Table of Contents

Volume 7, Issue 4

  April 2012

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SHORT COMMUNICATIONS

Rice transgenic plants with suppressed expression of the β subunit of the heterotrimeric G protein

Yuzuko Utsunomiya, Chihiro Samejima, Yukiko Fujisawa, Hisaharu Kato and Yukimoto Iwasaki
Abstract | Full Text

The deficient mutant for the rice heterotrimeric G protein α subunit gene (RGA1), d1, showed dwarfism and set small seed due to a reduced cell number. Mutants for the rice heterotrimeric G protein β subunit gene (RGB1) have not been isolated. To determine the functions of RGB1, transgenic rice plants with suppressed expression of RGB1 were studied using the RNAi method. RGB1 knock-down lines showed browning of the lamina joint regions and nodes and reduced fertility, but these abnormality were not observed in d1. Transgenic plants in which the G protein β subunit was greatly decreased were not obtained, suggesting that the complete suppression of RGB1 mRNA may be lethal. In contrast, the d1 mutants, with complete loss of the G protein α subunit, were fertile and half the size of the WT. These studies suggest that RGB1 has different functions than RGA1.

Ecological role of transgenerational resistance against biotic threats

Sergio Rasmann, Martin De Vos and Georg Jander
Abstract | Full Text | PDF

Plants in their natural environments are constantly subjected to biotic stress. In addition to possessing physical barriers and anti-nutritive toxins, plants can be primed to respond more efficiently against future attack via faster and stronger gene activation. Here we discuss recent findings showing that plants can pass signatures of attack to the next generation, thus rendering the progeny more resistant against insect and pathogen attack. A combination of phytohormone signaling, small RNA-mediated gene silencing, and DNA methylation are involved in transgenerational induced resistance. Epiallelic variation against biotic threats should be under positive selection in populations of plants where the environment is predictable over time. Similarly, in very genetically homogenous populations, such as during range expansion, epigenome reorganization is a likely mechanism for faster plant adaptation to novel biotic attack. Further research is needed to understand the relative role of the genome vs. the epigenome for the evolution of increased plant resistance.

Multivesicular bodies in developing tobacco seed and mung bean are functionally equivalent

Yu Chung Tse, Junqi Wang and Liwen Jiang
Abstract | Full Text | PDF

Protein storage vacuoles (PSVs) are the primarily storage organelles in cotyledon cells for protein preservation in seeds. Storage proteins are transported from the endoplasmic reticulum (ER) to the Golgi apparatus for subsequent delivery to PSVs via presumably Golgi-derived dense vesicles (DVs). However, recent studies demonstrated that storage proteins in early stage of developing cotyledon of mung beans reached the multivesicular bodies (MVBs) prior to the detection of DVs, indicating the possible involvement of MVBs in mediating transport of storage proteins during the early stage of seed development. Here, we further show that the MVBs in developing tobacco seeds are functionally and biochemically equivalent to those in developing mung beans. Thus, MVBs in developing tobacco seeds are structurally distinct from DVs, contain both vacuolar sorting receptors (VSRs) and storage proteins, and they are insensitive to treatments of wortmannin and brefeldin A (BFA).

Membrane microdomain may be a platform for immune signaling

Yiping Qi and Fumiaki Katagiri
Abstract | Full Text | PDF

Arabidopsis RPS2 is a typical disease resistance (R) protein with nucleotide-binding leucine-rich repeats (NB-LRR). Previously, we reported that RPS2 is physically associated with some Arabidopsis hypersensitive induced reaction (AtHIR) proteins, which are enriched in membrane microdomains. Biochemical and genetic analyses suggested that members of the AtHIR gene family have a function in RPS2-mediated immune signaling. Here, we provide evidence that the pattern recognition receptor (PRR) FLS2 is also physically associated with AtHIR2 in a N. benthamiana transient expression system. We thus speculate that PM microdomains provide a platform for both types of immune receptors, R proteins and PRRs, and that the activation of the receptors is facilitated by AtHIR proteins.

Ethylene response pathway is essential for ARABIDOPSIS A-FIFTEEN function in floral induction and leaf senescence

Guan-Hong Chen, Yuan-Li Chan, Chia-Ping Liu and Long-Chi Wang
Abstract | Full Text | PDF

ARABIDOPSIS A-FIFTEEN (AAF) encodes a plastid protein and was originally identified as a SENESCENCE-ASSOCIATED GENE. Previously, we found that overexpression of AAF (AAF-OX) in Arabidopsis led to accumulated reactive oxygen species and promoted leaf senescence induced by oxidative stress, which was suppressed by a null mutant, ein2–5, in ethylene response pathway. Whether AAF function is involved in ethylene biosynthesis and/or the response pathway remained unknown. Here we showed that neither overexpression (AAF-OX) nor a null mutant (aaf-KO) of AAF generated a higher level of ethylene than the wild type and displayed a typical triple-response phenotype in etiolated seedlings treated with 1-aminocyclopropane-1-carboxylic acid (ACC). Nevertheless, ein2–5 suppressed the phenotypes of early flowering and age-dependent leaf senescence in AAF-OX plants. We reveal that a functional ethylene response is essential for AAF function in leaf senescence and floral induction, but AAF is unlikely a regulatory component integral to the ethylene pathway.

Cell type-specific protein and transcription profiles implicate periarbuscular membrane synthesis as an important carbon sink in the mycorrhizal symbiosis

Nicole Gaude, Waltraud X. Schulze, Philipp Franken and Franziska Krajinski
Abstract | Full Text | PDF

The development of an arbuscular mycorrhizal (AM) symbiosis is a non-synchronous process with typical mycorrhizal root containing different symbiotic stages at one time. Methods providing cell type-specific resolution are therefore required to separate these stages and analyze each particular structure independently from each other.

 

We established an experimental system for analyzing specific proteomic changes in arbuscule-containing cells of Glomus intraradices colonized Medicago truncatula roots. The combination of laser capture microdissection (LCM) and liquid chromatography-tandem mass chromatography (LC-MS/MS) allowed the identification of proteins with specific or increased expression in arbuscule-containing cells. Consistent with previous transcriptome data, the proteome of arbuscule-containing cells showed an increased number of proteins involved in lipid metabolism, most likely related to the synthesis of the periarbuscular membrane. In addition, transcriptome data of non-colonized cells of mycorrhizal roots suggest mobilization of carbon resources and their symplastic transport toward arbuscule-containing cells for the synthesis of periarbuscular membranes. This highlights the periarbuscular membrane as important carbon sink in the mycorrhizal symbiosis.

Function of Arabidopsis SWAP70 GEF in immune response

Koji Yamaguchi and Tsutomu Kawasaki
Abstract | Full Text | PDF

In animals, major classes of Rho guanine nucleotide exchange factors (GEFs) possess a Dbl (diffuse B-cell lymphoma)- homology (DH) domain that functions as a GEF-catalytic domain. However, no GEFs with the DH domain had been identified in plants. Recently, we found that the rice homolog of human SWAP70, Oryza sative (Os) SWAP70, containing the DH domain, exhibited GEF activity toward the rice Rho GTPase OsRac1, and regulates chitin-induced production of reactive oxygen species and defense gene expression in rice.¹ Arabidopsis contains a single SWAP70 gene. A T-DNA insertion mutant of Arabidopsis SWAP70 was morphologically wild type. Measurement of in planta growth of Pseudomonas syringae DC3000 hrcC, a mutant incapable of type III effector delivery, revealed enhanced growth of the pathogen in the atswap70 mutant, indicating that AtSWAP70 is required for PAMP-triggered immunity. In addition, the atswap70 mutation reduced the RPM1-mediated hypersensitive response. These results suggested that AtSWAP70 plays a role in both PAMP- and effector-triggered immunity in Arabidopsis.




MINI-REVIEWS

Interactions between nitric oxide and plant hormones in aluminum tolerance

Huyi He, Longfei He and Minghua Gu
Abstract | Full Text | PDF

Nitric oxide (NO) is involved, together with plant hormones, in the adaptation to Al stress in plants. However, the mechanism by which NO and plant hormones interplay to improve Al tolerance are still unclear. We have recently shown that patterns of plant hormones alteration differ between rye and wheat under Al stress. NO may enhance Al tolerance by regulating hormonal equilibrium in plants, as a regulator of plant hormones signaling. In this paper, some unsolved issues are discussed based on recent studies and the complex network of NO and plant hormones in inducing Al tolerance of plants are proposed.

Does diacylglycerol serve as a signaling molecule in plants?

Wei Dong, Hongjun Lv, Guangmin Xia and Mengcheng Wang
Abstract | Full Text | PDF

Diacylglycerol (DAG) is an important signaling phospholipid in animals, specifically binding to the C1 domain of proteins such as protein kinase C. In most plant species, however, DAG is present at low abundance, and no interacting proteins have yet been identified. As a result, it has been proposed that the signaling function of DAG has been discarded by plants during their evolution. In this mini-review, we summarize the accumulating experimental evidence which supports that notion that changes in DAG content in response to particular cues are a feature of plant cells. This behavior suggests that DAG does indeed act as a signaling molecule during plant development and in response to certain environmental stimuli.




REVIEW

Nitric oxide counters ethylene effects on ripening fruits

Girigowda Manjunatha, Kapuganti J. Gupta, Veeresh Lokesh, Luis AJ Mur and Bhagyalakshmi Neelwarne
Abstract | Full Text

Ethylene plays a key role in promoting fruit ripening so altering its biosynthesis/signaling could be an important means to delaying this process. Nitric oxide (NO)-generated signals are now being show to regulate ethylene pathways. NO signals have been shown to transcriptionally repress the expression of genes involved in ethylene biosynthesis enzymes and post-translationally modify methionine adenosyl transferase (MAT) activity through S-nitrosylation to reduce the availably of methyl groups required to produce ethylene. Additionally, NO cross-talk with plant hormones and other signal molecules acts orchestrate the suppression of ethylene effects by modulating enzymes/proteins that are generally triggered by ethylene signaling at post-climacteric stage. Thus, medication of endogenous NO production is suggested as strategy to postpone the climacteric stage of many tropical fruits.




RESEARCH PAPERS

microRNAs responsive to ozone-induced oxidative stress in Arabidopsis thaliana

Niranjani J Iyer, Xiaoyun Jia, Ramanjulu Sunkar, Guiliang Tang and Ramamurthy Mahalingam
Abstract | Full Text | PDF

Ozone is a model abiotic elicitor of reactive oxygen species (ROS). ROS are important oxidative signaling molecules coordinating plant development and responses to biotic and abiotic stresses. Recently, microRNAs have been described as important players in regulating stress responses in plants. In this research we examined the miRNAs that are differentially expressed early in response to ozone in the Arabidopsis thaliana ecotype Col-0 that is tolerant to this oxidant. We used a plant miRNA array to identify 22 miRNA families that are differentially expressed within one hour of ozone fumigation. Majority of these miRNAs were also reported in response to UV-B stress. Analysis of the miRNA target genes showed a strong negative correlation to the miRNA expression. In silico promoter analysis of miRNA genes identified several stress responsive cis-elements that were enriched in the promoters of ozone responsive genes. Majority of the target genes of ozone responsive miRNAs were associated with developmental processes. Based on these results we suggest that post-transcriptional gene regulation via miRNAs may aid in resource allocation by downregulating developmental processes to cater to the oxidative stress demands on plants.

Architectural plasticity in a Mediterranean winter annual

Hagai Shemesh, Benjamin Zaitchik, Tania Acuña and Ariel Novoplansky
Abstract | Full Text

Size variability in plants may be underlain by overlooked components of architectural plasticity. In annual plants, organ sizes are expected to depend on the availability and reliability of resources and developmental time. Given sufficient resources and developmental time, plants are expected to develop a greater number of large branches, which would maximize fitness in the long run. However, under restrictive growth conditions and environmental reliability, developing large branches might be risky and smaller branches are expected to foster higher final fitness. Growth and architecture of Trifolium purpureum (Papilionaceae) plants from both Mediterranean (MED) and semi-arid (SAR) origins were studied, when plants were subjected to variable water availability, photoperiod cues and germination timing. Although no clear architectural plasticity could be found in response to water availability, plants subjected to photoperiod cuing typical to late spring developed fewer basal branches. Furthermore, plants that germinated late were significantly smaller, with fewer basal branches, compared with plants which grew for the same time, starting at the beginning of the growing season. The results demonstrate an intricate interplay between size and architectural plasticities, whereby size modifications are readily induced by environmental factors related to prevalent resource availability but architectural plasticity is only elicited following the perception of reliable anticipatory cues.

Overexpression of Rab16A gene in indica rice variety for generating enhanced salt tolerance

Moumita Ganguly, Karabi Datta, Aryadeep Roychoudhury, Dipak Gayen, Dibyendu N. Sengupta and Swapan K. Datta
Abstract | Full Text | PDF

We report here the overexpression of Rab16A full length gene (promoter + ORF), from the salt-tolerant indica rice Pokkali, in the salt-susceptible indica rice variety Khitish, via particle bombardment. Molecular analysis of the transgenics revealed stable integration of the transgene upto T2 generation. High level of expression of the transgene (driven by its own stress-inducible promoter), as well as the protein, was detectable in the leaves under simulated salinity stress (250 mM NaCl, 24 h); the expression level being higher than wild type (WT) plants. The Rab16A transcript also increased gradually with seed maturity, with its maximal accumulation at 30 d after pollination (DAP) i.e., fully matured seeds, explaining the protective role of Rab16A gene during seed maturation. Enhanced tolerance to salinity was observed in the plants transformed with Rab16A. The superior physiological performances of the transgenics under salt treatment were also reflected in lesser shoot or root length inhibition, reduced chlorophyll damages, lesser accumulation of Na⁺ and reduced loss of K⁺, increased proline content as compared with the WT plants. All these results indicated that the overproduction of RAB16A protein in the transgenics enable them to display enhanced tolerance to salinity stress with improved physiological traits. Our work demonstrates the profound potential of Group 2 LEA proteins (to which RAB16A belongs to) in conferring stress tolerance in crop plants through their genetic manipulation.




ARTICLE ADDENDA

Branching of the PIF3 regulatory network in Arabidopsis: Roles of PIF3-regulated MIDAs in seedling development in the dark and in response to light

Maria Sentandreu, Pablo Leivar, Guiomar Martín and Elena Monte
Abstract | Full Text

Plants need to accurately adjust their development after germination in the underground darkness to ensure survival of the seedling, both in the dark and in the light upon reaching the soil surface. Recent studies have established that the photoreceptors phytochromes and the bHLH phytochrome interacting factors PIFs regulate seedling development to adjust it to the prevailing light environment during post-germinative growth. However, complete understanding of the downstream regulatory network implementing these developmental responses is still lacking. In a recent work, published in The Plant Cell, we report a subset of PIF3-regulated genes in dark-grown seedlings that we have named MIDAs (MISREGULATED IN DARK). Analysis of their functional relevance using mutants showed that four of them present phenotypic alterations in the dark, and that each affected a particular facet of seedling development, suggesting organ-specific branching in the signal that PIF3 relays downstream. Furthermore, our results also showed an altered response to light in seedlings with an impaired PIF3/MIDA regulatory network, indicating that these factors might also be essential to initiate and optimize the developmental adjustment of the seedling to the light environment.

Blue light regulated shade avoidance

Diederik H. Keuskamp, Mercedes M. Keller, Carlos L. Ballaré and Ronald Pierik
Abstract | Full Text

Most plants grow in dense vegetation with the risk of being out-competed by neighboring plants. These neighbors can be detected not only through the depletion in light quantity that they cause, but also through the change in light quality, which plants perceive using specific photoreceptors. Both the reduction of the red:far-red ratio and the depletion of blue light are signals that induce a set of phenotypic traits, such as shoot elongation and leaf hyponasty, which increase the likelihood of light capture in dense plant stands. This set of phenotypic responses are part of the so called “shade avoidance syndrome” (SAS). This addendum discusses recent findings on the regulation of the SAS of Arabidopsis thaliana upon blue light depletion. Keller et al. and Keuskamp et al. show that the low blue light attenuation induced shade avoidance response of seedling and rosette-stage A. thaliana plants differ in their hormonal regulation. These studies also show there is a regulatory overlap with the R:FR-regulated SAS.

A model for combinatorial regulation of the petunia R2R3-MYB transcription factor ODORANT1

Alex Van Moerkercke, Michel A. Haring and Robert C. Schuurink
Abstract | Full Text | PDF

The emission of floral volatiles requires coordinated expression of biosynthetic genes. In the regulatory network of the volatile benzenoid/phenylpropanoid pathway in Petunia hybrida two master regulators of the pathway have been identified. The R2R3-MYB transcription factor EMISSION OF BENZENOIDS II (EOBII) utilizes a specific MYB binding site to activate the expression of the R2R3-MYB ODORANT1 (ODO1). However, because EOBII is expressed early in flower development, when ODO1 is not, there must be other factors that play a role in regulating expression of ODO1. Through functional analyses of ODO1 promoter fragments from fragrant and non-fragrant flowers, we provide evidence for additional players and present a model for combinatorial regulation of ODO1 expression in Petunia.

Investigation of copper homeostasis in plant cells by fluorescence lifetime imaging microscopy

Benjamin Hötzer, Rumen Ivanov, Petra Bauer and Gregor Jung
Abstract | Full Text

Copper ions play a fundamental role in plant metabolism where its uptake and distribution within the organism is highly regulated, allowing the cells to sustain an adequate concentration. Shortage or excess of Cu can cause severe damage to the organisms endangering their survival. We recently reported a non-invasive method to follow the intracellular uptake of bivalent copper ion concentration by fluorescence lifetime microscopy of green fluorescent protein within plant cells. Measuring the fluorescence lifetime has the advantage of being independent on the fluorophore concentration and the excitation intensity. The use of GFP is beneficial because the protein can be introduced nondestructively. Here, we discuss the benefits of this approach as well as the possibility of applying this concept for the investigation of Cu redistribution and storage at the subcellular level. The fluorescence lifetime-encoded microscopic images are envisioned to map the copper distribution within plant cells not only qualitatively but even quantitatively. Time-lapse microscopy enables the following of cellular processes and the study of relevant transport mechanisms of copper in plant cells. Perspectives and necessary improvements are discussed.

Insight into plant annexin function: From shoot to root signaling

Marie Baucher, David Pérez-Morga and Mondher El Jaziri
Abstract | Full Text

The multifunctionality of plant annexins and their importance for coordinating development and responses to biotic and abiotic environment have been largely reviewed. We recently described a tobacco annexin, named Ntann12, which is mainly localized in the nucleus of root cells when the plant is grown under light conditions. We also found that auxin and polar auxin transport are essential for Ntann12 accumulation in root cells. Under dark condition, Ntann12 is no longer detected in the root system. In the present addendum, light, regulating auxin signaling, is evidenced as an essential determinant for the synchronization of growth and development between the shoot and the root during light/dark cycle. A speculative model for Ntann12 is described and discussed with regards to relevant literature data.

Salt stress triggers enhanced cycling of Arabidopsis root plasma-membrane aquaporins

Alexandre Martinière, Xiaojuan Li, John Runions, Jinxing Lin, Christophe Maurel and Doan-Trung Luu
Abstract | Full Text

Aquaporins of the Plasma membrane Intrinsic Protein (PIP) subfamily are channels which facilitate the diffusion of water across the plant plasma membrane (PM). Although PIPs have been considered as canonical protein markers of this compartment, their endomembrane trafficking is still not well documented. We recently obtained insights into the constitutive cycling of PIPs in Arabidopsis root cells by means of fluorescence recovery after photobleaching (FRAP). This work also uncovered the behavior of the model isoform AtPIP2;1 in response to NaCl. The present addendum connects these findings to another recent work which describes the dynamic properties of AtPIP2;1 in the PM in normal and salt stress conditions by means of single particle tracking (SPT) and fluorescence correlation spectroscopy (FCS). The results suggest that membrane rafts play an important role in the partitioning of AtPIP2;1 in normal conditions and that clathrin-mediated endocytosis is predominant. In salt stress conditions, the rate of AtPIP2;1 cycling was enhanced and endocytosis was cooperated by a membrane raft-associated salt-induced pathway and a clathrin-dependent pathway.