Sumoylation stabilizes RACK1B and enhance its interaction with RAP2.6 in the abscisic acid response

COP1-ERF1-SCE1 regulatory module fine-tunes stress response under light-dark cycle in Arabidopsis

ETHYLENE RESPONSE FACTOR 1 (ERF1) plays an important role in integrating hormone crosstalk and stress responses. Previous studies have shown that ERF1 is unstable in the dark and its degradation is mediated by UBIQUITIN-CONJUGATING ENZYME 18. However, whether there are other enzymes regulating ERF1's stability remains unclear. Here, we use various in vitro and in vivo biochemical, genetic and stress-tolerance tests to demonstrate that both CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and SUMO-CONJUGATING ENZYME 1 (SCE1) regulate the stability of ERF1. We also performed transcriptomic analyses to understand their common regulatory pathways. We show that COP1 mediates ERF1 ubiquitination in the dark while SCE1 mediates ERF1 sumoylation in the light. ERF1 stability is positively regulated by SCE1 and negatively regulated by COP1. Upon abiotic stress, SCE1 plays a positive role in stress defence by regulating the expression of ERF1's downstream stress-responsive genes, whereas COP1 plays a negative role in stress response. Moreover, ERF1 also promotes photomorphogenesis and the expression of light-responsive genes. Our study reveals the molecular mechanism of how COP1 and SCE1 counteract to regulate ERF1's stability and light-stress signalling crosstalk.

The converging path of protein SUMOylation in phytohormone signalling: highlights and new frontiers

The intersection of phytohormone signalling pathways with SUMOylation, a key post-translational modification, offers an additional layer of control to the phytohormone signalling for sophisticated regulation of plant development. Plants live in a constantly changing environment that are often challenging for the growth and development of plants. Phytohormones play a critical role in modulating molecular-level changes for enabling plants to resist climatic aberrations. The orchestration of such effective molecular responses entails rapid regulation of phytohormone signalling at transcriptional, translational and post-translational levels. Post-translational modifications have emerged as a key player in modulating hormonal pathways. The current review lays emphasis on the role of SUMOylation, a key post-translational modification, in manipulating individual hormone signalling pathways for better plant adaptability. Here, we discuss the recent advancement in the field and highlights how SUMO targets key signalling intermediates including transcription factors to provide a quick response to different biotic or abiotic stresses, sometimes even prior to changes in hormone levels. The understanding of the convergence of SUMOylation and hormonal pathways will offer an additional layer of control to the phytohormone signalling for an intricate and sophisticated regulation of plant development and can be utilised as a tool to generate climate-resilient crops.

Upstream Open Reading Frame Mediated Translation of WNK8 Is Required for ABA Response in Arabidopsis

With no lysine (K) (WNK) kinases comprise a family of serine/threonine kinases belonging to an evolutionary branch of the eukaryotic kinome. These special kinases contain a unique active site and are found in a wide range of eukaryotes. The model plant Arabidopsis has been reported to have 11 WNK members, of which WNK8 functions as a negative regulator of abscisic acid (ABA) signaling. Here, we found that the expression of WNK8 is post-transcriptionally regulated through an upstream open reading frame (uORF) found in its 5′ untranslated region (5′-UTR). This uORF has been predicted to encode a conserved peptide named CPuORF58 in both monocotyledons and dicotyledons. The analysis of the published ribosome footprinting studies and the study of the frameshift CPuORF58 peptide with altered repression capability suggested that this uORF causes ribosome stalling. Plants transformed with the native WNK8 promoter driving WNK8 expression were comparable with wild-type plants, whereas the plants transformed with a similar construct with mutated CPuORF58 start codon were less sensitive to ABA. In addition, WNK8 and its downstream target RACK1 were found to synergistically coordinate ABA signaling rather than antagonistically modulating glucose response and flowering in plants. Collectively, these results suggest that the WNK8 expression must be tightly regulated to fulfill the demands of ABA response in plants.

SUMO Modification of OsFKBP20-1b Is Integral to Proper Pre-mRNA Splicing upon Heat Stress in Rice

OsFKBP20-1b, a plant-specific cyclophilin protein, has been implicated to regulate pre-mRNA splicing under stress conditions in rice. Here, we demonstrated that OsFKBP20-1b is SUMOylated in a reconstituted SUMOylation system in E.coli and in planta, and that the SUMOylation-coupled regulation was associated with enhanced protein stability using a less SUMOylated OsFKBP20-1b mutant (5KR_OsFKBP20-1b). Furthermore, OsFKBP20-1b directly interacted with OsSUMO1 and OsSUMO2 in the nucleus and cytoplasm, whereas the less SUMOylated 5KR_OsFKBP20-1b mutant had an impaired interaction with OsSUMO1 and 2 in the cytoplasm but not in the nucleus. Under heat stress, the abundance of an OsFKBP20-1b-GFP fusion protein was substantially increased in the nuclear speckles and cytoplasmic foci, whereas the heat-responsiveness was remarkably diminished in the presence of the less SUMOylated 5KR_OsFKBP20-1b-GFP mutant. The accumulation of endogenous SUMOylated OsFKBP20-1b was enhanced by heat stress in planta. Moreover, 5KR_OsFKBP20-1b was not sufficiently associated with the U snRNAs in the nucleus as a spliceosome component. A protoplast transfection assay indicated that the low SUMOylation level of 5KR_OsFKBP20-1b led to inaccurate alternative splicing and transcription under heat stress. Thus, our results suggest that OsFKBP20-1b is post-translationally regulated by SUMOylation, and the modification is crucial for proper RNA processing in response to heat stress in rice.

Non-TZF Protein AtC3H59/ZFWD3 Is Involved in Seed Germination, Seedling Development, and Seed Development, Interacting with PPPDE Family Protein Desi1 in Arabidopsis

Despite increasing reports on the function of CCCH zinc finger proteins in plant development and stress response, the functions and molecular aspects of many non-tandem CCCH zinc finger (non-TZF) proteins remain uncharacterized. AtC3H59/ZFWD3 is an Arabidopsis non-TZF protein and belongs to the ZFWD subfamily harboring a CCCH zinc finger motif and a WD40 domain. In this study, we characterized the biological and molecular functions of AtC3H59, which is subcellularly localized in the nucleus. The seeds of AtC3H59-overexpressing transgenic plants (OXs) germinated faster than those of wild type (WT), whereas atc3h59 mutant seeds germinated slower than WT seeds. AtC3H59 OX seedlings were larger and heavier than WT seedlings, whereas atc3h59 mutant seedlings were smaller and lighter than WT seedlings. Moreover, AtC3H59 OX seedlings had longer primary root length than WT seedlings, whereas atc3h59 mutant seedlings had shorter primary root length than WT seedlings, owing to altered cell division activity in the root meristem. During seed development, AtC3H59 OXs formed larger and heavier seeds than WT. Using yeast two-hybrid screening, we isolated Desi1, a PPPDE family protein, as an interacting partner of AtC3H59. AtC3H59 and Desi1 interacted via their WD40 domain and C-terminal region, respectively, in the nucleus. Taken together, our results indicate that AtC3H59 has pleiotropic effects on seed germination, seedling development, and seed development, and interacts with Desi1 in the nucleus via its entire WD40 domain. To our knowledge, this is the first report to describe the biological functions of the ZFWD protein and Desi1 in Arabidopsis.

Systematic analysis of SmWD40s, and responding of SmWD40-170 to drought stress by regulation of ABA- and H2O2-induced stomal movement in Salvia miltiorrhiza bunge

WD40 proteins play crucial roles in response to abiotic stress. By screening the genome sequences of Salvia miltiorrhiza Bunge, 225 SmWD40 genes were identified and divided into 9 subfamilies (I-IX). Physiological, biochemical, gene structure, conserved protein motif and GO annotation analyses were performed on SmWD40 family members. The SmWD40-170 was found in 110 SmWD40 genes that contain drought response elements, SmWD40-170 was one of these genes whose response in terms of expression under drought was significant. The expression of SmWD40-170 was also up-regulated by ABA and H₂O₂. Through observed the stomatal phenotype of SmWD40-170 transgenic lines, the stomatal closure was abolished under dehydration, ABA and H₂O₂ treatment in SmWD40-170 knockdown lines. Abscisic acid (ABA), as the key phytohormone, elevates reactive oxygen species (ROS) levels under drought stress. The ABA-ROS interaction mediated the generation of H₂O₂ and the activation of anion channel in guard cells. The osmolality alteration of guard cells further accelerated the stomatal closure. As a second messenger, nitric oxide (NO) regulated ABA signaling, the NO stimulated protein kinase activity inhibited the K⁺ influx which result in stomatal closure. These NO-relevant events were essential for ABA-induced stomatal closure. The reduction of NO production was also observed in the guard cells of SmWD40-170 knockdown lines. The abolished of stomatal closure attributed to the SmWD40-170 deficiency induced the reduction of NO content. In general, the SmWD40-170 is a critical drought response gene in SmWD40 gene family and regulates ABA- and H₂O₂-induced stomatal movement by affecting the synthesis of NO.

Specific TCP transcription factors interact with and stabilize PRR2 within different nuclear sub-domains

Plants possess a large set of transcription factors both involved in the control of plant development or in plant stress responses coordination. We previously identified PRR2, a Pseudo-Response Regulator, as a plant-specific CML-interacting partner. We reported that PRR2 acts as a positive actor of plant defense by regulating the production of antimicrobial compounds. Here, we report new data on the interaction between PRR2 and transcription factors belonging to the Teosinte branched Cycloidea and PCF (TCP) family. TCPs have been described to be involved in plant development and immunity. We evaluated the ability of PRR2 to interact with seven TCPs representative of the different subclades of the family. PRR2 is able to interact with TCP13, TCP15, TCP19 and TCP20 in yeast two-hybrid system and in planta interactions were validated for TCP19 and TCP20. Transient expression in tobacco highlighted that PRR2 protein is more easily detected when co-expressed with TCP19 or TC20. This stabilization is associated with a specific sub-nuclear localization of the complex in Cajal bodies or in nuclear speckles according to the interaction of PRR2 with TCP19 or TCP20 respectively. The interaction between PRR2 and TCP19 or TCP20 would contribute to the biological function in specific nuclear compartments.

Tyrosine Kinase-Dependent Defense Responses Against Herbivory in Arabidopsis

Tyrosine (Tyr) phosphorylation (TP) is important for promotion of plants' signaling. Arabidopsis calcium-dependent protein kinase related protein kinases (CRK2 and CRK3) phosphorylate Tyr residues of a subset of transcription factors (TFs), including herbivory-responsive ethylene response factor 13 (ERF13), but the in vivo functions of these kinases in plant defense responses and development remain to be clarified. We show that when CRKs were coexpressed with ERF13 in Arabidopsis leaf protoplasts, the transcription activity regulated via ERF13 was elevated by CRK2 but not CRK3 or their kinase-dead form mutants. Moreover, this elevation was abolished when a Tyr-phosphorylation mutant of ERF was coexpressed with CRK2, indicating that CRK2 serves as an effector of ERF13 mediated by Tyr-phosphorylation. Moreover, CRK2 and CRK3 acted as effectors of RAP2.6 and WRKY14, respectively. CRK-overexpressing lines and knockout mutants of Arabidopsis plants showed increased and decreased expression levels of the defensin gene PDF1.2 in leaves, respectively, conferring on the plants modulated defense properties against the generalist herbivore Spodoptera litura. However, these lines did not show any obvious developmental defects, indicating that CRKs play a role in defense responses but not in the ordinary growth or development of plants. Transcription of both CRK2 and CRK3 was positively regulated by jasmonate signaling and abscisic acid (ABA) signaling upon herbivory. Our findings suggest that these phytohormone-responsive CRKs work coordinately for plant defense responses via Tyr phosphorylation of herbivory-responsive regulators.

Integration of RACK1 and ethylene signaling regulates plant growth and development in Arabidopsis

Arabidopsis RACK1 (Receptors for Activated C Kinase 1) are versatile scaffold proteins that have been shown to be involved in the regulation of plant response to plant hormones including auxin, ABA, gibberellin and brassinosteroid, but not ethylene. By characterizing the double and triple mutants of RACK1 genes, we found that rack1 mutants showed reduced sensitivity to ethylene. By characterizing double and high order mutants generated between ein2, a loss-of-function mutant of the key ethylene signaling regulator gene EIN2 (Ethylene INsensitive 2), and rack1 mutants, we found that loss-of-function of EIN2 partially recovered some phenotypes observed in the rack1 mutants, such as low-fertility and reduced root length and rosette size. On the other hand, the ein2 rack1 mutants produced more rosette leaves, and flowered late when compared with ein2 and the corresponding rack1 mutants. We also found that the curled leaves and twisted petioles phenotypes observed in the ein2 mutants were enhanced in the ein2 rack1 mutants. However, assays in yeast indicated that EIN2 may not physically interact with RACK1. On the other hand, RT-PCR results showed that the expression level of EIN2 was reduced in the rack1 mutants. Taken together, our results suggest that RACKl may integrate ethylene signaling to regulate plant growth and development in Arabidopsis.

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.

OsRACK1A, encodes a circadian clock-regulated WD40 protein, negatively affect salt tolerance in rice

The receptor for activated C kinase 1 (RACK1) is a WD40 type protein that is involved in multiple signaling pathways and is conserved from prokaryotes to eukaryotes. Here we report that rice RACK1A (OsRACK1A) is regulated by circadian clocks and plays an important role in the salt stress response. OsRACK1A was found to follow a rhythmic expression profile under circadian conditions at both the transcription and the translation levels, although the expression was arrhythmic under salt stress. Analysis of plant survival rates, fresh weight, proline content, malondialdehyde, and chlorophyll showed that suppression of OsRACK1A enhanced tolerance to salt stress. The ion concentration in both roots and leaves revealed that OsRACK1A-suppressed transgenic rice could maintain low Na⁺ and high K⁺ concentrations. Furthermore, OsRACK1A-suppressed transgenic rice accumulated significantly more abscisic acid (ABA) and more transcripts of ABA- and stress-inducible genes compared with the wild-type plants. Real-time quantitative polymerase chain reaction analysis revealed that many stress-related genes, including APETALA 2/Ethylene Responsive Factor (AP2/ERF) transcription factors, were upregulated in the OsRACK1A-suppressed transgenic rice line. We identified putative interactors of OsRACK1A, and found that OsRACK1A interacted with many salt stress-responsive proteins directly. These results suggest that OsRACK1A is regulated by circadian rhythm, and involved in the regulation of salt stress responses.