSPO (Translocator protein)-related protein is transiently induced by abiotic stresses and ABA treatment. Our recent work1 aiming at understanding the function and regulation of At-TSPO yielded exciting insights into the interplay among a stress-regulated protein, ABA responses, tetrapyrrole biosynthesis/scavenging, and autophagy.2 We discuss these findings in relation to tetrapyrroles metabolism/trafficking and the regulation of ABA-dependent signaling by the plant cell.

Open Access Article

Changes in thylakoid membrane thickness associated with the reorganization of photosystem II light harvesting complexes during photoprotective energy dissipation

Matthew P. Johnson, Anthony P. R. Brain and Alexander V. Ruban
Pages 1386 - 1390
http://dx.doi.org/10.4161/psb.6.9.16503
Abstract | PDF

Using freeze-fracture electron microscopy we have recently shown that non-photochemical quenching (NPQ), a mechanism of photoprotective energy dissipation in higher plant chloroplasts, involves a reorganization of the pigment-protein complexes within the stacked grana thylakoids1. Photosystem II light harvesting complexes (LHCII) are reorganized in response to the amplitude of the light driven transmembrane proton gradient (ΔpH) leading to their dissociation from photosystem II reaction centers and their aggregation within the membrane1. This reorganization of the PSII-LHCII macrostructure was found to be enhanced by the formation of zeaxanthin and was associated with changes in the mobility of the pigment-protein complexes therein1. We suspected that the structural changes we observed were linked to the ΔpH-induced changes in thylakoid membrane thickness that were first observed by Murikami and Packer2,3. Here using thin-section electron microscopy we show that the changes in thylakoid membrane thickness do not correlate with ΔpH per se but rather the amplitude of NPQ and is thus affected by the de-epoxidation of the LHCII bound xanthophyll violaxanthin to zeaxanthin. We thus suggest that the change in thylakoid membrane thickness occurring during NPQ reflects the conformational change within LHCII proteins brought about by their protonation and aggregation within the membrane

Open Access Article

Tissue-specific organelle DNA degradation mediated by DPD1 exonuclease

Lay Yin Tang and Wataru Sakamoto
Pages 1391 - 1393
http://dx.doi.org/10.4161/psb.6.9.16595
Abstract | PDF

Organelle DNA in plastids and mitochondria is present in multiple copies and undergoes degradation developmentally. For example, organelle DNA that is detectable cytologically using DNA-fluorescent dye disappears during pollen development. Nevertheless, nucleases involved in this degradation process remain unknown. Our recent study identified the organelle nuclease, DPD1, which has Mg2^+-dependent exonuclease activity in vitro. The discovery of DPD1 emerged from Arabidopsis mutant screening and concomitant isolation of dpd1 mutants that retain organelle DNA in mature pollen. DPD1 is conserved only in angiosperms: not in other photosynthetic organisms. Despite these findings, the physiological significance of organelle DNA degradation during pollen development remains unclear because dpd1 exhibits no apparent defects in pollen viability or in the maternal inheritance of organelle DNA. We discuss a possible role of organelle DNA degradation mediated by DPD1, based on a DPD1 expression profile studied using in silico analyses.

Open Access Article

Both negative and positive G1 cell cycle regulators undergo proteasome-dependent degradation during sucrose starvation in Arabidopsis

Hiroto Hirano , Atsuhiko Shinmyo and Masami Sekine
Pages 1394 - 1396
http://dx.doi.org/10.4161/psb.6.9.16877
Abstract | PDF

The proteasome pathway regulates many aspects of biological processes in plants, such as plant hormone signaling, light responses, the circadian clock and regulation of cell division. Key cell-cycle regulatory proteins including B-type cyclins, Cdc6, cyclin-dependent kinase inhibitors and E2Fc undergo proteasome-dependent degradation. We used the proteasome inhibitor MG132 to show that proteolysis of Arabidopsis RETINOBLASTOMA-RELATED 1 (AtRBR1) and three E2Fs is mediated by the proteasome pathway during sucrose starvation in Arabidopsis suspension MM2d cells. We found previously that estrogen-inducible RNAi-mediated downregulation of AtRBR1 leads to a higher frequency of arrest in G2 phase, instead of G1-phase arrest in the uninduced control, after sucrose starvation. Degradation of not only negative (AtRBR1 and E2Fc) but also positive (E2Fa and E2Fb) cell cycle regulators after sucrose starvation may be required for arrest in G1 phase, when cells integrate a variety of nutritional, hormonal and developmental signals to decide whether or not to commit to entry into the cell cycle.

Open Access Article

The motors of protein import into chloroplasts

Lan-Xin Shi and Steven M. Theg
Pages 1397 - 1401
http://dx.doi.org/10.4161/psb.6.9.16916
Abstract | PDF

Chloroplast function is largely dependent on its resident proteins, most of which are encoded by the nuclear genome and are synthesized in cytosol. Almost all of these are imported through the translocons located in the outer and inner chloroplast envelope membranes. The motor protein that provides the driving force for protein import has been proposed to be Hsp93, a member of the Hsp100 family of chaperones residing in the stroma. Combining in vivo and in vitro approaches, recent publications have provided multiple lines of evidence demonstrating that a stromal Hsp70 system is also involved in protein import into this organelle. Thus it appears that protein import into chloroplasts is driven by two motor proteins, Hsp93 and Hsp70. A perspective on collaboration between these two chaperones is discussed.

Open Access Article

The higher expression levels of dehydroascorbate reductase and glutathione reductase in salicylic acid-deficient plants may contribute to their alleviated symptom infected with RNA viruses

Feng Zhu, Shu Yuan, Shao-Dong Wang, De-Hui Xi and Hong-Hui Lin
Pages 1402 - 1404
http://dx.doi.org/10.4161/psb.6.9.16538
Abstract | PDF

Salicylic acid (SA) is a critical signal for activation of both local and systemic resistance responses. However, SA-deficient plants adapt to RNA virus infections better, which show a less-severe symptom and less reactive oxygen species (ROS) accumulation. The higher levels of reduced glutathione (GSH) and reduced ascorbic acid (AsA) in SA-deficient plants may contribute to their alleviated symptoms, which are consistent with their higher expression levels of dehydroascorbate reductase gene (DHAR) and glutathione reductase gene (GR). High-dose AsA or GSH treatment could alleviate the symptom and inhibit virus replication after 20 days, but ROS eliminators could not imitate the effect of AsA or GSH. The data show a new link between SA and AsA/GSH-mediated redox homeostasis.

Open Access Article

Auxin distribution and lenticel formation in determinate nodule of Lotus japonicus

Kojiro Takanashi, Akifumi Sugiyama and Kazufumi Yazaki
Pages 1405 - 1407
http://dx.doi.org/10.4161/psb.6.9.16934
Abstract | PDF

Legumes can establish a symbiosis with rhizobia and form root nodules that function as an apparatus for nitrogen fixation. Nodule development is regulated by several phytohormones including auxin. Although accumulation of auxin is necessary to initiate the nodulation of indeterminate nodules, the functions of auxin on the nodulation of determinate nodules have been less characterized. In this study, the functions of auxin in nodule development in Lotus japonicus have been demonstrated using an auxin responsive promoter and auxin inhibitors. We found that the lenticel formation on the nodule surface was sensitive to the auxin defect. Further analysis indicated that failure in the development of the vascular bundle of the determinate nodule, which was regulated by auxin, was the cause of the disappearance of lenticels.

Open Access Article

The autophagy gene, ATG18a, plays a negative role in powdery mildew resistance and mildew-induced cell death in Arabidopsis

Yiping Wang, Yingying Wu and Dingzhong Tang
Pages 1408 - 1410
http://dx.doi.org/10.4161/psb.6.9.16967
Abstract | PDF

Autophagy is a conserved intracellular recycling system that traffics cellular organelles and cytosolic proteins within lysosomes for reuse or breakdown in eukaryotes. Increased evidence indicates that autophagy is involved in programmed cell death and disease resistance in plants. We recently showed that atg2, atg5, atg7 and atg10 displayed early senescence and cell death in later growth stage under nutrient-rich conditions in Arabidopsis thaliana. These mutants also exhibited powdery mildew resistance and mildew-induced cell death. Salicylic acid (SA) signaling is required for atg2-mediated powdery mildew resistance, however, inactivation of SA signaling is not sufficient to fully suppress powdery mildew-induced cell death in atg2 mutant1. Here, we show that atg18a-2 is also resistant to the powdery mildew pathogen, Golovinomyces cichoracearum, and it shows mildew-induced cell death similar to the atg2 mutant. Taken together, our study reveals that autophagy plays important roles in suppression of cell death and defense response to the biotrophic pathogen, the powdery mildew fungus. Future work on autophagy in plants will shine light on how autophagy is involved in cell death and defense response in plants.

Open Access Article

Integrating circadian and gibberellin signaling in Arabidopsis: Possible links between the circadian clock and the AtGID transcription

Nora Marín-de la Rosa, David Alabadí, Miguel Ángel Blázquez and María Verónica Arana
Pages 1411 - 1413
http://dx.doi.org/10.4161/psb.6.9.17209
Abstract | PDF

The circadian clock acts as central coordinator of plant activity, and it regulates key traits for plant fitness such as flowering time, gas exchange, growth, and stress responses. In the May issue of the Proceedings of the National Academy of Science we describe the circadian regulation of gibberellin (GA) signaling, through transcriptional control of GA receptor genes (GID1a and GID1b). We show that, in short day photocycles, the expression of GA receptors oscillates in seedlings, yielding a window of strong GA activity at the end of the night that overlaps with the period of maximum growth. This clock-mediated control of GA signaling is not only crucial for the establishment of rhythmic patterns of growth but also affects the expression of many circadian-controlled genes that participate in a wide range of biological processes. Here we propose a possible mechanism that might operate for the transcriptional control of GID1 expression by the circadian clock.

Open Access Article

Responses of root hair development to elevated CO2

Yao Fang Niu, Gu Lei Jin, Ru Shan Chai, Huan Wang and Yong Song Zhang
Pages 1414 - 1417
http://dx.doi.org/10.4161/psb.6.9.17302
Abstract | PDF

This review highlights a potential signaling pathway of CO₂-dependent stimulation in root hair development. Elevated CO₂ firstly increases the carbohydrates production, which triggers the auxin or ethylene responsive signal transduction pathways and subsequently stimulates the generation of intracellular nitric oxide (NO). The NO acts on target Ca²⁺ and ion channels and induces activation of MAPK. Meanwhile, reactive oxygen species (ROS) activates cytoplasmic Ca²⁺ channels at the plasma membrane in the apex of the root tip. This complex pathway involves transduction cascades of multiple signals that lead to the fine tuning of epidermal cell initiation and elongation. The results suggest that elevated CO₂ plays an important role in cell differentiation processes at the root epidermis. 

Open Access Article

Smoke signals and seed dormancy: Where next for MAX2?

Mark T. Waters, Steven M. Smith and David C. Nelson
Pages 1418 - 1422
http://dx.doi.org/10.4161/psb.6.9.17303
Abstract | PDF

The Arabidopsis thaliana F-box protein MAX2 has been discovered in four separate genetic screens, indicating that it has roles in leaf senescence, seedling photosensitivity, shoot outgrowth, and seed germination. Both strigolactones and karrikins can regulate A. thaliana seed germination and seedling photomorphogenesis in a MAX2-dependent manner, but only strigolactones inhibit shoot branching. How MAX2 mediates specific responses to both classes of structurally-related signals, and the origin of its dual role remains unknown. The moss Physcomitrella patens utilizes strigolactones and MAX2 orthologs are present across the land plants, suggesting that this signaling system could have an ancient origin. The seed of parasitic Orobanchaceae species germinate preferentially in response to strigolactones over karrikins, and putative Orobanchaceae MAX2 orthologs form a sub-clade distinct from those of other dicots. These observations suggest that lineage-specific evolution of MAX2 may have given rise to specialized responses to these signaling molecules.