A basic helix-loop-helix transcription factor in Arabidopsis, MYC2, acts as a repressor of blue light-mediated photomorphogenic growth

A critical role of STAYGREEN/Mendel's I locus in controlling disease symptom development during Pseudomonas syringae pv tomato infection of Arabidopsis

Production of disease symptoms represents the final phase of infectious diseases and is a main cause of crop loss and/or marketability. However, little is known about the molecular basis of disease symptom development. In this study, a genetic screening was conducted to identify Arabidopsis (Arabidopsis thaliana) mutants that are impaired specifically in the development of disease symptoms (leaf chlorosis and/or necrosis) after infection with the bacterial pathogen Pseudomonas syringae pv tomato (Pst) DC3000. An ethyl methanesulfonate-induced Arabidopsis mutant (no chlorosis1 [noc1]) was identified. In wild-type plants, the abundance of chlorophylls decreased markedly after Pst DC3000 infection, whereas the total amount of chlorophylls remained relatively unchanged in the noc1 mutant. Interestingly, noc1 mutant plants also exhibited reduced disease symptoms in response to the fungal pathogen Alternaria brassicicola. Genetic and molecular analyses showed that the nuclear gene STAYGREEN (SGR; or Mendel's I locus) is mutated (resulting in the aspartic acid to tyrosine substitution at amino acid position 88) in noc1 plants. Transforming wild-type SGR cDNA into the noc1 mutant rescued the chlorosis phenotype in response to Pst DC3000 infection. The SGR transcript was highly induced by Pst DC3000, A. brassicicola, or coronatine (COR), a bacterial phytotoxin that promotes chlorosis. The induction of SGR expression by COR is dependent on COI1, a principal component of the jasmonate receptor complex. These results suggest that pathogen/COR-induced expression of SGR is a critical step underlying the development of plant disease chlorosis.

The Arabidopsis extracellular UNUSUAL SERINE PROTEASE INHIBITOR functions in resistance to necrotrophic fungi and insect herbivory

Protease inhibitors (PIs) function in the precise regulation of proteases, and are thus involved in diverse biological processes in many organisms. Here, we studied the functions of the Arabidopsis UNUSUAL SERINE PROTEASE INHIBITOR (UPI) gene, which encodes an 8.8 kDa protein of atypical sequence relative to other PIs. Plants harboring a loss-of-function UPI allele displayed enhanced susceptibility to the necrotrophic fungi Botrytis cinerea and Alternaria brassicicola as well as the generalist herbivore Trichoplusia ni. Further, ectopic expression conferred increased resistance to B. cinerea and T. ni. In contrast, the mutant has wild-type responses to virulent, avirulent and non-pathogenic strains of Pseudomonas syringae, thus limiting the defense function of UPI to necrotrophic fungal infection and insect herbivory. Expression of UPI is significantly induced by jasmonate, salicylic acid and abscisic acid, but is repressed by ethylene, indicating complex phytohormone regulation of UPI expression. The upi mutant also shows significantly delayed flowering, associated with decreased SOC1 expression and elevated levels of MAF1, two regulators of floral transition. Recombinant UPI strongly inhibits the serine protease chymotrypsin but also weakly blocks the cysteine protease papain. Interestingly, jasmonate induces intra- and extracellular UPI accumulation, suggesting a possible role in intercellular or extracellular functions. Overall, our results show that UPI is a dual-specificity PI that functions in plant growth and defense, probably through the regulation of endogenous proteases and/or those of biotic invaders.

Root-localized phytochrome chromophore synthesis is required for photoregulation of root elongation and impacts root sensitivity to jasmonic acid in Arabidopsis

Plants exhibit organ- and tissue-specific light responses. To explore the molecular basis of spatial-specific phytochrome-regulated responses, a transgenic approach for regulating the synthesis and accumulation of the phytochrome chromophore phytochromobilin (PΦB) was employed. In prior experiments, transgenic expression of the BILIVERDIN REDUCTASE (BVR) gene was used to metabolically inactivate biliverdin IXα, a key precursor in the biosynthesis of PΦB, and thereby render cells accumulating BVR phytochrome deficient. Here, we report analyses of transgenic Arabidopsis (Arabidopsis thaliana) lines with distinct patterns of BVR accumulation dependent upon constitutive or tissue-specific, promoter-driven BVR expression that have resulted in insights on a correlation between root-localized BVR accumulation and photoregulation of root elongation. Plants with BVR accumulation in roots and a PΦB-deficient elongated hypocotyl2 (hy2-1) mutant exhibit roots that are longer than those of wild-type plants under white illumination. Additional analyses of a line with root-specific BVR accumulation generated using a GAL4-dependent bipartite enhancer-trap system confirmed that PΦB or phytochromes localized in roots directly impact light-dependent root elongation under white, blue, and red illumination. Additionally, roots of plants with constitutive plastid-localized or root-specific cytosolic BVR accumulation, as well as phytochrome chromophore-deficient hy1-1 and hy2-1 mutants, exhibit reduced sensitivity to the plant hormone jasmonic acid (JA) in JA-dependent root inhibition assays, similar to the response observed for the JA-insensitive mutants jar1 and myc2. Our analyses of lines with root-localized phytochrome deficiency or root-specific phytochrome depletion have provided novel insights into the roles of root-specific PΦB, or phytochromes themselves, in the photoregulation of root development and root sensitivity to JA.

Analysis of IIId, IIIe and IVa group basic-helix-loop-helix proteins expressed in Arabidopsis root epidermis

Differentiation of Arabidopsis epidermal cells into root hairs and trichomes is a functional model system for understanding plant cell development. Previous studies showed that one of the Arabidopsis basic-helix-loop-helix (AtbHLH) proteins, GLABRA3 (GL3), is involved in root-hair and trichome differentiation. We analyzed 11 additional AtbHLH genes with homology to GL3. Estimation of the phylogeny based on amino acid sequences of the bHLH region suggests that 11 AtbHLH genes used in this study evolved by duplications of a single common GL3 ancestor. Promoter-GUS analysis showed that AtbHLH006, AtbHLH013, AtbHLH017 and AtbHLH020 were expressed in roots. Among them, AtbHLH006 and AtbHLH020 were preferentially expressed in root epidermal non-hair cells. Consistent with the expression patterns from promoter-GUS analysis, GFP fluorescence was observed in the nuclei of root epidermal non-hair cells of AtbHLH006p::AtbHLH006:GFP and AtbHLH020p::AtbHLH020:GFP transgenic plants. However, AtbHLH006 and AtbHLH0020 proteins did not interact with epidermis-specific MYB proteins and TTG1. Taken together, AtbHLH006 and AtbHLH020 may function in root epidermal cells, but other GL3-like bHLH proteins may have evolved to regulate different processes.

MYC genes with differential responses to tapping, mechanical wounding, ethrel and methyl jasmonate in laticifers of rubber tree (Hevea brasiliensis Muell. Arg.)

MYC2 transcription factor is a key component of the core module COI1-JAZ-MYC2 of jasmonate signaling in Arabidopsis, but the MYC transcription factor (s) associated with jasmonate signaling in jasmonate-responsive laticifer cells remains to be identified. Two full-length cDNAs, designated HblMYC1 and HblMYC2, were isolated from laticifer cells in Hevea brasiliensis by the method of RACE. HblMYC1 contained 1431bp ORF encoding a putative protein of 476 amino acids while HblMYC2 contained 1428bp ORF encoding a putative protein of 475 amino acids. Bioinformatic analysis showed that the putative proteins, HblMYC1 and HblMYC2, possessed a bHLH domain and were most related to the MYC2 among the selected 27 MYC members with identified functions in Arabidopsis. In addition to the presence of cis-regulatory elements involving jasmonate responsiveness in the promoter regions of HblMYC1 and HblMYC2, the abscisic acid-, salicylic acid- and gibberellin-responsive elements were found in the promoter region of HblMYC1. Transcripts of HblMYC1 and HblMYC2 were most abundant in latex, relatively low in male flowers and nearly undetected in bark tissues and roots by real-time RT-PCR analysis. Regular tapping, mechanical wounding, and ethrel remarkably up-regulated HblMYC1 expression, but had little effect on the expression of HblMYC2 in laticifer cells. Successive tapping, however, significantly down-regulated the expression of HblMYC2 while up-regulating the expression of HblMYC1. The HblMYC2 expression took a mutual ebb and flow relationship with the HblMYC1 expression upon treatment with methyl jasmonate. Characterization of HblMYC1 and HblMYC2 will contribute to the understanding of jasmonate signaling in laticifiers, a kind of specialized tissue for natural rubber biosynthesis in Hevea brasiliensis.

Arabidopsis ocp3 mutant reveals a mechanism linking ABA and JA to pathogen-induced callose deposition

In the present study, we evaluated the role of the defense-related gene OCP3 in callose deposition as a response to two necrotrophic fungal pathogens, Botrytis cinerea and Plectosphaerella cucumerina. ocp3 plants exhibited accelerated and intensified callose deposition in response to fungal infection associated with enhanced disease resistance to the two pathogens. A series of double mutant analyses showed potentiation of callose deposition and the heightened disease resistance phenotype in ocp3 plants required the plant hormone abscisic acid (ABA) and the PMR4 gene encoding a callose synthase. This finding was congruent with an observation that ocp3 plants exhibited increased ABA accumulation, and ABA was rapidly synthesized following fungal infection in wild-type plants. Furthermore, we determined that potentiation of callose deposition in ocp3 plants, including enhanced disease resistance, also required jasmonic acid (JA) recognition though a COI1 receptor, however JA was not required for basal callose deposition following fungal infection. In addition, potentiation of callose deposition in ocp3 plants appeared to follow a different mechanism than that proposed for callose β-amino-butyric acid (BABA)-induced resistance and priming, because ocp3 plants responded to BABA-induced priming for callose deposition and induced resistance of a magnitude similar to that observed in wild-type plants. Our results point to a model in which OCP3 represents a specific control point for callose deposition regulated by JA yet ultimately requiring ABA. These results provide new insights into the mechanism of callose deposition regulation in response to pathogen attack; however the complexities of the processes remain poorly understood.

Plant adaptation to dynamically changing environment: the shade avoidance response

The success of competitive interactions between plants determines the chance of survival of individuals and eventually of whole plant species. Shade-tolerant plants have adapted their photosynthesis to function optimally under low-light conditions. These plants are therefore capable of long-term survival under a canopy shade. In contrast, shade-avoiding plants adapt their growth to perceive maximum sunlight and therefore rapidly dominate gaps in a canopy. Daylight contains roughly equal proportions of red and far-red light, but within vegetation that ratio is lowered as a result of red absorption by photosynthetic pigments. This light quality change is perceived through the phytochrome system as an unambiguous signal of the proximity of neighbors resulting in a suite of developmental responses (termed the shade avoidance response) that, when successful, result in the overgrowth of those neighbors. Shoot elongation induced by low red/far-red light may confer high relative fitness in natural dense communities. However, since elongation is often achieved at the expense of leaf and root growth, shade avoidance may lead to reduction in crop plant productivity. Over the past decade, major progresses have been achieved in the understanding of the molecular basis of shade avoidance. However, uncovering the mechanisms underpinning plant response and adaptation to changes in the ratio of red to far-red light is key to design new strategies to precise modulate shade avoidance in time and space without impairing the overall crop ability to compete for light.

The interplay between light and jasmonate signalling during defence and development

During their evolution, plants have acquired diverse capabilities to sense their environment and modify their growth and development as required. The versatile utilization of solar radiation for photosynthesis as well as a signal to coordinate developmental responses to the environment is an excellent example of such a capability. Specific light quality inputs are converted to developmental outputs mainly through hormonal signalling pathways. Accordingly, extensive interactions between light and the signalling pathways of every known plant hormone have been uncovered in recent years. One such interaction that has received recent attention and forms the focus of this review occurs between light and the signalling pathway of the jasmonate hormone with roles in regulating plant defence and development. Here the recent research that revealed new mechanistic insights into how plants might integrate light and jasmonate signals to modify their growth and development, especially when defending themselves from either pests, pathogens, or encroaching neighbours, is discussed.

ABA-mediated transcriptional regulation in response to osmotic stress in plants

The plant hormone abscisic acid (ABA) plays a pivotal role in a variety of developmental processes and adaptive stress responses to environmental stimuli in plants. Cellular dehydration during the seed maturation and vegetative growth stages induces an increase in endogenous ABA levels, which control many dehydration-responsive genes. In Arabidopsis plants, ABA regulates nearly 10% of the protein-coding genes, a much higher percentage than other plant hormones. Expression of the genes is mainly regulated by two different families of bZIP transcription factors (TFs), ABI5 in the seeds and AREB/ABFs in the vegetative stage, in an ABA-responsive-element (ABRE) dependent manner. The SnRK2-AREB/ABF pathway governs the majority of ABA-mediated ABRE-dependent gene expression in response to osmotic stress during the vegetative stage. In addition to osmotic stress, the circadian clock and light conditions also appear to participate in the regulation of ABA-mediated gene expression, likely conferring versatile tolerance and repressing growth under stress conditions. Moreover, various other TFs belonging to several classes, including AP2/ERF, MYB, NAC, and HD-ZF, have been reported to engage in ABA-mediated gene expression. This review mainly focuses on the transcriptional regulation of ABA-mediated gene expression in response to osmotic stress during the vegetative growth stage in Arabidopsis.

Tissue- and isoform-specific phytochrome regulation of light-dependent anthocyanin accumulation in Arabidopsis thaliana

Phytochromes regulate light- and sucrose-dependent anthocyanin synthesis and accumulation in many plants. Mesophyll-specific phyA alone has been linked to the regulation of anthocyanin accumulation in response to far-red light in Arabidopsis thaliana. However, multiple mesophyll-localized phytochromes were implicated in the photoregulation of anthocyanin accumulation in red-light conditions. Here, we report a role for mesophyll-specific phyA in blue-light-dependent regulation of anthocyanin levels and novel roles for individual phy isoforms in the regulation of anthocyanin accumulation under red illumination. These results provide new insight into spatial- and isoform-specific regulation of pigmentation by phytochromes in A. thaliana.

Hormone crosstalk in plant disease and defense: more than just jasmonate-salicylate antagonism

Until recently, most studies on the role of hormones in plant-pathogen interactions focused on salicylic acid (SA), jasmonic acid (JA), and ethylene (ET). It is now clear that pathogen-induced modulation of signaling via other hormones contributes to virulence. A picture is emerging of complex crosstalk and induced hormonal changes that modulate disease and resistance, with outcomes dependent on pathogen lifestyles and the genetic constitution of the host. Recent progress has revealed intriguing similarities between hormone signaling mechanisms, with gene induction responses often achieved by derepression. Here, we report on recent advances, updating current knowledge on classical defense hormones SA, JA, and ET, and the roles of auxin, abscisic acid (ABA), cytokinins (CKs), and brassinosteroids in molding plant-pathogen interactions. We highlight an emerging theme that positive and negative regulators of these disparate hormone signaling pathways are crucial regulatory targets of hormonal crosstalk in disease and defense.

DBB1a, involved in gibberellin homeostasis, functions as a negative regulator of blue light-mediated hypocotyl elongation in Arabidopsis

Double B-box 1a (DBB1a) belongs to the zinc-finger family proteins in Arabidopsis thaliana. Transcriptional analysis uncovered that the DBB1a gene expression was blue light-dependently regulated, and the transcript level of DBB1a in cry1cry2 was decreased but not in phyAphyB compared to wild type under blue light conditions. Transgenic plants containing pDBB1a:GUS (β-glucuronidase) displayed GUS activity in the vascular system of leaves and petioles. Green fluorescent protein (GFP)-fused DDB1a (DBB1a-GFP) protein was found in the nucleus in transient transformation assays with onion epidermal cells as well as in stable transgenic Arabidopsis plants. To investigate the function of DBB1a, we generated DBB1a over-expressing and under-expressing transgenic Arabidopsis plants. Analysis of hypocotyl growth of these lines indicated that DBB1a promoted hypocotyl elongation under blue light condition. The phenotype of transgenic plants with DBB1a over-expression could be impaired by a gibberellin (GA)-biosynthesis inhibitor. Moreover, the expression analysis of GA metabolic and catabolic genes in DBB1a transgenic lines indicated that the DBB1a suppressed GA2-oxidase1 (GA2ox1) and GA2-oxidase8 (GA2ox8) expression, but induced GA3β-hydroxygenase1 (GA3ox1) and GA20-oxidase1 (GA20ox1) expression under blue light. Taken together, we concluded that DBB1a promotes hypocotyl elongation under blue light condition through an increase in bioactive GA levels in Arabidopsis.

Kinome profiling reveals an interaction between jasmonate, salicylate and light control of hyponastic petiole growth in Arabidopsis thaliana

Plants defend themselves against infection by biotic attackers by producing distinct phytohormones. Especially jasmonic acid (JA) and salicylic acid (SA) are well known defense-inducing hormones. Here, the effects of MeJA and SA on the Arabidopsis thaliana kinome were monitored using PepChip arrays containing kinase substrate peptides to analyze posttranslational interactions in MeJA and SA signaling pathways and to test if kinome profiling can provide leads to predict posttranslational events in plant signaling. MeJA and SA mediate differential phosphorylation of substrates for many kinase families. Also some plant specific substrates were differentially phosphorylated, including peptides derived from Phytochrome A, and Photosystem II D protein. This indicates that MeJA and SA mediate cross-talk between defense signaling and light responses. We tested the predicted effects of MeJA and SA using light-mediated upward leaf movement (differential petiole growth also called hyponastic growth). We found that MeJA, infestation by the JA-inducing insect herbivore Pieris rapae, and SA suppressed low light-induced hyponastic growth. MeJA and SA acted in a synergistic fashion via two (partially) divergent signaling routes. This work demonstrates that kinome profiling using PepChip arrays can be a valuable complementary ∼omics tool to give directions towards predicting behavior of organisms after a given stimulus and can be used to obtain leads for physiological relevant phenomena in planta.

MYC2, a bHLH transcription factor, modulates the adult phenotype of SPA1

MYC2 and SPA1 are two key regulatory proteins that negatively regulate light-controlled Arabidopsis seedling development. We have recently demonstrated the genetic and molecular relationships of MYC2 and SPA1 in light and JA (jasmonic acid) signaling pathways. Here, we have further shown the genetic interactions between these two proteins in flowering time and lateral root development.

Functional interconnection of MYC2 and SPA1 in the photomorphogenic seedling development of Arabidopsis

MYC2 is a basic helix-loop-helix transcription factor that cross talks with light, abscisic acid (ABA), and jasmonic acid (JA) signaling pathways. Here, we have shown that Arabidopsis (Arabidopsis thaliana) MYC2 directly binds to the G-box present in the SUPPRESSOR OF PHYTOCHROME A1 (SPA1) promoter and that it controls the expression of SPA1 in a COP1-dependent manner. Analyses of atmyc2 spa1 double mutants suggest that whereas MYC2 and SPA1 act redundantly to suppress photomorphogenic growth in the dark, they function synergistically for the suppression of photomorphogenic growth in the light. Our studies have also revealed that MYC2-mediated ABA and JA responses are further modulated by SPA1. Taken together, this study demonstrates the molecular and physiological interrelations of MYC2 and SPA1 in light, ABA, and JA signaling pathways.

Jasmonate and Phytochrome A Signaling inArabidopsisWound and Shade Responses Are Integrated through JAZ1 Stability

Jasmonate (JA) activates plant defense, promotes pollen maturation, and suppresses plant growth. An emerging theme in JA biology is its involvement in light responses; here, we examine the interdependence of the JA- and light-signaling pathways in Arabidopsis thaliana. We demonstrate that mutants deficient in JA biosynthesis and signaling are deficient in a subset of high irradiance responses in far-red (FR) light. These mutants display exaggerated shade responses to low, but not high, R/FR ratio light, suggesting a role for JA in phytochrome A (phyA) signaling. Additionally, we demonstrate that the FR light–induced expression of transcription factor genes is dependent on CORONATINE INSENSITIVE1 (COI1), a central component of JA signaling, and is suppressed by JA. phyA mutants had reduced JA-regulated growth inhibition and VSP expression and increased content of cis-(+)-12-oxophytodienoic acid, an intermediate in JA biosynthesis. Significantly, COI1-mediated degradation of JASMONATE ZIM DOMAIN1-β-glucuronidase (JAZ1-GUS) in response to mechanical wounding and JA treatment required phyA, and ectopic expression of JAZ1-GUS resulted in exaggerated shade responses. Together, these results indicate that JA and phyA signaling are integrated through degradation of the JAZ1 protein, and both are required for plant responses to light and stress.

DWA1 and DWA2, two Arabidopsis DWD protein components of CUL4-based E3 ligases, act together as negative regulators in ABA signal transduction

To elucidate potential roles of CUL4-DDB1-DWD (for Cullin 4-Damaged DNA Binding1-DDB1 binding WD40) E3 ligases in abscisic acid (ABA) signaling, we examined ABA sensitivities of T-DNA mutants of a number of Arabidopsis thaliana DWD genes, which encode substrate receptors for CUL4 E3 ligases. Mutants in two DWD genes, DWA1 and DWA2 (DWD hypersensitive to ABA1 and 2), had ABA-hypersensitive phenotypes. Both proteins interacted with DDB1 in yeast two-hybrid assays and associated with DDB1 and CUL4 in vivo, implying they could form CUL4-based complexes. Several ABA-responsive genes were hyperinduced in both mutants, and the ABA-responsive transcription factors ABA INSENSITIVE 5 (ABI5) and MYC2 accumulated to high levels in the mutants after ABA treatment. Moreover, ABI5 interacted with DWA1 and DWA2 in vivo. Cell-free degradation assays showed ABI5 was degraded more slowly in dwa1 and dwa2 than in wild-type cell extracts. Therefore, DWA1 and/or DWA2 may be the substrate receptors for a CUL4 E3 ligase that targets ABI5 for degradation. Our data indicate that DWA1 and DWA2 can directly interact with each other, and their double mutants exhibited enhanced ABA and NaCl hypersensitivities, implying they can act together. This report thus describes a previously unknown heterodimeric cooperation between two independent substrate receptors for CUL4-based E3 ligases.

Origin and diversification of basic-helix-loop-helix proteins in plants

Basic helix-loop-helix (bHLH) proteins are a class of transcription factors found throughout eukaryotic organisms. Classification of the complete sets of bHLH proteins in the sequenced genomes of Arabidopsis thaliana and Oryza sativa (rice) has defined the diversity of these proteins among flowering plants. However, the evolutionary relationships of different plant bHLH groups and the diversity of bHLH proteins in more ancestral groups of plants are currently unknown. In this study, we use whole-genome sequences from nine species of land plants and algae to define the relationships between these proteins in plants. We show that few (less than 5) bHLH proteins are encoded in the genomes of chlorophytes and red algae. In contrast, many bHLH proteins (100-170) are encoded in the genomes of land plants (embryophytes). Phylogenetic analyses suggest that plant bHLH proteins are monophyletic and constitute 26 subfamilies. Twenty of these subfamilies existed in the common ancestors of extant mosses and vascular plants, whereas six further subfamilies evolved among the vascular plants. In addition to the conserved bHLH domains, most subfamilies are characterized by the presence of highly conserved short amino acid motifs. We conclude that much of the diversity of plant bHLH proteins was established in early land plants, over 440 million years ago.

Jasmonates

ARABIDOPSIS IS A SUPERB MODEL FOR THE STUDY OF AN IMPORTANT SUBGROUP OF OXYLIPINS: the jasmonates. Jasmonates control many responses to cell damage and invasion and are essential for reproduction. Jasmonic acid (JA) is a prohormone and is conjugated to hydrophobic amino acids to produce regulatory ligands. The major receptor for active jasmonate ligands is closely related to auxin receptors and, as in auxin signaling, jasmonate signaling requires the destruction of repressor proteins. This chapter uses a frequently asked question (FAQ) approach and concludes with a practical section.

Top hits in contemporary JAZ: an update on jasmonate signaling

The phytohormone jasmonate (JA) regulates a wide range of growth, developmental, and defense-related processes during the plant life cycle. Identification of the JAZ family of proteins that repress JA responses has facilitated rapid progress in understanding how this lipid-derived hormone controls gene expression. Recent analysis of JAZ proteins has provided insight into the nature of the JA receptor, the chemical specificity of signal perception, and cross-talk between JA and other hormone response pathways. Functional diversification of JAZ proteins by alternative splicing, together with the ability of JAZ proteins to homo- and heterodimerize, provide mechanisms to enhance combinatorial diversity and versatility in gene regulation by JA.

Plant oxylipins: COI1/JAZs/MYC2 as the core jasmonic acid-signalling module

Jasmonic acid (JA) and its derivates, collectively known as jasmonates (JAs), are essential signalling molecules that coordinate the plant response to biotic and abiotic challenges, in addition to several developmental processes. The COI1 F-box and additional SCF modulators have long been known to have a crucial role in the JA-signalling pathway. Downstream JA-dependent transcriptional re-programming is regulated by a cascade of transcription factors and MYC2 plays a major role. Recently, JAZ family proteins have been identified as COI1 targets and repressors of MYC2, defining the 'missing link' in JA signalling. JA-Ile has been proposed to be the active form of the hormone, and COI1 is an essential component of the receptor complex. These recent discoveries have defined the core JA-signalling pathway as the module COI1/JAZs/MYC2.

Expression of the Arabidopsis jasmonate signalling repressor JAZ1/TIFY10A is stimulated by auxin

Plant hormones have pivotal roles in almost every aspect of plant development. Over the past decades, physiological and genetic studies have revealed that hormone action in plants is determined by complex interactions between hormonal signalling pathways. Evidence is accumulating for the existence of crosstalk between the auxin and jasmonate (JA) signalling pathways. Recently, the JASMONATE ZIM-domain (JAZ) proteins have been identified as the long-sought repressors of JA signalling. Here, we show that expression of JAZ1/TIFY10A is not solely inducible by JA, but that it is also an early auxin-responsive gene. Furthermore, we could show that the auxin-inducible expression of JAZ1/TIFY10A is independent of the JA signalling pathway but is controlled by the auxin/indole-3-acetic acid-auxin response transcription factor signalling pathway. Our results provide evidence for the existence of at least two different input signals regarding JAZ1/TIFY10A expression and thus support the idea of an intimate molecular interplay between auxin and JA signalling.

The multifaceted role of ABA in disease resistance

Long known only for its role in abiotic stress tolerance, recent evidence shows that abscisic acid (ABA) also has a prominent role in biotic stress. Although it acts as a negative regulator of disease resistance, ABA can also promote plant defense and is involved in a complicated network of synergistic and antagonistic interactions. Its role in disease resistance depends on the type of pathogen, its specific way of entering the host and, hence, the timing of the defense response and the type of affected plant tissue. Here, we discuss the controversial evidence pointing to either a repression or a promotion of resistance by ABA. Furthermore, we propose a model in which both possibilities are integrated.

Herbivore-induced jasmonic acid bursts in leaves of Nicotiana attenuata mediate short-term reductions in root growth

Root growth in Nicotiana attenuata is transiently reduced after application of oral secretions (OS) of Manduca sexta larvae to wounds in leaves. Feeding of M. sexta or OS elicitation is known to result in jasmonic acid (JA) and ethylene bursts, and activates a suite of defence responses. Because both plant hormones are known to strongly reduce root growth, their activation might account for the observed reduction of root growth following herbivory. To test this hypothesis, we measured primary root growth with digital image sequence processing at high temporal resolution in antisense-lipoxygenase 3 (asLOX3) and inverted repeat-coronatin-insensitive 1 (irCOI1) seedlings which are impaired in JA biosynthesis and perception, respectively, and wild-type (WT) seedlings. Higher root growth rates in irCOI1 compared with WT were observed after OS elicitation. The dynamics of wound-induced root growth reduction coincide with the dynamics of root growth reduction induced by external application of methyl JA. In an experiment with 1-methylcyclopropen (1-MCP), a potent ethylene receptor blocker, no wounding-specific difference between growth of 1-MCP-treated plants and non-treated plants was observed, suggesting that wound-induced endogenous JA and not ethylene mediates the wounding-specific reduction in root growth. Yet, inhibiting the ethylene response by applying 1-MCP led to markedly increased root growth compared with that of control plants, indicating that ethylene normally suppresses plant growth in N. attenuata seedlings.

A light-independent allele of phytochrome B faithfully recapitulates photomorphogenic transcriptional networks

Dominant gain-of-function alleles of Arabidopsis phytochrome B were recently shown to confer light-independent, constitutive photomorphogenic (cop) phenotypes to transgenic plants (Su and Lagarias, 2007). In the present study, comparative transcription profiling experiments were performed to assess whether the pattern of gene expression regulated by these alleles accurately reflects the process of photomorphogenesis in wild-type Arabidopsis. Whole-genome transcription profiles of dark-grown phyAphyB seedlings expressing the Y276H mutant of phyB (YHB) revealed that YHB reprograms about 13% of the Arabidopsis transcriptome in a light-independent manner. The YHB-regulated transcriptome proved qualitatively similar to but quantitatively greater than those of wild-type seedlings grown under 15 or 50 micromol m(-2) m(-1) continuous red light (Rc). Among the 2977 genes statistically significant two-fold (SSTF) regulated by YHB in the absence of light include those encoding components of the photosynthetic apparatus, tetrapyrrole/pigment biosynthetic pathways, and early light-responsive signaling factors. Approximately 80% of genes SSTF regulated by Rc were also YHB-regulated. Expression of a notable subset of 346 YHB-regulated genes proved to be strongly attenuated by Rc, indicating compensating regulation by phyC-E and/or other Rc-dependent processes. Since the majority of these 346 genes are regulated by the circadian clock, these results suggest that phyA- and phyB-independent light signaling pathway(s) strongly influence clock output. Together with the unique plastid morphology of dark-grown YHB seedlings, these analyses indicate that the YHB mutant induces constitutive photomorphogenesis via faithful reconstruction of phyB signaling pathways in a light-independent fashion.

The alpha-subunit of the heterotrimeric G-protein affects jasmonate responses in Arabidopsis thaliana

Heterotrimeric G-proteins have been implicated in having a role in many plant signalling pathways. To understand further the role of G-proteins, a preliminary experiment was performed to assess the impact of the G alpha subunit loss-of-function mutation gpa1-1 on the Arabidopsis transcriptome. The analysis indicated that the G alpha subunit may play a role in response to jasmonic acid (JA). Consistent with this, G alpha mutants showed a reduced response to JA in inhibition of chlorophyll accumulation and root growth, whilst G alpha gain-of-function plants overexpressing G alpha showed the opposite phenotype. The levels of JA and related compounds were unaffected in the gpa1-1 mutant, as was autoregulation of the Allene Oxide Synthase (AOS) gene that encodes a key enzyme for JA biosynthesis. In contrast, further analyses using G alpha loss- and gain-of-function Arabidopsis lines indicated that G alpha positively modulates the expression of the Vegetative Storage Protein (VSP) gene. This indicates that the G alpha subunit regulates a subset of JA-regulated genes defining a branch point in this signalling pathway in Arabidopsis. Further analysis of the impact of G alpha loss of function upon the JA-regulated transcriptome using Arabidopsis full genome arrays indicated that up to 29% of genes that are >2-fold regulated by JA in the wild type are misregulated in the G alpha mutant. This supports the observation that a significant proportion of, but not all, JA-regulated gene expression is mediated by G alpha.

Calcium and calmodulin-mediated regulation of gene expression in plants

Sessile plants have developed a very delicate system to sense diverse kinds of endogenous developmental cues and exogenous environmental stimuli by using a simple Ca2+ ion. Calmodulin (CaM) is the predominant Ca2+ sensor and plays a crucial role in decoding the Ca2+ signatures into proper cellular responses in various cellular compartments in eukaryotes. A growing body of evidence points to the importance of Ca2+ and CaM in the regulation of the transcriptional process during plant responses to endogenous and exogenous stimuli. Here, we review recent progress in the identification of transcriptional regulators modulated by Ca2+ and CaM and in the assessment of their functional significance during plant signal transduction in response to biotic and abiotic stresses and developmental cues.

GBF1, a transcription factor of blue light signaling in Arabidopsis, is degraded in the dark by a proteasome-mediated pathway independent of COP1 and SPA1

Arabidopsis GBF1/ZBF2 is a bZIP transcription factor that plays dual but opposite regulatory roles in cryptochrome-mediated blue light signaling. Here, we show the genetic and molecular interrelation of GBF1 with two well characterized negative regulators of light signaling, COP1 and SPA1, in photomorphogenic growth and light-regulated gene expression. Our results further reveal that GBF1 protein is less abundant in the dark-grown seedlings and is degraded by a proteasome-mediated pathway independent of COP1 and SPA1. Furthermore, COP1 physically interacts with GBF1 and is required for the optimum accumulation of GBF1 protein in light-grown seedlings. Taken together, this study provides a mechanistic view of concerted function of three important regulators in Arabidopsis seedling development.

SHW1, a common regulator of abscisic acid (ABA) and light signaling pathways

In our recent paper in Plant Physiology, we have reported the identification and functional characterization of a unique regulator, SHW1, a serine-arginine-aspartate rich protein in Arabidopsis seedling development.1 Genetic and molecular analyses have revealed that SHW1 functions in an independent and interdependent manner with COP1, and differentially regulates photomorphogenic growth and light regulated gene expression. Here, we show the involvement of photoreceptors in the function of SHW1. Our results have further revealed that SHW1 is a common regulator of light and ABA signaling pathways. These results along with some data described in Plant Physiology paper have been discussed here in a broader perspective.

LZF1/SALT TOLERANCE HOMOLOG3, an Arabidopsis B-box protein involved in light-dependent development and gene expression, undergoes COP1-mediated ubiquitination

B-box containing proteins play an important role in light signaling in plants. Here, we identify LIGHT-REGULATED ZINC FINGER1/SALT TOLERANCE HOMOLOG3 (STH3), a B-box encoding gene that genetically interacts with two key regulators of light signaling, ELONGATED HYPOCOTYL5 (HY5) and CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1). STH3 physically interacts with HY5 in vivo and shows a COP1-dependent localization to nuclear speckles when coexpressed with COP1 in plant cells. A T-DNA insertion mutant, sth3, is hyposensitive to high fluence blue, red, and far-red light and has elongated hypocotyls under short days. Analyses of double mutants between sth3, sth2, and hy5 suggest that they have partially overlapping functions. Interestingly, functional assays in protoplasts suggest that STH3 can activate transcription both independently and together with STH2 through the G-box promoter element. Furthermore, sth3 suppresses the cop1 hypocotyl phenotype in the dark as well as the anthocyanin accumulation in the light. Finally, COP1 ubiquitinates STH3 in vitro, suggesting that STH3 is regulated by COP1. In conclusion, we have identified STH3 as a positive regulator of photomorphogenesis acting in concert with STH2 and HY5, while also being a target of COP1-mediated ubiquitination.

CRY1 inhibits COP1-mediated degradation of BIT1, a MYB transcription factor, to activate blue light-dependent gene expression in Arabidopsis

Cryptochromes (CRY) are one of the two major classes of photoreceptors that perceive light stimuli in the UV-A to blue light region and they are involved in multiple aspects of plant growth and development. However, knowledge regarding their signaling transduction components and mechanisms remains limited. Here, we report that a MYB transcription factor Blue Insensitive Trait 1 (BIT1), plays an important role in controlling blue light responses. Hypocotyl growth responses indicate that BIT1 functions as a positive element in blue light signaling, since BIT1 antisense and knock-out lines show a reduced light response in blue light. BIT1 controls blue light-dependent expression of various genes such as PsbS, a member of the light-harvesting complex gene family. A transactivation assay showed that BIT1 regulates promoter activity of PsbS in a blue light-dependent manner and that it requires CRY1 for activation of the PsbS promoter. BIT1 undergoes degradation in darkness and CRY1 functions to stabilize BIT1 in a blue light-dependent manner. In contrast, COP1 binds to BIT1 and mediates its degradation. We propose that the PsbS promoter is activated in blue light via the blue light-dependent stabilization of BIT1 by CRY1, while in darkness BIT1 is degraded by COP1-mediated proteolysis.

The protein phosphatase 7 regulates phytochrome signaling in Arabidopsis

The psi2 mutant of Arabidopsis displays amplification of the responses controlled by the red/far red light photoreceptors phytochrome A (phyA) and phytochrome B (phyB) but no apparent defect in blue light perception. We found that loss-of-function alleles of the protein phosphatase 7 (AtPP7) are responsible for the light hypersensitivity in psi2 demonstrating that AtPP7 controls the levels of phytochrome signaling. Plants expressing reduced levels of AtPP7 mRNA display reduced blue-light induced cryptochrome signaling but no noticeable deficiency in phytochrome signaling. Our genetic analysis suggests that phytochrome signaling is enhanced in the AtPP7 loss of function alleles, including in blue light, which masks the reduced cryptochrome signaling. AtPP7 has been found to interact both in yeast and in planta assays with nucleotide-diphosphate kinase 2 (NDPK2), a positive regulator of phytochrome signals. Analysis of ndpk2-psi2 double mutants suggests that NDPK2 plays a critical role in the AtPP7 regulation of the phytochrome pathway and identifies NDPK2 as an upstream element involved in the modulation of the salicylic acid (SA)-dependent defense pathway by light. Thus, cryptochrome- and phytochrome-specific light signals synchronously control their relative contribution to the regulation of plant development. Interestingly, PP7 and NDPK are also components of animal light signaling systems.

Calmodulin7 plays an important role as transcriptional regulator in Arabidopsis seedling development

Although calmodulin (CaM) is known to play multiple regulatory roles in eukaryotes, its direct function as transcriptional regulator is unknown. Furthermore, the physiological functions of CaM are largely unknown in plants. Here, we show that one of the four Arabidopsis thaliana CaM isoforms, CAM7, is a transcriptional regulator that directly interacts with the promoters of light-inducible genes and promotes photomorphogenesis. CAM7 overexpression causes hyperphotomorphogenic growth and an increase in the expression of light-inducible genes. Mutations in CAM7 produce no visible effects on photomorphogenic growth, indicating likely redundant gene functions. However, cam7 mutants display reduced expression of light-inducible genes, and cam7 hy5 double mutants show an enhancement of the hy5 phenotype. Moreover, overexpression of CAM7 can partly suppress the hy5 phenotype, indicating that the two factors work together to control light-induced seedling development. The mutational and transgenic studies, together with physiological analyses, illustrate the concerted function of CAM7 and HY5 basic leucine zipper transcription factor in Arabidopsis seedling development.

Rice JASMONATE RESISTANT 1 is involved in phytochrome and jasmonate signalling

Jasmonic acid (JA) is an important negative regulator of light-regulated coleoptile elongation in rice. We isolated rice JASMONATE RESISTANT 1 (osjar1) mutants from the Tos17 mutant panel by BLAST search. In far-red and blue lights, osjar1 coleoptiles were longer if compared with the wild type (WT), indicating that OsJar1 participates in the suppression of coleoptile elongation in these light conditions, while the mutant did not show a clear phenotype in red light. The analysis of OsJar1 expression in phytochrome (phy) mutants revealed that phytochrome A (phyA) and phytochrome B (phyB) act redundantly to induce this gene by red light, presumably. Unexpectedly, blue light-induced expression of OsJar1 gene was impaired in phyA-deficient mutants, indicating the involvement of phyA in the blue light signalling. In WT seedlings, OsJar1 transcripts were up-regulated transiently in response to treatment with exogenous methyl-jasmonic acid (MeJA). The dose-response curve of the MeJA treatment showed a characteristic pattern: concentrations as low as 4.5 nM could induce OsJar1 transcription, while the gene was superinduced at a concentration of 450 microM MeJA. In summary, this paper demonstrated that OsJar1 modulates light and JA signalling in the photomorphogenesis of rice.

SHORT HYPOCOTYL IN WHITE LIGHT1, a serine-arginine-aspartate-rich protein in Arabidopsis, acts as a negative regulator of photomorphogenic growth

Light is an important factor for plant growth and development. We have identified and functionally characterized a regulatory gene SHORT HYPOCOTYL IN WHITE LIGHT1 (SHW1) involved in Arabidopsis (Arabidopsis thaliana) seedling development. SHW1 encodes a unique serine-arginine-aspartate-rich protein, which is constitutively localized in the nucleus of hypocotyl cells. Transgenic analyses have revealed that the expression of SHW1 is developmentally regulated and is closely associated with the photosynthetically active tissues. Genetic and molecular analyses suggest that SHW1 acts as a negative regulator of light-mediated inhibition of hypocotyl elongation, however, plays a positive regulatory role in light-regulated gene expression. The shw1 mutants also display shorter hypocotyl in dark, and analyses of shw1 cop1 double mutants reveal that SHW1 acts nonredundantly with COP1 to control hypocotyl elongation in the darkness. Taken together, this study provides evidences that SHW1 is a regulatory protein that is functionally interrelated to COP1 and plays dual but opposite regulatory roles in photomorphogenesis.

The F-box protein ACRE189/ACIF1 regulates cell death and defense responses activated during pathogen recognition in tobacco and tomato

Virus-induced gene silencing identified the Avr9/Cf-9 RAPIDLY ELICITED gene ACRE189 as essential for the Cf-9- and Cf-4-mediated hypersensitive response (HR) in Nicotiana benthamiana. We report a role for ACRE189 in disease resistance in tomato (Solanum lycopersicum) and tobacco (Nicotiana tabacum). ACRE189 (herein renamed Avr9/Cf-9-INDUCED F-BOX1 [ACIF1]) encodes an F-box protein with a Leu-rich-repeat domain. ACIF1 is widely conserved and is closely related to F-box proteins regulating plant hormone signaling. Silencing of tobacco ACIF1 suppressed the HR triggered by various elicitors (Avr9, Avr4, AvrPto, Inf1, and the P50 helicase of Tobacco mosaic virus [TMV]). ACIF1 is recruited to SCF complexes (a class of ubiquitin E3 ligases), and the expression of ACIF1 F-box mutants in tobacco compromises the HR similarly to ACIF1 silencing. ACIF1 affects N gene-mediated responses to TMV infection, including lesion formation and salicylic acid accumulation. Loss of ACIF1 function also reduced confluent cell death induced by Pseudomonas syringae pv tabaci. ACIF1 silencing in Cf9 tomato attenuated the Cf-9-dependent HR but not Cf-9 resistance to Cladosporium fulvum. Resistance conferred by the Cf-9 homolog Cf-9B, however, was compromised in ACIF1-silenced tomato. Analysis of public expression profiling data suggests that Arabidopsis thaliana homologs of ACIF1 (VFBs) regulate defense responses via methyl jasmonate- and abscisic acid-responsive genes. Together, these findings support a role of ACIF1/VFBs in plant defense responses.

Identification and characterization of COI1-dependent transcription factor genes involved in JA-mediated response to wounding in Arabidopsis plants

The phytohormone jasmonic acid (JA) is an important signaling molecular involved in many developmental and physiological processes, especially in the response of plants to wounding. In this study, we adopted a new strategy, taking into consideration the microarray data of the CHX treatment, to identify 15 COI1-dependent JA-inducible transcription factors (JCTFs) that have distinct expression patterns in response to wounding. After the analysis on the JCTFs over-expressor plants, we identified four JCTFs, i.e., WRKY18, At1g74930 and At3g53600 in addition to AtMYC2, as the positive regulators in the JA-mediated signaling pathway in response to Arabidopsis wounding.

Manganese deficiency alters the patterning and development of root hairs in Arabidopsis

Manganese (Mn) is the second most prevalent transition metal in the Earth's crust but its availability is often limited due to rapid oxidation and low mobility of the oxidized forms. Acclimation to low Mn availability was studied in Arabidopsis seedlings subjected to Mn deficiency. As reported here, Mn deficiency caused a thorough change in the arrangement and characteristics of the root epidermal cells. A proportion of the extra hairs formed upon Mn deficiency were located in atrichoblast positions, indicative of a post-embryonic reprogramming of the cell fate acquired during embryogenesis. When plants were grown under a light intensity of >50 micromol m(-2) s(-1) in the presence of manganese root hair elongation was substantially inhibited, whereas Mn-deficient seedlings displayed stimulated root hair development. GeneChip analysis revealed several candidate genes with potential roles in the reprogramming of rhizodermal cells. None of the genes that function in epidermal cell fate specification were affected by Mn deficiency, indicating that the patterning mechanism which controls the differentiation of rhizodermal cells during embryogenesis have been bypassed under Mn-deficient conditions. This assumption is supported by the partial rescue of the hairless cpc mutant by Mn deficiency. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis revealed that, besides the anticipated reduction in Mn concentration, Mn deficiency caused an increase in iron concentration. This increase was associated with a decreased transcript level of the iron transporter IRT1, indicative of a more efficient transport of iron in the absence of Mn.

Opposite ends of the spectrum: plant and animal g-protein signaling

Different classes of biotic (e.g., plant hormones) and abiotic (e.g., different wavelengths of light) signals act through specific signal transduction mechanisms to coordinate all aspects of plant development. Full signal transduction chains have not yet been described for most light or hormonal-mediated events despite the wide range of events early in development which are dependent upon hormonal and light signals. We recently reported a single signal transduction chain which can be initiated by both blue light (BL) and ABA, and which leads to the expression of specific members of the Lhcb gene family in the apical bud of etiolated Arabidopsis seedlings. The signal transduction chain consists of GCR1 (one of two Arabidopsis proteins coding for a potential G-protein coupled receptor), GPA1 (the sole Arabidopsis Ga subunit), PRN1 (Pirin1, one of four members of an iron-containing subgroup of the cupin superfamily), and a Nuclear Factor -Y (NF-Y) heterotrimer comprised of A5, B9 and possibly C9. The same signaling proteins control ABA-mediated delay of germination.

The bHLH-type transcription factor AtAIB positively regulates ABA response in Arabidopsis

The phytohormone ABA was known to play a vital role in modulating plant responses to drought stress. Here, we report that a nuclear-localized basic helix-loop-helix (bHLH)-type protein, AtAIB, positively regulates ABA response in Arabidopsis. The expression of AtAIB was transitorily induced by ABA and PEG, although its transcripts were accumulated in various organs. We provided evidence showing that AtAIB has transcriptional activation activity in yeast. Knockdown of AtAIB expression caused reduced sensitivity to ABA, whereas overexpression of this gene led to elevated sensitivity to ABA in cotyledon greening and seedling root growth. Furthermore, soil-grown plants overexpressing AtAIB showed increased drought tolerance. Taken together, these results suggested that AtAIB functions as a transcription activator involved in the regulation of ABA signaling in Arabidopsis.

SALT TOLERANCE HOMOLOG2, a B-box protein in Arabidopsis that activates transcription and positively regulates light-mediated development

CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) and ELONGATED HYPOCOTYL5 (HY5) are two major regulators of light signaling in plants. Here, we identify SALT TOLERANCE HOMOLOG2 (STH2) as a gene that interacts genetically with both of these key regulators. STH2 encodes a B-box-containing protein that interacts physically with HY5 in yeast and in plant cells. Whereas STH2 is uniformly nuclear by itself, it shows a COP1-dependent localization to speckles when coexpressed with COP1. We identified two independent T-DNA insertion lines in STH2. Both alleles are hyposensitive to blue, red, and far-red light. The sth2 mutant, like hy5, shows an enhanced number of lateral roots and accumulates less anthocyanin. Analysis of double mutants between sth2 and hy5 indicates that STH2 has both HY5-dependent and -independent functions. Furthermore, besides partially suppressing the hypocotyl phenotype of dark-grown cop1 alleles, sth2 also suppresses the reduced number of lateral roots and high anthocyanin levels in light-grown cop1 alleles. Interestingly, we found that STH2 can activate transcription. Transient transfection assays in protoplasts using a LUC reporter driven by the chalcone isomerase promoter show that the B-boxes in STH2 and a functional G-box element in the promoter are required for this activity. In conclusion, we have identified STH2, a B-box protein in Arabidopsis thaliana, as a positive regulator of photomorphogenesis and report that the B-box domain plays a direct role in activating transcription in plants.

Phytochrome chromophore deficiency leads to overproduction of jasmonic acid and elevated expression of jasmonate-responsive genes in Arabidopsis

An Arabidopsis mutant line named hy1-101 was isolated because it shows stunted root growth on medium containing low concentrations of jasmonic acid (JA). Subsequent investigation indicated that even in the absence of JA, hy1-101 plants exhibit shorter roots and express higher levels of a group of JA-inducible defense genes. Here, we show that the hy1-101 mutant has increased production of JA and its jasmonate-related phenotype is suppressed by the coi1-1 mutation that interrupts JA signaling. Gene cloning and genetic complementation analyses revealed that the hy1-101 mutant contains a mutation in the HY1 gene, which encodes a heme oxygenase essential for phytochrome chromophore biosynthesis. These results support a hypothesis that phytochrome chromophore deficiency leads to overproduction of JA and activates COI1-dependent JA responses. Indeed, we show that, like hy1-101, independent alleles of the phytochrome chromophore-deficient mutants, including hy1-100 and hy2 (CS68), also show elevated JA levels and constant expression of JA-inducible defense genes. We further provide evidence showing that, on the other hand, JA inhibits the expression of a group of light-inducible and photosynthesis-related genes. Together, these data imply that the JA-signaled defense pathway and phytochrome chromophore-mediated light signaling might have antagonistic effects on each other.

MYC2 differentially modulates diverse jasmonate-dependent functions in Arabidopsis

The Arabidopsis thaliana basic helix-loop-helix Leu zipper transcription factor (TF) MYC2/JIN1 differentially regulates jasmonate (JA)-responsive pathogen defense (e.g., PDF1.2) and wound response (e.g., VSP) genes. In this study, genome-wide transcriptional profiling of wild type and mutant myc2/jin1 plants followed by functional analyses has revealed new roles for MYC2 in the modulation of diverse JA functions. We found that MYC2 negatively regulates Trp and Trp-derived secondary metabolism such as indole glucosinolate biosynthesis during JA signaling. Furthermore, MYC2 positively regulates JA-mediated resistance to insect pests, such as Helicoverpa armigera, and tolerance to oxidative stress, possibly via enhanced ascorbate redox cycling and flavonoid biosynthesis. Analyses of MYC2 cis binding elements and expression of MYC2-regulated genes in T-DNA insertion lines of a subset of MYC2-regulated TFs suggested that MYC2 might modulate JA responses via differential regulation of an intermediate spectrum of TFs with activating or repressing roles in JA signaling. MYC2 also negatively regulates its own expression, and this may be one of the mechanisms used in fine-tuning JA signaling. Overall, these results provide new insights into the function of MYC2 and the transcriptional coordination of the JA signaling pathway.

The GCR1, GPA1, PRN1, NF-Y signal chain mediates both blue light and abscisic acid responses in Arabidopsis

Different classes of biotic (e.g. plant hormones) and abiotic (e.g. different wavelengths of light) signals act through specific signal transduction mechanisms to coordinate higher plant development. While a great deal of progress has been made, full signal transduction chains have not yet been described for most blue light- or abscisic acid-mediated events. Based on data derived from T-DNA insertion mutants and yeast (Saccharomyces cerevisiae) two-hybrid and coprecipitation assays, we report a signal transduction chain shared by blue light and abscisic acid leading to light-harvesting chlorophyll a/b-binding protein expression in etiolated Arabidopsis (Arabidopsis thaliana) seedlings. The chain consists of GCR1 (the sole Arabidopsis protein coding for a potential G-protein-coupled receptor), GPA1 (the sole Arabidopsis Galpha-subunit), Pirin1 (PRN1; one of four members of an iron-containing subgroup of the cupin superfamily), and a nuclear factor Y heterotrimer comprised of A5, B9, and possibly C9. We also demonstrate that this mechanism is present in imbibed seeds wherein it affects germination rate.

Light-regulated transcriptional networks in higher plants

Plants have evolved complex and sophisticated transcriptional networks that mediate developmental changes in response to light. These light-regulated processes include seedling photomorphogenesis, seed germination and the shade-avoidance and photoperiod responses. Understanding the components and hierarchical structure of the transcriptional networks that are activated during these processes has long been of great interest to plant scientists. Traditional genetic and molecular approaches have proved powerful in identifying key regulatory factors and their positions within these networks. Recent genomic studies have further revealed that light induces massive reprogramming of the plant transcriptome, and that the early light-responsive genes are enriched in transcription factors. These combined approaches provide new insights into light-regulated transcriptional networks.

Analysis of transcription factor HY5 genomic binding sites revealed its hierarchical role in light regulation of development

The transcription factor LONG HYPOCOTYL5 (HY5) acts downstream of multiple families of the photoreceptors and promotes photomorphogenesis. Although it is well accepted that HY5 acts to regulate target gene expression, in vivo binding of HY5 to any of its target gene promoters has yet to be demonstrated. Here, we used a chromatin immunoprecipitation procedure to verify suspected in vivo HY5 binding sites. We demonstrated that in vivo association of HY5 with promoter targets is not altered under distinct light qualities or during light-to-dark transition. Coupled with DNA chip hybridization using a high-density 60-nucleotide oligomer microarray that contains one probe for every 500 nucleotides over the entire Arabidopsis thaliana genome, we mapped genome-wide in vivo HY5 binding sites. This analysis showed that HY5 binds preferentially to promoter regions in vivo and revealed >3000 chromosomal sites as putative HY5 binding targets. HY5 binding targets tend to be enriched in the early light-responsive genes and transcription factor genes. Our data thus support a model in which HY5 is a high hierarchical regulator of the transcriptional cascades for photomorphogenesis.

GER1, a GDSL motif-encoding gene from rice is a novel early light- and jasmonate-induced gene

The reaction of the rice mutant HEBIBA differs from that of wild-type rice in that the mutant responds inversely to red light and is defective in the light-triggered biosynthesis of jasmonic acid (JA). Using the wild type and the HEBIBA mutant of rice in a differential display screen, we attempted to identify genes that act in or near the convergence point of light and JA signalling. We isolated specifically regulated DNA fragments from approximately 10 000 displayed bands, and identified a new early light- and JA-induced gene. This gene encodes an enzyme containing a GDSL motif, showing 38 % identity at the amino acid level to lipase Arab-1 in Arabidopsis thaliana. The GDSL CONTAINING ENZYME RICE 1 gene (GER1) is rapidly induced by both red (R) and far-red (FR) light and by JA. The results are discussed with respect to a possible role for GER1 as a negative regulator of coleoptile elongation in the context of recent findings on the impact of JA on light signalling.