Table of Contents

Volume 5, Issue 2

  February 2010

  Pages 87 - 207

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 Issue Catalogue (MARC XML)

PERSPECTIVES

Open Access Article

Recent surprising similarities between plant cells and neurons

František Baluška
Pages 87 - 89
http://dx.doi.org/10.4161/psb.5.2.11237
Abstract | PDF

Plant cells and neurons share several similarities, including non-centrosomal microtubules, motile post-Golgi organelles, separated both spatially/structurally and functionally from the Golgi apparatus and involved in vesicular endocytic recycling, as well as cell-cell adhesion domains based on the actin/myosin cytoskeleton which serve for cell-cell communication. Tip-growing plant cells such as root hairs and pollen tubes also resemble neurons extending their axons. Recently, surprising discoveries have been made with respect to the molecular basis of neurodegenerative disorders known as Hereditary Spastic Paraplegias and tip-growth of root hairs. All these advances are briefly discussed in the context of other similarities between plant cells and neurons.




SHORT COMMUNICATION

Open Access Article

The plant as a biomechatronic system

Barbara Mazzolai, Cecilia Laschi, Paolo Dario, Sergio Mugnai and Stefano Mancuso
Pages 90 - 93
http://dx.doi.org/10.4161/psb.5.2.10457
Abstract | PDF

Our vision of plants is changing dramatically: from insensitive and static objects to complex living beings able to sense the environment and to use the information collected to adapt their behavior. At all times humans imitate ideas and concepts from nature to resolve technological problems. Solutions coming from plants have the potential to face challenges and difficulties of modern engineering design. Characteristic concepts of the plant world such as reiteration, modularity and swarm behavior could be of great help resolving technological problems. On the other hand a biorobotic approach would facilitate the resolution of many biological problems. In this paper, the concept of a plant-inspired robot is proposed for the investigation of both biological and technological issues.




MINI-REVIEW

Open Access Article

Probing the actin-auxin oscillator

Peter Nick
Pages 94 - 98
http://dx.doi.org/10.4161/psb.5.2.10337
Abstract | PDF

The directional transport of the plant hormone auxin depends on transcellular gradients of auxin-efflux carriers that continuously cycle between plasma membrane and intracellular compartments. This cycling has been proposed to depend on actin filaments. However, the role of actin for the polarity of auxin transport has been disputed. To get insight into this question, actin bundling was induced by overexpression of the actin-binding domain of talin in tobacco BY-2 cells and in rice plants. This bundling can be reverted by addition of auxins, which allows to address the role of actin organization on the flux of auxin. In both systems, the reversion of a normal actin configuration can be restored by addition of exogenous auxins and this fully restores the respective auxin-dependent functions. These findings lead to a model of a self-referring regulatory circuit between polar auxin transport and actin organization. To further dissect the actin-auxin oscillator, we used photoactivated release of caged auxin in tobacco cells to demonstrate that auxin gradients can be manipulated at a subcellular level.




REVIEWS

Open Access Article

Functional diversity of the plant glycine-rich proteins superfamily

Amanda Mangeon, Ricardo Magrani Junqueira and Gilberto Sachetto-Martins
Pages 99 - 104
http://dx.doi.org/10.4161/psb.5.2.10336
Abstract | PDF

The first plant glycine–rich proteins (GRPs) have been isolated more than 20 years ago based on their specific expression pattern and/or modulation by several biotic and abiotic factors. This superfamily is characterized by the presence of a glycine-rich domain arranged in (Gly)n-X repeats. The presence of additional motifs, as well as the nature of the glycine repeats, groups them in different classes. The diversity in structure as well as in expression pattern, modulation and sub-cellular localization have always indicated that these proteins, although classified as members of the same superfamily, would perform different functions in planta. Only now, two decades later, with the first functional characterizations of plant GRPs their involvement in diverse biological and biochemical processes are being uncovered. Here, we review the so far ascribed functions of plant GRPs.

Open Access Article

Redox-regulation of protein import into chloroplasts and mitochondria: Similarities and differences

Anna Stengel, J. Philipp Benz, Jürgen Soll and Bettina Bölter
Pages 105 - 109
http://dx.doi.org/10.4161/psb.5.2.10525
Abstract | PDF

Redox signals play important roles in many developmental and metabolic processes, in particular in chloroplasts and mitochondria. Furthermore, redox reactions are crucial for protein folding via the formation of inter- or intramolecular disulfide bridges. Recently, redox signals were described to be additionally involved in regulation of protein import: in mitochondria, a disulfide relay system mediates retention of cystein-rich proteins in the intermembrane space by oxidizing them. Two essential proteins, the redox-activated receptor Mia40 and the sulfhydryl oxidase Erv1 participate in this pathway. In chloroplasts, it becomes apparent that protein import is affected by redox signals on both the outer and inner envelope: at the level of the Toc complex (translocon at the outer envelope of chloroplasts), the formation/reduction of disulfide bridges between the Toc components has a strong influence on import yield. Moreover, the stromal metabolic redox state seems to be sensed by the Tic complex (translocon at the inner envelope of chloroplasts) that is able to adjust translocation efficiency of a subgroup of redox-related preproteins accordingly. This review summarizes the current knowledge of these redox-regulatory pathways and focuses on similarities and differences between chloroplasts and mitochondria.




RESEARCH PAPER

Open Access Article

Thermo and pH stable ATP-independent chaperone activity of heat-inducible Hsp70 from Pennisetum glaucum

J.L. Uma Maheswar Rao, Palakolanu Sudhakar Reddy, Rabi N. Mishra, Dinesh Gupta, Dinkar Sahal, Narendra Tuteja, Sudhir K. Sopory and Malireddy K. Reddy
Pages 110 - 121
http://dx.doi.org/10.4161/psb.5.2.10547
Abstract | PDF

Heat shock proteins (Hsps) are a class of molecular chaperones that play an essential role in preserving cellular functions under stressful conditions. The over production of recombinant proteins often causes cellular stress that results in aggregation /misfolding of proteins, which sometimes leads to the formation of inclusion bodies. Here we report the cloning and characterization of heat-inducible PgHsp70 from Pennisetum glaucum, a heat and drought tolerant plant that showed stability and chaperone activity at elevated temperatures. The predicted amino acid sequence of PgHsp70 revealed a high homology with Hsp70 from other plants, and the overall 3D structure homology modeling is similar to that of the constitutively expressed bovine cytosolic Heat Shock Cognate (HSC)-70. The purified recombinant protein had an apparent molecular mass of 70 kDa and displayed optimal chaperone activity at 50oC, and pH 8.0. Under these conditions, the T1/2 of PgHsp70 increased from 10 to 15 h in the presence of glycerol. The PgHsp70 exhibited a higher chaperone activity towards glutamate dehydrogenase than alcohol dehydrogenase. The expression of recombinant carbonic anhydrase (CA) in E. coli in a catalytically active soluble form rather than in inclusion bodies was made feasible by co-expression of PgHsp70. Circular dichroism (CD) studies of the recombinant PgHsp70 did not reveal any discernible changes in the α-helix content, with increase in temperature from 35 to 85^oC, thus suggesting a critical role of α-helix content in maintaining the chaperone activity.




ARTICLE ADDENDA

Open Access Article

Thigmomorphogenesis in Solanum lycopersicum: Morphological and biochemical responses in stem after mechanical stimulation

Issam Saidi, Saïda Ammar, Nathalie Demont-CauletSaïda, Johanne Thévenin, Catherine Lapierre, Sadok Bouzid and Lise Jouanin
Pages 122 - 125
http://dx.doi.org/10.4161/psb.5.2.10302
Abstract | PDF

The activation of the phenylpropanoid pathway in plants by environmental stimuli is one of the most universal biochemical stress responses known. In tomato plant, rubbing applied to a young internode inhibit elongation of the rubbed internode and his neighboring one. These morphological changes were correlated with an increase in lignification enzyme activities, phenylalanine ammonia-lyase (PAL), cinnamyl alcohol dehydrogenase (CAD) and peroxidases (POD), 24 hours after rubbing of the forth internode. Furthermore, a decrease in indole-3-acetic acid (IAA) content was detected in the rubbed internode and the upper one. Taken together, our results suggest that decrease in rubbed internode length is a consequence of IAA oxidation, increases in enzyme activities (PAL, CAD and POD), and cell wall rigidification associated with induction of lignification process.

Open Access Article

Mitochondrial protease AtFtsH4 protects ageing Arabidopsis rosettes against oxidative damage under short-day photoperiod

Marta Kicia, Edyta M. Gola and Hanna Janska
Pages 126 - 128
http://dx.doi.org/10.4161/psb.5.2.10303
Abstract | PDF

Mitochondrial AtFtsH4 protease, whose catalytic site is exposed to the intermembrane space, is one of four inner membrane-bound FtsH proteases in Arabidopsis. We found that the loss of AtFtsH4 altered Arabidopsis leaf morphology at the late stage of rosette growth under short-day photoperiod, while such changes were not observed in ftsh4 mutants grown under long days. These morphological changes were correlated with elevated levels of both reactive oxygen species (ROS) and carbonylated proteins, which strongly suggested that ageing ftsh4 plants experienced oxidative stress. This view was supported by the accumulation of electron-dense material, presumably containing aggregated oxidized proteins, in mitochondria of ftsh4 plants with the most strongly malformed leaf blades. Taken together, our data published in the May issue of Plant J ¹ suggest a link between the lack of AtFtsH4 protease, oxidative stress and altered leaf morphology at the late rosette stage under short days. Here, we present evidence that the onset of altered leaf morphology in ftsh4 correlates with an increase in the abundance of AtFtsH4 transcript observed in wild-type Arabidopsis growing under the same conditions. We also discuss how the lack of AtFtsH4 may cause oxidative stress towards the end of the vegetative growth in short days.

Open Access Article

Is there leaderless protein secretion in plants?

Fang-yi Cheng and John D. Williamson
Pages 129 - 131
http://dx.doi.org/10.4161/psb.5.2.10304
Abstract | PDF

The sugar alcohol mannitol and its catabolic enzyme mannitol dehydrogenase (MTD), in addition to well-documented roles in metabolism and osmoprotection, may play roles in host-pathogen interactions. Research suggests that in response to the mannitol that pathogenic fungi secrete to suppress reactive oxygen-mediated host defenses, plants make MTD to catabolize fungal mannitol. Yet previous work suggested that pathogen-secreted mannitol is extracellular, while in healthy plants MTD is cytoplasmic. We have presented results showing that the normally cytoplasmic MTD is exported into the cell wall or extracellular space in response to the endogenous inducer of plant defense responses salicylic acid (SA). This SA-induced secretion is insensitive to brefeldin A, an inhibitor of Golgi-mediated protein transport. Together with the absence of MTD in Golgi stacks and the lack of a documented extracellular targeting sequence in the MTD protein, this suggests MTD is secreted by a non-Golgi, pathogen-activated secretion mechanism in plants. Here we discuss the potential significance of non-Golgi secretion in response to stress.

Open Access Article

The ever expanding role of aquaglyceroporins: Confirmation of protein-facilitated boron transport

Kate L. Fitzpatrick and Robert Reid
Pages 132 - 133
http://dx.doi.org/10.4161/psb.5.2.10305
Abstract | PDF

The exact mechanism of transport of boron (B) entering the plant cell as boric acid B(OH)3, has become hotly debated with evidence for both passive and protein facilitated transport. Here we put the controversy to rest by confirming that boron influx into plants can be partially controlled by opening and closing of channel-like transport proteins. Using treatments that were likely to inhibit membrane transporters capable of facilitating B transport, we confirmed that at least 50 % of B transport could be contributed by a transporter of some type in barley roots. Based on the physiochemical similarities between B(OH)3 and other solutes that were known to be transported via aquaglyceroporins, we hypothesised that aquaglyceroporins would be likely candidates to facilitate B(OH)3 transport into the cytoplasm. We demonstrated using functional yeast complementation that two barley root aquaglyceroporins, HvPIP1;3 and HvPIP1;4, were both capable of facilitating B transport. This finding has demonstrated yet another function of aquaglyceroporins.

Open Access Article

Similarities and differences between phytochrome-mediated growth inhibition of coleoptiles and seminal roots in rice seedlings

Hisayo Shimizu, Tomoko Shinomura and Kotaro T. Yamamoto
Pages 134 - 135
http://dx.doi.org/10.4161/psb.5.2.10335
Abstract | PDF

In rice, light is known to inhibit the growth of coleoptiles and seminal roots of seedlings through phytochrome. Here we investigated the light-induced growth inhibition of seminal roots and compared the results with those recently determined for coleoptiles. Although three rice phytochromes, phyA, phyB and phyC functioned in a similar manner in coleoptile and seminal root, the Bunsen-Roscoe law of reciprocity was not observed in the growth inhibition of seminal root. We also found coiling of the seminal root at the root tip which appeared to be associated with the photoinhibition of seminal root growth. This could be a new light-induced phenomenon in certain cultivars of rice.

Open Access Article

Rice plants response to the disruption of OsCSLD4 gene

Rui Li, Guangyan Xiong, Baocai Zhang and Yihua Zhou
Pages 136 - 139
http://dx.doi.org/10.4161/psb.5.2.10369
Abstract | PDF

Plant cell wall is a complex polysaccharide network and performs important developmental and physiological functions far beyond supplying the physical constrains. Plant cells have the ability to react to cell wall defects as exhibited by changes in gene expression, accumulation of ectopic lignin, stress responses, and growth arrest. It is a major challenge to understand how plants sense and response to wall integrity since very little is known about the signaling involved in the responses. Cellulose synthase-like D (CSLD) proteins mediating the biosynthesis of a wall polysaccharide polymer make up a common subfamily to all plants. Recently, we have reported the functional characterization of CSLD4 in rice. Mutation in OsCSLD4 shows morphological alterations and pleiotropic effects on wall compositions and structure. Our study demonstrates that OsCSLD4 plays critical roles in cell wall formation and plant growth. Here we show the subtle wall alterations through separating the culm residues into five fractions. Quantitative RT-PCR analysis further revealed that the expression of various genes involved in xylan synthesis and cell-cycle regulation was altered in mutant plants, as the responses to OsCSLD4 disruption. Therefore, plants may have a fine sensory machinery to react to wall defects and modulate growth for adapting to the changes.

Open Access Article

Brassinosteroid negatively regulates jasmonate inhibition of root growth in Arabidopsis

Ying Huang, Chengyun Han, Wen Peng, Zhihong Peng, Xingyao Xiong, Qi Zhu, Bida Gao, Daoxin Xie and Chunmei Ren
Pages 140 - 142
http://dx.doi.org/10.4161/psb.5.2.10399
Abstract | PDF

Jasmonate (JA) inhibits root growth of Arabidopsis thaliana seedlings. The mutation in COI1 that plays a central role in JA signaling displays insensitivity to JA inhibition of root growth. To dissect JA signaling pathway, we recently isolated one mutant named psc1, which partially suppresses coi1 insensitivity to JA inhibition of root growth. As we identified the PSC1 gene as an allele of DWF4 that encodes a key enzyme in brassinosteroid (BR) biosynthesis, we hypothesized and demonstrated that BR is involved in JA signaling and negatively regulates JA inhibition of root growth. In our Plant Physiology paper, we analyzed effects of psc1 or exogenous BR on the inhibition of root growth by JA. Here we show that treatment with brassinazole (Brz), a BR biosynthesis inhibitor, increased JA sensitivity in both coi1-2 and wild type, which further confirms that BR negatively regulates JA inhibition of root growth. Since effects of psc1, Brz, and exogenous BR on JA inhibition of root growth were mild, we suggests that BR negatively finely regulates JA inhibition of root growth in Arabidopsis.

Open Access Article

Radical-induced cell death1 and similar to RCD one1 and the stress-induced morphogenetic response

Sachin Teotia, Sivaramakrishnan Muthuswamy and Rebecca Lamb
Pages 143 - 145
http://dx.doi.org/10.4161/psb.5.2.10400
Abstract | PDF

Plants exposed to abiotic stress show a range of morphogenetic responses, sometimes termed the Stress-Induced Morphogenetic Response (SIMR). SIMR is principally composed of three components: inhibition of cell elongation, alterations in cell differentiation, and stimulus of cell division in localized areas. An explanation proposed for SIMR has been increased accumulation of reactive oxygen species (ROS) and alterations in hormone signalling. Mutations in the Arabidopsis thaliana RADICAL-INDUCED CELL DEATH1 (RCD1) gene have altered abiotic stress responses and ROS accumulation. Even in the absence of exogenous stress, these plants show many morphological changes also seen in SIMR. In the September Issue of Plant Physiology we reported an in depth analysis of the phenotype of rcd1-3 plants as well as the phenotype of a mutations in the previously uncharacterized paralog of RCD1, SIMILAR TO RCD ONE1 (SRO1). sro1-1 plants have mild morphological changes and abiotic stress response defects while rcd1-3; sro1-1 double mutant plants have severe developmental defects, including less cell elongation. In this Addendum, we hypothesize that rcd1, sro1, and rcd1; sro1 mutant plants are under constitutive stress, and that this stress is responsible for at least some of the developmental defects seen in these plants.

Open Access Article

Co-localization of mitochondria with chloroplasts is a light-dependent reversible response

Md. Sayeedul Islam and Shingo Takagi
Pages 146 - 147
http://dx.doi.org/10.4161/psb.5.2.10410
Abstract | PDF

Co-localization of mitochondria with chloroplasts in plant cells has long been noticed as beneficial interactions of the organelles to active photosynthesis. Recently, we have found that mitochondria in mesophyll cells of Arabidopsis thaliana expressing mitochondrion-targeted green fluorescent protein (GFP) change their distribution in a light-dependent manner. Mitochondria occupy the periclinal and anticlinal regions of palisade cells under weak and strong blue light, respectively. Redistributed mitochondria seem to be rendered static through co-localization with chloroplasts. Here we further demonstrated that distribution patterns of mitochondria, together with chloroplasts, returned back to those of dark-adapted state during dark incubation after blue-light illumination. Reversible association of the two organelles may underlie flexible adaptation of plants to environmental fluctuations.

Open Access Article

Role of cytokinin responsive two-component system in ABA and osmotic stress signalings

Lam-Son Phan Tran, Kazuo Shinozaki and Kazuko Yamaguchi-Shinozaki
Pages 148 - 150
http://dx.doi.org/10.4161/psb.5.2.10411
Abstract | PDF

The two-component signaling systems (TCSs), which mediate the histidine-aspartate signaling, control diverse biological processes of many organisms, including cell division, cell growth and proliferation, and responses to environmental stimuli and growth regulators. We have provided in planta evidence that the cytokinin (CK) responsive TCS mediates abscisic acid (ABA) and osmotic stress responses. By using loss-of-function approach we have demonstrated that the three cytokinin (CK) receptor histidine kinases AHK2, AHK3 and AHK4/CRE1 act as negative regulators in ABA, drought and high salinity stress signalings. Genome-wide expression profiling of the stress-tolerant double mutant suggested that CK receptor kinases mediate osmotic stress response in both an ABA-dependent and ABA-independent manner. Additionally, we showed evidence as for the role of CK in mediating stress responses, judging from the fact that AHK4 requires the CK to function as a negative regulator in osmotic stress response. Our results suggested that cross-talks exist among CK, ABA and osmotic stress signaling pathways, and that CK signaling and CK metabolism may play crucial roles not only in plant growth and development but also in osmotic stress signaling.

Open Access Article

In vivo subcellular targeting analysis validates a novel peroxisome targeting signal type 2 and the peroxisomal localization of two proteins with putative functions in defense in Arabidopsis

Sheng Quan, Robert Switzenberg, Sigrun Reumann and Jianping Hu
Pages 151 - 153
http://dx.doi.org/10.4161/psb.5.2.10412
Abstract | PDF

Plant peroxisomes are essential organelles that house diverse metabolic activities. To understand the full spectrum of peroxisomal functions, we recently performed a proteomic analysis of Arabidopsis leaf peroxisomes followed by in vivo subcellular targeting validation of some novel proteins. Here, we continue with the targeting analysis and demonstrate that the nonapeptide RVx5HF, which is present near the N terminus of the newly identified peroxisomal protein, HIT3 (histidine triad family protein 3), is a functional peroxisome targeting signal type 2 (PTS2). In addition, we have confirmed the peroxisomal localization of UP6 (unknown protein 6) and GLX1 (glyoxalase 1 homolog), two proteins with possible roles in stress responses and glutathione-dependent detoxification, respectively. These data, together with results from our previous analysis of the peroxisomal proteome, reinforce the notion that peroxisomes are involved in various stress responses and suggest glutathione as a major antioxidant in plant peroxisomes.

Open Access Article

Plant SUN domain proteins: Components of putative plant LINC complexes?

Katja Graumann and David E. Evans
Pages 154 - 156
http://dx.doi.org/10.4161/psb.5.2.10458
Abstract | PDF

We have recently reported the identification and characterization of Sad1/UNC84 (SUN) domain proteins in various plant species. In animals and yeasts, SUN domain proteins are localized at the inner nuclear membrane and form a bridge across the nuclear envelope (NE) by interacting with outer nuclear membrane-localized Klarsicht/Anc-1/Syne-1 homology (KASH) domain proteins. This bridge physically connects cytoskeletal elements with chromatin and nucleoskeletal components. These multiprotein complexes are essential for various cellular and nuclear processes. The identification of SUN domain proteins provides the first evidence of putative NE bridging complexes in plants. Here we speculate on the composition and functions of these in regards to our current understanding of plant SUN domain proteins.

Open Access Article

Dual regulation of iron deficiency response mediated by the transcription factor IDEF1

Takanori Kobayashi, Hiromi Nakanishi and Naoko K. Nishizawa
Pages 157 - 159
http://dx.doi.org/10.4161/psb.5.2.10459
Abstract | PDF

Higher plants respond to fluctuating Fe availability by regulating the expression of genes involved in Fe homeostasis. Transcriptional responses to Fe deficiency in plants are mediated via various interactions between cis-acting elements and trans-acting factors. The transcription factor IDEF1 regulates the response to Fe deficiency in Oryza sativa (rice) by recognizing the CATGC sequence within the Fe deficiency-responsive cis-acting element IDE1. We recently presented evidence that IDEF1 mediates two-phase responses to Fe deficiency. During the early stages of Fe deficiency, the majority of known Fe uptake/utilization-related genes are positively regulated by IDEF1. In subsequent stages, IDEF1-mediated regulation of these Fe uptake/utilization-related genes are less obvious. In turn, expression of several Fe deficiency-induced genes encoding late embryogenesis abundant proteins is increasingly regulated by IDEF1 at the subsequent stages. We propose a dual function of IDEF1 for Fe deficiency response, namely, (1) the coordinated transactivation of Fe utilization-related genes via CATGC-containing IDE1-like elements, especially at the early stage, and (2) the transactivation of seed maturation-related genes via RY elements, especially during the subsequent stages of Fe deficiency. IDEF1 appears to have evolved to mediate an interface between Fe deficiency-inducible and seed maturation-related gene expression.

Open Access Article

The PP2C–SnRK2 complex: The central regulator of an abscisic acid signaling pathway

Takashi Hirayama and Taishi Umezawa
Pages 160 - 163
http://dx.doi.org/10.4161/psb.5.2.10460
Abstract | PDF

The phytohormone abscisic acid (ABA), an important bioactive compound in plants, is implicated in several essential processes such as development and the abiotic stress response. Many components have been reported to have roles in these processes. Although 2C-type protein phosphatases (PP2C) and SNF1-related protein kinases (SnRK2) are known to be important signal mediators, the molecular mechanisms by which these components regulate the ABA signaling pathway have not been elucidated. Recent identification of soluble ABA receptors, PYR/PYL/RCAR, has provided a major breakthrough in understanding the signaling mechanisms of ABA and revealed the importance of PP2Cs. In addition, the physical, biochemical, and physiological connections between PP2C and SnRK2 have been clearly demonstrated. Taken together, the molecular basis of the major ABA signaling pathway has been established, from perception to gene expression. In this addendum, we discuss this emerging ABA signaling pathway, which has a conventional protein phosphorylation/dephosphorylation regulatory circuit and consider its physiological and functional relevance.

Open Access Article

The evolution of the regulatory mechanism of chloroplast division

Kumiko Okazaki, Yukihiro Kabeya and Shin-ya Miyagishima
Pages 164 - 167
http://dx.doi.org/10.4161/psb.5.2.10461
Abstract | PDF

Chloroplasts arose from a cyanobacterial endosymbiont and multiply by division, reminiscent of their free-living ancestor. However, chloroplasts can not divide by themselves, and the division is performed and controlled by proteins that are encoded by the host nucleus. The continuity of chloroplasts was originally established by synchronization of endosymbiotic cell division with host cell division, as seen in existent algae. In contrast, land plant cells contain multiple chloroplasts, the division of which is not synchronized, even in the same cell. Land plants have evolved cell and chloroplast differentiation systems in which the size and number of chloroplasts (or other types of plastids) change along with their respective cellular function by changes in the division rate. We recently reported that PLASTID DIVISION (PDV) proteins, land-plant specific components of the chloroplast division apparatus, determined the rate of chloroplast division. The level of PDV protein is regulated by the cell differentiation program based on cytokinin, and the increase or decrease of the PDV level gives rise to an increase or decrease in the chloroplast division rate. Thus, the integration of PDV proteins into the chloroplast division machinery enabled land plant cells to change chloroplast size and number in accord with the fate of cell differentiation.

Open Access Article

RFC3 regulates cell proliferation and pathogen resistance in Arabidopsis

Shitou Xia, Langtao Xiao, Patrick Gannon and Xin Li
Pages 168 - 170
http://dx.doi.org/10.4161/psb.5.2.10526
Abstract | PDF

Replication factor C subunit 3 (RFC3) is one of the small subunits of the RFC complex originally purified from the HeLa cells that is essential for the in vitro replication of Simian virus 40 (SV40). Although RFC has been reported to be involved in DNA replication, DNA repair and check-point control of cell cycle progression in yeast, little is known about the precise function of each subunit of the RFC in plants. We recently reported the identification of rfc3-1, which carries a point mutation leading to plants with enhanced expression of Pathogenesis-Related (PR) genes and resistance against the virulent oomycete Hyaloperonospora arabidopsidis (H. a.) Noco2. The mutant is hypersensitive to SA and has enhanced pathogen resistance independent of Nonexpressor of PR genes 1 (NPR1). The rfc3-1 mutation caused a substitution from a nonpolar aliphatic amino acid (Gly-84) to a negatively charged amino acid (Asp) in functional domain III, which is one of eight conserved domains in the RFC. This may interfere with the interaction between RFC3 and other subunits, compromising the function of the protein complex, and leading to cell proliferation defects in the leaves and roots of Arabidopsis. Furthermore, enhanced expression of PR genes and induction of systemic acquired resistance in rfc3-1 may be caused by a partial loss of RFC function through its involvement in replication-coupled chromatin assembling.

Open Access Article

Arabidopsis glutathione reductase 1 is dually targeted to peroxisomes and the cytosol

Amr R. A. Kataya and Sigrun Reumann
Pages 171 - 175
http://dx.doi.org/10.4161/psb.5.2.10527
Abstract | PDF

We recently established a proteome methodology for Arabidopsis leaf peroxisomes and identified more than 90 putative novel proteins of the organelle. These proteins included glutathione reductase isoform 1 (GR1), a major enzyme of the antioxidative defense system that was previously reported to be cytosolic. In this follow-up study, we validated the proteome data by analyzing the in vivo subcellular targeting of GR1 and the function of its C-terminal tripeptide, TNL>, as a putative novel peroxisome targeting signal type 1 (PTS1). The full-length protein was targeted to peroxisomes in onion epidermal cells when fused N-terminally with the reporter protein. The efficiency of peroxisome targeting, however, was weak upon expression from a strong promoter, consistent with the idea that the enzyme is dually targeted to peroxisomes and the cytosol in vivo. The reporter protein that was extended C-terminally by 10 amino acid residues of GR1 was directed to peroxisomes, characterizing TNL> as a novel PTS1. The data thus identify plant peroxisomal GR at the molecular level in the first plant species and complete the plant peroxisomal ascorbateglutathione cycle. Moreover, GR1 is the first plant protein that is dually targeted to peroxisomes and the cytosol. The evolutionary origin and regulatory mechanisms of dual targeting are discussed.

Open Access Article

Microgravity environment uncouples cell growth and cell proliferation in root meristematic cells: The mediator role of auxin

Francisco Javier Medina and Raúl Herranz
Pages 176 - 179
http://dx.doi.org/10.4161/psb.5.2.10966
Abstract | PDF

Experiments performed in space have evidenced that, in root meristematic cells, the absence of gravity results in the uncoupling of cell growth and cell proliferation, two essential cellular functions that support plant growth and development, which are strictly coordinated under normal ground gravity conditions. In space, cell proliferation appears enhanced whereas cell growth is depleted. Since coordination of cell growth and proliferation is a major feature of meristematic cells, the observed uncoupling is a serious stress condition for these cells producing important alterations in the developmental pattern of the plant. Auxin plays a major role in these processes both by assuring the coupling of cell growth and proliferation under normal conditions and by exerting a decisive influence in the uncoupling under altered gravity conditions. Auxin is a mediator of the transduction of the gravitropic signal and its distribution in the root is altered subsequent to a change in the gravity conditions. This altered distribution may produce changes in the expression of specific growth coordinators leading to the alteration of cell cycle and protein synthesis. Therefore, available data indicate that the effects of altered gravity on cell growth and proliferation are the consequence of the transduction of the gravitropic signal perceived by columella cells, in the root tip.

Open Access Article

Structural features of plant subtilases

Rolf Rose, Andreas Schaller and Christian Ottmann
Pages 180 - 183
http://dx.doi.org/10.4161/psb.5.2.11069
Abstract | PDF

Serine proteases of the subtilase family are present in Archaea, Bacteria, and Eukarya. Many more subtilases are found in plants as compared to other organisms, implying adaptive significance for the expansion of the subtilase gene family in plants. Structural data, however, were hitherto available only for non-plant subtilases. We recently solved the first structure of a plant subtilase, SlSBT3 from tomato (Solanum lycopersicum). SlSBT3 is a multidomain enzyme displaying a subtilisin, a Protease-Associated (PA) and a fibronectin (Fn) III-like domain. Two prominent features set SlSBT3 apart from other structurally elucidated subtilases: (1) activation by PA domain-mediated homo-dimerization and (2) calcium-independent activity and thermostability. To address the question whether these characteristics are unique features of SlSBT3, or else, general properties of plant subtilases, homology models were calculated for representative proteases from tomato and Arabidopsis using the SlSBT3 structure as template. We found the major structural elements required for the stabilization of the subtilisin domain to be conserved among all enzymes analyzed. PA domain-mediated dimerization as an auto-regulatory mechanism of enzyme activation, on the other hand, appears to be operating in only a subset of the analyzed subtilases.

Open Access Article

Subcellular localization and turnover of Arabidopsis phototropin 1

Stuart Sullivan, Eirini Kaiserli, Tong-Seung Tseng and John M. Christie
Pages 184 - 186
http://dx.doi.org/10.4161/psb.5.2.11082
Abstract | PDF

We reported recently that internalization of the plant blue light receptor phototropin 1 (phot1) from the plasma membrane in response to irradiation is reliant on receptor autophosphorylation. Pharmacological interference and co-immunoprecipitation analyses also indicated that light-induced internalization of phot1 involves clathrin-dependent processes. Here, we describe additional pharmacological studies that impact the subcellular localization and trafficking of Arabidopsis phot1. Alterations in the microtububle cytoskeleton led to dramatic differences in phot1 localization and function. Likewise, inhibition of phosphatidic acid (PA) signaling was found to impair phot1 localization and function. However, action of PA inhibition on phot1 function may be attributed to pleiotropic effects on cell growth. While phot1 kinase activation is necessary to stimulate its internalization, autophosphorylation is not required for phot1 turnover in response to prolonged blue light irradiation. The implications of these findings in regard to phot1 localization and function are discussed.

Open Access Article

Sequence divergences between cyst nematode effector protein orthologs may contribute to host specificity

Tarek Hewezi and Thomas J. Baum
Pages 187 - 189
http://dx.doi.org/10.4161/psb.5.2.11084
Abstract | PDF

The soybean cyst nematode (Heterodera glycines) and the closely related sugar beet cyst nematode (Heterodera schachtii) are devastating pathogens of plant roots that use secreted effector proteins to engage in sophisticated host-parasite interactions. While H. schachtii infects and reproduces readily on the roots of Arabidopsis thaliana, H. glycines rarely is able to infect this model plant. The molecular basis for differing host ranges remains obscure but likely involves differences between nematode effector proteins and the recognition of host factors. Recently we reported that constitutive expression of the H. schachtii 10A06 effector protein gene (Hs-10A06) in Arabidopsis affected plant morphology and increased susceptibility to H. schachtii and that the 10A06 protein functions through its interaction with Arabidopsis spermidine synthase 2 (SPDS2). Therefore, we investigated whether differences between cyst nematode effector protein orthologs in two nematode species have a role in mediating host specificity. Here, we show that, similar to Hs-10A06, ectopic expression of H. glycines 10A06 (Hg-10A06) in Arabidopsis affected leaf number and root length, however, to a much lesser extent. More importantly, no effect of Hg-10A06 overexpression on Arabidopsis susceptibility to H. schachtii was observed. While we found that Hg-10A06 can weakly interact with Arabidopsis SPDS2 in yeast-two hybrid assays, this ability to interact with SPDS2 was decreased approximately 5-fold compared with Hs-10A06. Collectively, these data suggest that sequence divergence between cyst nematode effector protein orthologs could contribute in determining cyst nematode host range.

Open Access Article

AtTZF gene family localizes to cytoplasmic foci

Marcelo Pomeranz, Pei-Chi Lin, John Finer and Jyan-Chyun Jang
Pages 190 - 192
http://dx.doi.org/10.4161/psb.5.2.10988
Abstract | PDF

In eukaryotes, mRNA turnover and translational repression represent important regulatory steps in gene expression. Curiously, when under cellular stresses, factors involved in these processes aggregate into cytoplasmic foci known as Processing bodies (P-bodies) and Stress Granules (SGs). In animals, CCCH Tandem Zinc Finger (TZF) proteins play important roles in mRNA decay within P-bodies. TTP, a P-body localized mammalian TZF, can bind to the 3’UTRs of mRNAs containing AU-rich elements (AREs) and recruit and activate RNA decay machineries. In plants, the function of CCCH TZF proteins is still unclear. We previously showed that AtTZF1, a member of the Arabidopsis CCCH TZF protein family, could bind both RNA and DNA in vitro, traffic between the nucleus and cytoplasmic foci, and co-localize with P-body markers AGO1, DCP2, and XRN4. In this report, we show that the other 10 members of the AtTZF gene family can similarly localize in the cytoplasmic foci. Together these 11 genes, though similar in TZF sequence and sub-cellular localization, appear to be differentially expressed in response to developmental and environmental cues. We propose that AtTZF1 and its gene family members may each play distinct roles at different developmental stages and environmental conditions.

Open Access Article

Evolution of serine carboxypeptidase-like acyltransferases in the monocots

Sam T Mugford and Anne Osbourn
Pages 193 - 195
http://dx.doi.org/10.4161/psb.5.2.11093
Abstract | PDF

The serine carboxypeptidases are a large family of proteases. In higher plants some members of this family have diversified and adopted new functions as acyltransferases required for the synthesis of natural products. We recently reported the first serine carboxypeptidase-like (SCPL) acyltransferase enzyme to be characterised from monocotyledonous plants.¹ This enzyme, AsSCPL1, is required for acylation of antimicrobial terpenes (avenacins) that are produced in the roots of oat (Avena spp.) and that provide protection against soil-borne pathogens. The SCPL enzyme family has undergone substantial expansion following the divergence of monocots and dicots. Here we discuss the evolution of the SCPL enzyme family in monocots, their contribution to metabolic diversity, and the roles of these enzymes in biotic and abiotic stress tolerance.

Open Access Article

New insights into the retrograde signaling pathway between the plastids and the nucleus

Tomohiro Kakizaki and Takehito Inaba
Pages 196 - 199
http://dx.doi.org/10.4161/psb.5.2.11107
Abstract | PDF

In plant cells, the genetic information required for biological activity is divided into 3 organelles—the nucleus, plastids, and mitochondria. These organelles require tightly coordinated gene expression to accomplish the appropriate biological processes. Chloroplasts harness light energy and use it for carbon fixation in photosynthesis. However, majority of the proteins involved in photosynthesis is encoded by the nucleus genome. Thus, nuclear-encoded photosynthesis-related proteins are targeted to plastids after their synthesis in the cytosol. Therefore, it is critical to regulate nuclear gene expression in response to the functional or metabolic state of the plastids; this process relies on signals from the plastids to the nucleus that are known as retrograde signals. Our genetic studies revealed that GENOMES UNCOUPLED 1 (GUN1) and Golden2-like1 (GLK1) mediate the retrograde signal that coordinates plastid protein import and nuclear gene expression. In this study, we propose a novel signaling pathway that regulates nuclear gene expression according to the rate of protein import into the plastid.

Open Access Article

Shedding light on the role of AT-hook/PPC domain protein in Arabidopsis thaliana

Kian-Hong Ng and Toshiro Ito
Pages 200 - 201
http://dx.doi.org/10.4161/psb.5.2.11111
Abstract | PDF

Flower reproductive development is a complex process involving well-coordinated control of transcriptional regulation cascades. AGAMOUS (AG) plays an instrumental role in the specification and differentiation of reproductive organs in Arabidopsis thaliana. We recently characterized a downstream target gene of AG, GIANT KILLER (GIK), which encodes for an AT-hook/plants and prokaryotes conserved (PPC) domain protein. We found that overexpression of GIK leads to severe reproductive defects and downregulation of genes involved in patterning and differentiation of reproductive floral organs. We showed that GIK is a matrix protein, and GIK-mediated gene regulation requires binding of GIK to matrix associated region (MAR) of the target genes. We further showed that GIK-mediated negative regulation of one of the target genes, ETTIN (ETT), is associated with changes of chromatin histone modification at ETT promoter, suggesting that GIK acts as a gene expression modulator through chromatin organization.

Open Access Article

Positioning of the auxin maximum affects the character of cells occupying the root stem cell niche

Keni Jiang and Lewis J. Feldman
Pages 202 - 204
http://dx.doi.org/10.4161/psb.5.2.11188
Abstract | PDF

Using a microarray-based approach the root meristem of maize was profiled.⁶ Three populations were characterized; the root cap, the quiescent center and the proximal meristem. From these data we here consider the central role of auxin in defining and establishing the root stem cell niche.

Open Access Article

Hormone-mediated promotion of trichome initiation in plants is conserved but utilizes species and trichome-specific regulatory mechanisms

Lies Maes and Alain Goossens
Pages 205 - 207
http://dx.doi.org/10.4161/psb.5.2.11214
Abstract | PDF

Plant trichome initiation is steered by diverse developmental and environmental cues, through molecular mechanisms that remain elusive in most plant species. Using a robust experimental method to investigate the molecular mechanisms by which phytohormones modulate leaf trichome formation, we verified the effect of jasmonates, cytokinins, and gibberellins in Arabidopsis (Arabidopsis thaliana). All three phytohormones promoted Arabidopsis trichome initiation, but caused divergent effects on trichome maturation and other leaf parameters. Molecular analysis indicated that the phytohormones mediated trichome initiation by the transcriptional regulation of the components of the TRANSPARENT TESTA GLABRA1 (TTG1) activator/inhibitor complex. In this addendum, we additionally studied the effects of jasmonates, cytokinins, and gibberellins on leaf trichome formation in a representative set of plant species, spanning the angiosperm lineage and covering different trichome types. We found that the general ability of the three phytohormones to impinge on trichome initiation is conserved across angiosperms, but that within a particular plant species distinct regulatory networks might be activated to steer the formation of the various trichome types.