H3K27 demethylase SsJMJ4 negatively regulates drought-stress responses in sugarcane.

Sugarcane (Saccharum spp.), a leading sugar and energy crop, is seriously impacted by drought stress. However, the molecular mechanisms underlying sugarcane drought resistance, especially the functions of epigenetic regulators, remain elusive. Here, we show that a S. spontaneum KDM4/JHDM3 group JmjC protein, SsJMJ4, negatively regulates drought-stress responses through its H3K27me3 demethylase activity. Ectopic overexpression of SsJMJ4 in Arabidopsis reduced drought resistance possibly by promoting expression of AtWRKY54 and AtWRKY70, encoding two negative regulators of drought stress. SsJMJ4 directly bound to AtWRKY54 and AtWRKY70, and reduced H3K27me3 levels at these loci to ensure their proper transcription under normal conditions. Drought stress down-regulated both transcription and protein abundance of SsJMJ4, which was correlated with the reduced occupancy of SsJMJ4 at AtWRKY54 and AtWRKY70 chromatin, increased H3K27me3 levels at these loci, as well as reduced transcription levels of these genes. In S. spontaneum, drought stress-repressed transcription of SsWRKY122, an ortholog of AtWRKY54 and AtWRKY70, was associated with increased H3K27me3 levels at these loci. Transient overexpression of SsJMJ4 in S. spontaneum protoplasts raised transcription of SsWRKY122, paralleled with reduced H3K27me3 levels at its loci. These results suggest that the SsJMJ4-mediated dynamic deposition of H3K27me3 is required for an appropriate response to drought stress.

JMJ27-mediated histone H3K9 demethylation positively regulates drought-stress responses in Arabidopsis.

Dimethylation of histone H3 at lysine 9 (H3K9me2) is associated with heterochromatinization and transcriptional gene silencing in plants. The activation of sets of genes by drought stress is correlated with reduced H3K9me2 levels, but the role of H3K9 methylation in the regulation of drought-stress responses remains elusive. Here, we show that the Jumonji domain-containing H3K9 demethylase JMJ27 positively regulates drought-stress responses through its histone demethylase activity. RNA-seq analysis identified JMJ27-regulated genes, including positive regulators of drought stress GALACTINOL SYNTHASE 2 (GOLS2) and RESPONSE TO DESICCATION 20 (RD20). Genetic analysis demonstrated that JMJ27 positively regulates drought-stress responses at least partly through GOLS2 and RD20. JMJ27 directly associated with GOLS2 and RD20, and protected these loci from silencing by reducing H3K9me2 levels under normal conditions. REGULATORY PARTICLE NON-ATPASE 1a (RPN1a), a subunit of the 26S proteasome, interacted with JMJ27 and negatively regulated JMJ27 accumulation. Drought stress diminished RPN1a abundance, resulting in increased JMJ27 abundance. The drought stress-promoted occupancy of JMJ27 at GOLS2 and RD20 chromatin may reinforce their transcriptional induction by locally reducing the H3K9me2 levels. These results indicate that the RPN1a-JMJ27 module precisely regulates dynamic H3K9me2 deposition plasticity, ensuring proper adaptation to drought stress in Arabidopsis.

SIZ1-Mediated SUMO Modification of SEUSS Regulates Photomorphogenesis in Arabidopsis.

Small ubiquitin-like modifier (SUMO) post-translational modification (SUMOylation) plays essential roles in regulating various biological processes; however, its function and regulation in the plant light signaling pathway are largely unknown. SEUSS (SEU) is a transcriptional co-regulator that integrates light and temperature signaling pathways, thereby regulating plant growth and development in Arabidopsis thaliana. Here, we show that SEU is a substrate of SUMO1, and that substitution of four conserved lysine residues disrupts the SUMOylation of SEU, impairs its function in photo- and thermomorphogenesis, and enhances its interaction with PHYTOCHROME-INTERACTING FACTOR 4 transcription factors. Furthermore, the SUMO E3 ligase SIZ1 interacts with SEU and regulates its SUMOylation. Moreover, SEU directly interacts with phytochrome B photoreceptors, and the SUMOylation and stability of SEU are activated by light. Our study reveals a novel post-translational modification mechanism of SEU in which light regulates plant growth and development through SUMOylation-mediated protein stability.

Reversible SUMOylation of FHY1 Regulates Phytochrome A Signaling in Arabidopsis.

In response to far-red light (FR), FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) transports the photoactivated phytochrome A (phyA), the primary FR photoreceptor, into the nucleus, where it initiates FR signaling in plants. Light promotes the 26S proteasome-mediated degradation of FHY1, which desensitizes FR signaling, but the underlying regulatory mechanism remains largely unknown. Here, we show that reversible SUMOylation of FHY1 tightly regulates this process. Lysine K32 (K32) and K103 are major SUMOylation sites of FHY1. We found that FR exposure promotes the SUMOylation of FHY1, which accelerates its degradation. Furthermore, we discovered that ARABIDOPSIS SUMO PROTEASE 1 (ASP1) interacts with FHY1 in the nucleus under FR and facilitates its deSUMOylation. FHY1 was strongly SUMOylated and its protein level was decreased in the asp1-1 loss-of-function mutant compared with that in the wild type under FR. Consistently, asp1-1 seedlings exhibited a decreased sensitivity to FR, suggesting that ASP1 plays an important role in the maintenance of proper FHY1 levels under FR. Genetic analysis further revealed that ASP1 regulates FR signaling through an FHY1- and phyA-dependent pathway. Interestingly, We found that continuous FR inhibits ASP1 accumulation, perhaps contributing to the desensitization of FR signaling. Taken together, these results indicate that FR-induced SUMOylation and ASP1-dependent deSUMOylation of FHY1 represent a key regulatory mechanism that fine-tunes FR signaling.

Arabidopsis histone H3K4 demethylase JMJ17 functions in dehydration stress response.

Under dehydration in plants, antagonistic activities of histone 3 lysine 4 (H3K4) methyltransferase and histone demethylase maintain a dynamic and homeostatic state of gene expression by orientating transcriptional reprogramming toward growth or stress tolerance. However, the histone demethylase that specifically controls histone methylation homeostasis under dehydration stress remains unknown. Here, we document that a histone demethylase, JMJ17, belonging to the KDM5/JARID1 family, plays crucial roles in response to dehydration stress and abscisic acid (ABA) in Arabidopsis thaliana. jmj17 loss-of-function mutants displayed dehydration stress tolerance and ABA hypersensitivity in terms of stomatal closure. JMJ17 specifically demethylated H3K4me1/2/3 via conserved iron-binding amino acids in vitro and in vivo. Moreover, H3K4 demethylase activity of JMJ17 was required for dehydration stress response. Systematic combination of genome-wide chromatin immunoprecipitation coupled with massively parallel DNA sequencing (ChIP-seq) and RNA-sequencing (RNA-seq) analyses revealed that a loss-of-function mutation in JMJ17 caused an ectopic increase in genome-wide H3K4me3 levels and activated a plethora of dehydration stress-responsive genes. Importantly, JMJ17 bound directly to the chromatin of OPEN STOMATA 1 (OST1) and demethylated H3K4me3 for the regulation of OST1 mRNA abundance, thereby modulating the dehydration stress response. Our results demonstrate a new function of a histone demethylase under dehydration stress in plants.

The Histone H3K4 Demethylase JMJ16 Represses Leaf Senescence in Arabidopsis.

Leaf senescence is governed by a complex regulatory network involving the dynamic reprogramming of gene expression. Age-dependent induction of senescence-associated genes (SAGs) is associated with increased levels of trimethylation of histone H3 at Lys4 (H3K4me3), but the regulatory mechanism remains elusive. Here, we found that JMJ16, an Arabidopsis (Arabidopsis thaliana) JmjC-domain containing protein, is a specific H3K4 demethylase that negatively regulates leaf senescence through its enzymatic activity. Genome-wide analysis revealed a widespread coordinated upregulation of gene expression and hypermethylation of H3K4me3 at JMJ16 binding genes associated with leaf senescence in the loss-of-function jmj16 mutant as compared with the wild type. Genetic analysis indicated that JMJ16 negatively regulates leaf senescence, at least partly through repressing the expression of positive regulators of leaf senescence, WRKY53 and SAG201 JMJ16 associates with WRKY53 and SAG201 and represses their precocious expression in mature leaves by reducing H3K4me3 levels at these loci. The protein abundance of JMJ16 gradually decreases during aging, which is correlated with increased H3K4me3 levels at WRKY53 and SAG201, suggesting that the age-dependent downregulation of JMJ16 is required for the precise transcriptional activation of SAGs during leaf senescence. Thus, JMJ16 is an important regulator of leaf senescence that demethylates H3K4 at SAGs in an age-dependent manner.

SIZ1-Mediated SUMOylation of TPR1 Suppresses Plant Immunity in Arabidopsis.

Plant immune responses are tightly regulated to ensure their appropriate deployment. Overexpression of TOPLESS-RELATED 1 (TPR1), a SUPPRESSOR OF npr1-1, CONSTITUTIVE 1 (SNC1)-interacting protein, results in autoimmunity that reduces plant growth and development. However, how TPR1 activity is regulated remains unknown. Loss of function of SIZ1, a (SUMO) E3 ligase, induces an autoimmune response, partially due to elevated SNC1 levels. Here we show that SNC1 expression is upregulated in Arabidopsis thaliana siz1-2 due to positive-feedback regulation by salicylic acid. SIZ1 physically interacts with TPR1 and facilitates its SUMO modification. The K282 and K721 residues in TPR1 serve as critical SUMO attachment sites. Simultaneous introduction of K282R and K721R substitutions in TPR1 blocked its SUMOylation, enhanced its transcriptional co-repressor activity, and increased its association with HISTONE DEACETYLASE 19 (HDA19), suggesting that SUMOylation of TPR1 represses its transcriptional co-repressor activity and inhibits its interaction with HDA19. In agreement with this finding, the simultaneous introduction of K282R and K721R substitutions enhanced TPR1-mediated immunity, and the tpr1 mutation partially suppressed autoimmunity in siz1-2. These results demonstrate that SIZ1-mediated SUMOylation of TPR1 represses plant immunity, which at least partly contributes to the suppression of autoimmunity under non-pathogenic conditions to ensure proper plant development.

Arabidopsis small ubiquitin-related modifier protease ASP1 positively regulates abscisic acid signaling during early seedling development.

The small ubiquitin-related modifier (SUMO) modification plays an important role in the regulation of abscisic acid (ABA) signaling, but the function of the SUMO protease, in ABA signaling, remains largely unknown. Here, we show that the SUMO protease, ASP1 positively regulates ABA signaling. Mutations in ASP1 resulted in an ABA-insensitive phenotype, during early seedling development. Wild-type ASP1 successfully rescued, whereas an ASP1 mutant (C577S), defective in SUMO protease activity, failed to rescue, the ABA-insensitive phenotype of asp1-1. Expression of ABI5 and MYB30 target genes was attenuated in asp1-1 and our genetic analyses revealed that ASP1 may function upstream of ABI5 and MYB30. Interestingly, ASP1 accumulated upon ABA treatment, and ABA-induced accumulation of ABI5 (a positive regulator of ABA signaling) was abolished, whereas ABA-induced accumulation of MYB30 (a negative regulator of ABA signaling) was increased in asp1-1. These findings support the hypothesis that increased levels of ASP1, upon ABA treatment, tilt the balance between ABI5 and MYB30 towards ABI5-mediated ABA signaling.

Epigenetic switch from repressive to permissive chromatin in response to cold stress.

Switching from repressed to active status in chromatin regulation is part of the critical responses that plants deploy to survive in an ever-changing environment. We previously reported that HOS15, a WD40-repeat protein, is involved in histone deacetylation and cold tolerance in Arabidopsis However, it remained unknown how HOS15 regulates cold responsive genes to affect cold tolerance. Here, we show that HOS15 interacts with histone deacetylase 2C (HD2C) and both proteins together associate with the promoters of cold-responsive COR genes, COR15A and COR47 Cold induced HD2C degradation is mediated by the CULLIN4 (CUL4)-based E3 ubiquitin ligase complex in which HOS15 acts as a substrate receptor. Interference with the association of HD2C and the COR gene promoters by HOS15 correlates with increased acetylation levels of histone H3. HOS15 also interacts with CBF transcription factors to modulate cold-induced binding to the COR gene promoters. Our results here demonstrate that cold induces HOS15-mediated chromatin modifications by degrading HD2C. This switches the chromatin structure status and facilitates recruitment of CBFs to the COR gene promoters. This is an apparent requirement to acquire cold tolerance.

Two SUMO Proteases SUMO PROTEASE RELATED TO FERTILITY1 and 2 Are Required for Fertility in Arabidopsis.

In plants, the posttranslational modification small ubiquitin-like modifier (SUMO) is involved in regulating several important developmental and cellular processes, including flowering time control and responses to biotic and abiotic stresses. Here, we report two proteases, SUMO PROTEASE RELATED TO FERTILITY1 (SPF1) and SPF2, that regulate male and female gamete and embryo development and remove SUMO from proteins in vitro and in vivo. spf1 mutants exhibit abnormal floral structures and embryo development, while spf2 mutants exhibit largely a wild-type phenotype. However, spf1 spf2 double mutants exhibit severe abnormalities in microgametogenesis, megagametogenesis, and embryo development, suggesting that the two genes are functionally redundant. Mutation of SPF1 and SPF2 genes also results in misexpression of generative- and embryo-specific genes. In vitro, SPF1 and SPF2 process SUMO1 precursors into a mature form, and as expected in vivo, spf1 and spf2 mutants accumulate SUMO conjugates. Using a yeast two-hybrid screen, we identified EMBRYO SAC DEVELOPMENT ARREST9 (EDA9) as an SPF1-interacting protein. In vivo, we demonstrate that EDA9 is sumolyated and that, in spf1 mutants, EDA9-SUMO conjugates increase in abundance, demonstrating that EDA9 is a substrate of SPF1. Together, our results demonstrate that SPF1 and SPF2 are two SUMO proteases important for plant development in Arabidopsis (Arabidopsis thaliana).

SUMO E3 Ligases GmSIZ1a and GmSIZ1b regulate vegetative growth in soybean .

SIZ1 is a small ubiquitin-related modifier (SUMO) E3 ligase that mediates post-translational SUMO modification of target proteins and thereby regulates developmental processes and hormonal and environmental stress responses in Arabidopsis. However, the role of SUMO E3 ligases in crop plants is largely unknown. Here, we identified and characterized two Glycine max (soybean) SUMO E3 ligases, GmSIZ1a and GmSIZ1b. Expression of GmSIZ1a and GmSIZ1b was induced in response to salicylic acid (SA), heat, and dehydration treatment, but not in response to cold, abscisic acid (ABA), and NaCl treatment. Although GmSIZ1a was expressed at higher levels than GmSIZ1b, both genes encoded proteins with SUMO E3 ligase activity in vivo. Heterologous expression of GmSIZ1a or GmSIZ1b rescued the mutant phenotype of Arabidopsis siz1-2, including dwarfism, constitutively activated expression of pathogen-related genes, and ABA-sensitive seed germination. Simultaneous downregulation of GmSIZ1a and GmSIZ1b (GmSIZ1a/b) using RNA interference (RNAi)-mediated gene silencing decreased heat shock-induced SUMO conjugation in soybean. Moreover, GmSIZ1RNAi plants exhibited reduced plant height and leaf size. However, unlike Arabidopsis siz1-2 mutant plants, flowering time and SA levels were not significantly altered in GmSIZ1RNAi plants. Taken together, our results indicate that GmSIZ1a and GmSIZ1b mediate SUMO modification and positively regulate vegetative growth in soybean.

Transcriptional Gene Silencing Maintained by OTS1 SUMO Protease Requires a DNA-Dependent Polymerase V-Dependent Pathway.

The expression of genes with aberrant structure is prevented at both the transcriptional and posttranscriptional regulation levels. Aberrant gene silencing at the posttranscriptional level is well studied; however, it is not well understood how aberrant genes are silenced at the transcriptional level. In this study, through genetic screening a transgenic report line that harbors an aberrant gene (35S-LUC, lacking 3'-untranslated region [3'-UTR]) and lacks luciferase (LUC) activity, we identify that the small ubiquitin-like modifier (SUMO) protease OTS1 gene is required for maintaining the silence of the reporter 35S-LUC and an endogenous mutator-like element MULE-F19G14 at the transcriptional level, which requires DNA-dependent RNA polymerase (Pol) V and DDR complex, but not Pol IV. The increased transcripts in ots1 mutants are terminated by the 3'-UTRs of downstream genes. In addition to ots1 mutations, mutations in several known or putative SUMO proteases and two SUMO E3 ligases, SIZ1 and MMS21, have similar effects on this silencing regulation. Taken together, our results reveal that the enzymes involved in the SUMOylation process restrain aberrant gene transcription by using a downstream gene 3'-UTR, and this regulation requires a functional Pol V-dependent pathway in Arabidopsis (Arabidopsis thaliana).

Arabidopsis SUMO protease ASP1 positively regulates flowering time partially through regulating FLC stability .

The initiation of flowering is tightly regulated by the endogenous and environment signals, which is crucial for the reproductive success of flowering plants. It is well known that autonomous and vernalization pathways repress transcription of FLOWERING LOCUS C (FLC), a focal floral repressor, but how its protein stability is regulated remains largely unknown. Here, we found that mutations in a novel Arabidopsis SUMO protease 1 (ASP1) resulted in a strong late-flowering phenotype under long-days, but to a lesser extent under short-days. ASP1 localizes in the nucleus and exhibited a SUMO protease activity in vitro and in vivo. The conserved Cys-577 in ASP1 is critical for its enzymatic activity, as well as its physiological function in the regulation of flowering time. Genetic and gene expression analyses demonstrated that ASP1 promotes transcription of positive regulators of flowering, such as FT, SOC1 and FD, and may function in both CO-dependent photoperiod pathway and FLC-dependent pathways. Although the transcription level of FLC was not affected in the loss-of-function asp1 mutant, the protein stability of FLC was increased in the asp1 mutant. Taken together, this study identified a novel bona fide SUMO protease, ASP1, which positively regulates transition to flowering at least partly by repressing FLC protein stability.

Light affects salt stress-induced transcriptional memory of P5CS1 in Arabidopsis.

To cope with environmental stresses, plants often adopt a memory response upon primary stress exposure to facilitate a quicker and stronger reaction to recurring stresses. However, it remains unknown whether light is involved in the manifestation of stress memory. Proline accumulation is a striking metabolic adaptation of higher plants during various environmental stresses. Here we show that salinity-induced proline accumulation is memorable and HY5-dependent light signaling is required for such a memory response. Primary salt stress induced the expression of Δ1-pyrroline-5-carboxylate synthetase 1 (P5CS1), encoding a proline biosynthetic enzyme and proline accumulation, which were reduced to basal level during the recovery stage. Reoccurring salt stress-induced stronger P5CS1 expression and proline accumulation were dependent upon light exposure during the recovery stage. Further studies demonstrated that salt-induced transcriptional memory of P5CS1 is associated with the retention of increased H3K4me3 level at P5CS1 during the recovery stage. HY5 binds directly to light-responsive element, C/A-box, in the P5CS1 promoter. Deletion of the C/A-box or hy5 hyh mutations caused rapid reduction of H3K4me3 level at P5CS1 during the recovery stage, resulting in impairment of the stress memory response. These results unveil a previously unrecognized mechanism whereby light regulates salt-induced transcriptional memory via the function of HY5 in maintaining H3K4me3 level at the memory gene.

An Arabidopsis SUMO E3 Ligase, SIZ1, Negatively Regulates Photomorphogenesis by Promoting COP1 Activity.

COP1 (CONSTITUTIVE PHOTOMORPHOGENIC 1), a ubiquitin E3 ligase, is a central negative regulator of photomorphogenesis. However, how COP1 activity is regulated by post-translational modifications remains largely unknown. Here we show that SUMO (small ubiquitin-like modifier) modification enhances COP1 activity. Loss-of-function siz1 mutant seedlings exhibit a weak constitutive photomorphogenic phenotype. SIZ1 physically interacts with COP1 and mediates the sumoylation of COP1. A K193R substitution in COP1 blocks its SUMO modification and reduces COP1 activity in vitro and in planta. Consistently, COP1 activity is reduced in siz1 and the level of HY5, a COP1 target protein, is increased in siz1. Sumoylated COP1 may exhibits higher transubiquitination activity than does non-sumoylated COP1, but SIZ1-mediated SUMO modification does not affect COP1 dimerization, COP1-HY5 interaction, and nuclear accumulation of COP1. Interestingly, prolonged light exposure reduces the sumoylation level of COP1, and COP1 mediates the ubiquitination and degradation of SIZ1. These regulatory mechanisms may maintain the homeostasis of COP1 activity, ensuing proper photomorphogenic development in changing light environment. Our genetic and biochemical studies identify a function for SIZ1 in photomorphogenesis and reveal a novel SUMO-regulated ubiquitin ligase, COP1, in plants.

A pair of light signaling factors FHY3 and FAR1 regulates plant immunity by modulating chlorophyll biosynthesis.

Light and chloroplast function is known to affect the plant immune response; however, the underlying mechanism remains elusive. We previously demonstrated that two light signaling factors, FAR-RED ELONGATED HYPOCOTYL 3 (FHY3) and FAR-RED IMPAIRED RESPONSE 1 (FAR1), regulate chlorophyll biosynthesis and seedling growth via controlling HEMB1 expression in Arabidopsis thaliana. In this study, we reveal that FHY3 and FAR1 are involved in modulating plant immunity. We showed that the fhy3 far1 double null mutant displayed high levels of reactive oxygen species and salicylic acid (SA) and increased resistance to Pseudomonas syringae pathogen infection. Microarray analysis revealed that a large proportion of pathogen-related genes, particularly genes encoding nucleotide-binding and leucine-rich repeat domain resistant proteins, are highly induced in fhy3 far1. Genetic studies indicated that the defects of fhy3 far1 can be largely rescued by reducing SA signaling or blocking SA accumulation, and by overexpression of HEMB1, which encodes a 5-aminolevulinic acid dehydratase in the chlorophyll biosynthetic pathway. Furthermore, we found that transgenic plants with reduced expression of HEMB1 exhibit a phenotype similar to fhy3 far1. Taken together, this study demonstrates an important role of FHY3 and FAR1 in regulating plant immunity, through integrating chlorophyll biosynthesis and the SA signaling pathway.

The Arabidopsis NAC transcription factor ANAC096 cooperates with bZIP-type transcription factors in dehydration and osmotic stress responses.

Multiple transcription factors (TFs) play essential roles in plants under abiotic stress, but how these multiple TFs cooperate in abiotic stress responses remains largely unknown. In this study, we provide evidence that the NAC (for NAM, ATAF1/2, and CUC2) TF ANAC096 cooperates with the bZIP-type TFs ABRE binding factor and ABRE binding protein (ABF/AREB) to help plants survive under dehydration and osmotic stress conditions. ANAC096 directly interacts with ABF2 and ABF4, but not with ABF3, both in vitro and in vivo. ANAC096 and ABF2 synergistically activate RD29A transcription. Our genome-wide gene expression analysis revealed that a major proportion of abscisic acid (ABA)-responsive genes are under the transcriptional regulation of ANAC096. We found that the Arabidopsis thaliana anac096 mutant is hyposensitive to exogenous ABA and shows impaired ABA-induced stomatal closure and increased water loss under dehydration stress conditions. Furthermore, we found the anac096 abf2 abf4 triple mutant is much more sensitive to dehydration and osmotic stresses than the anac096 single mutant or the abf2 abf4 double mutant. Based on these results, we propose that ANAC096 is involved in a synergistic relationship with a subset of ABFs for the transcriptional activation of ABA-inducible genes in response to dehydration and osmotic stresses.

CYCLIN H;1 regulates drought stress responses and blue light-induced stomatal opening by inhibiting reactive oxygen species accumulation in Arabidopsis.

Arabidopsis (Arabidopsis thaliana) CYCLIN-DEPENDENT KINASE Ds (CDKDs) phosphorylate the C-terminal domain of the largest subunit of RNA polymerase II. Arabidopsis CYCLIN H;1 (CYCH;1) interacts with and activates CDKDs; however, the physiological function of CYCH;1 has not been determined. Here, we report that CYCH;1, which is localized to the nucleus, positively regulates blue light-induced stomatal opening. Reduced-function cych;1 RNA interference (cych;1 RNAi) plants exhibited a drought tolerance phenotype. CYCH;1 is predominantly expressed in guard cells, and its expression was substantially down-regulated by dehydration. Transpiration of intact leaves was reduced in cych;1 RNAi plants compared with the wild-type control in light but not in darkness. CYCH;1 down-regulation impaired blue light-induced stomatal opening but did not affect guard cell development or abscisic acid-mediated stomatal closure. Microarray and real-time polymerase chain reaction analyses indicated that CYCH;1 did not regulate the expression of abscisic acid-responsive genes or light-induced stomatal opening signaling determinants, such as MYB60, MYB61, Hypersensitive to red and blue1, and Protein phosphatase7. CYCH;1 down-regulation induced the expression of redox homeostasis genes, such as LIPOXYGENASE3 (LOX3), LOX4, ARABIDOPSIS GLUTATHIONE PEROXIDASE 7 (ATGPX7), EARLY LIGHT-INDUCIBLE PROTEIN1 (ELIP1), and ELIP2, and increased hydrogen peroxide production in guard cells. Furthermore, loss-of-function mutations in CDKD;2 or CDKD;3 did not affect responsiveness to drought stress, suggesting that CYCH;1 regulates the drought stress response in a CDKD-independent manner. We propose that CYCH;1 regulates blue light-mediated stomatal opening by controlling reactive oxygen species homeostasis.

A vacuolar ß-glucosidase homolog that possesses glucose-conjugated abscisic acid hydrolyzing activity plays an important role in osmotic stress responses in Arabidopsis.

The phytohormone abscisic acid (ABA) plays a critical role in various physiological processes, including adaptation to abiotic stresses. In Arabidopsis thaliana, ABA levels are increased both through de novo biosynthesis and via ß-glucosidase homolog1 (BG1)-mediated hydrolysis of Glc-conjugated ABA (ABA-GE). However, it is not known how many different ß-glucosidase proteins produce ABA from ABA-GE and how the multiple ABA production pathways are coordinated to increase ABA levels. Here, we report that a previously undiscovered ß-glucosidase homolog, BG2, produced ABA by hydrolyzing ABA-GE and plays a role in osmotic stress response. BG2 localized to the vacuole as a high molecular weight complex and accumulated to high levels under dehydration stress. BG2 hydrolyzed ABA-GE to ABA in vitro. In addition, BG2 increased ABA levels in protoplasts upon application of exogenous ABA-GE. Overexpression of BG2 rescued the bg1 mutant phenotype, as observed for the overexpression of NCED3 in bg1 mutants. Multiple Arabidopsis bg2 alleles with a T-DNA insertion in BG2 were more sensitive to dehydration and NaCl stress, whereas BG2 overexpression resulted in enhanced resistance to dehydration and NaCl stress. Based on these observations, we propose that, in addition to the de novo biosynthesis, ABA is produced in multiple organelles by organelle-specific ß-glucosidases in response to abiotic stresses.

Mutation in SUMO E3 ligase, SIZ1, disrupts the mature female gametophyte in Arabidopsis.

Female gametophyte is the multicellular haploid structure that can produce embryo and endosperm after fertilization, which has become an attractive model system for investigating molecular mechanisms in nuclei migration, cell specification, cell-to-cell communication and many other processes. Previous reports found that the small ubiquitin-like modifier (SUMO) E3 ligase, SIZ1, participated in many processes depending on particular target substrates and suppression of salicylic acid (SA) accumulation. Here, we report that SIZ1 mediates the reproductive process. SIZ1 showed enhanced expression in female organs, but was not detected in the anther or pollen. A defect in the siz1-2 maternal source resulted in reduced seed-set regardless of high SA concentration within the plant. Moreover, aniline blue staining and scanning electron microscopy revealed that funicular and micropylar pollen tube guidance was arrested in siz1-2 plants. Some of the embryo sacs of ovules in siz1-2 were also disrupted quickly after stage FG7. There was no significant affects of the siz1-2 mutation on expression of genes involved in female gametophyte development- or pollen tube guidance in ovaries. Together, our results suggest that SIZ1 sustains the stability and normal function of the mature female gametophyte which is necessary for pollen tube guidance.