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, enhaneed 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 nonpathogenic 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, ASPI positively regulates ABA signaling. Mutations in ASPI resulted in an ABA-insensitive phenotype, during early seedling develop- ment. Wild-type ASP1 successfully rescued, whereas an ASPI mutant （C577S）, defective in SUMO protease activity, failed to rescue, the ABA-insensitive phenotype of asp1-1. Expression of ABI5 and MYB3o target genes was attenuated in asp^-I and our genetic analyses revealed that ASP1 may function upstream of ABI5 and MYB3o.

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 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.

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 Na Cl 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, GmSIZ1 RNAi 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 GmSIZ1 RNAi plants. Taken together, our results indicate that GmSIZ1a and GmSIZ1b mediate SUMO modification and positively regulate vegetative growth in soybean.