Small ubiquitin-related modifier (SUMO) is a small polypeptide that modulates protein activity and regulates hormone signalling, abiotic and biotic responses in plants. Here we show that AtSIZ regulates nitrogen assimilation in Arabidopsis through its E3 SUMO ligase function. Dwarf plants of siz1-2 flower early, show abnormal seed development and have high salicylic acid content and enhanced resistance to bacterial pathogens. These mutant phenotypes are reverted to wild-type phenotypes by exogenous ammonium but not by nitrate, phosphate or potassium. Decreased nitrate reductase activity in siz1-2 plants resulted in low nitrogen concentrations, low nitric oxide production and high nitrate content in comparison with wild-type plants. The nitrate reductases, NIA1 and NIA2, are sumoylated by AtSIZ1, which dramatically increases their activity. Both sumoylated and non-sumoylated NIA1 and NIA2 can form dimers. Our results indicate that AtSIZ1 positively controls nitrogen assimilation by promoting sumoylation of NRs in Arabidopsis.

Seed size is one of the most important traits determining the yield of cereal crops. Many studies have been performed to uncover the mechanism of seed development. However, much remains to be understood, especially at the molecular level, although several genes involved in seed size have been identified. Here, we show that rice Grain Width 2 (GW2), a RING-type E3 ubiquitin ligase, can control seed development by catalyzing the ubiquitination of expansin-like 1 (EXPLA1), a cell wall-loosening protein that increases cell growth. Microscopic examination revealed that a GW2 mutant had a chalky endosperm due to the presence of loosely packed, spherical starch granules, although the grain shape was normal. Yeast two-hybrid and in vitro pull-down assays showed a strong interaction between GW2 and EXPLA1. In vitro ubiquitination analysis demonstrated that EXPLA1 was ubiquitinated by GW2 at lysine 279 (K279). GW2 and EXPLA1 colocalized to the nucleus when expressed simultaneously. These results suggest that GW2 negatively regulates seed size by targeting EXPLA1 for degradation through its E3 ubiquitin ligase activity.

Gibberellins affect various plant development processes including germination, cell division and elongation, and flowering. A large number of studies have been carried out to address the molecular mechanisms that mediate gibberellin signalling effects on plant growth. However, such studies have been limited to DELLA protein degradation; the regulatory mechanisms controlling how the stability and function of SLEEPY1 (SLY1), a protein that interacts with target DELLA proteins as components of the Skp, Cullin, F-box (SCF)SLY1 complex, are modulated at the post-translational level have not been addressed. In the present study, we show that the E3 SUMO (small ubiquitin-related modifier) ligase AtSIZ1 regulates gibberellic acid signalling in Arabidopsis species by sumoylating SLY1. SLY1 was less abundant in siz1–2 mutants than in wild-type plants, but the DELLA protein repressor of ga1–3 (RGA) was more abundant in siz1–2 mutants than in wild-type plants. SLY1 also accumulated to a high level in the SUMO protease mutant esd4. Transgenic sly1–13 mutants over-expressing SLY1 were phenotypically similar to wild-type plants; however, sly1–13 plants over-expressing a mutated mSLY1 protein (K122R, a mutation at the sumoylation site) retained the mutant dwarfing phenotype. Over-expression of SLY1 in sly1–13 mutants resulted in a return of RGA levels to wild-type levels, but RGA accumulated to high levels in mutants over-expressing mSLY1. RGA was clearly detected in Arabidopsis co-expressing AtSIZ1 and mSLY1, but not in plants co-expressing AtSIZ1 and SLY1. In addition, sumoylated SLY1 interacted with RGA and SLY1 sumoylation was significantly increased by GA. Taken together, our results indicate that, in Arabidopsis, AtSIZ1 positively controls GA signalling through SLY1 sumoylation.

Photoreceptors perceive different wavelengths of light and transduce light signals downstream via a range of proteins. COP1, an E3 ubiquitin ligase, regulates light signaling by mediating the ubiquitination and subsequent proteasomal degradation of photoreceptors such as phytochromes and cryptochromes, as well as various development-related proteins including other light-responsive proteins. COP1 is itself regulated by direct interactions with several signaling molecules that modulate its activity. The control of photomorphogenesis by COP1 is also regulated by its localization to the cytoplasm in response to light. COP1 thus acts as a tightly regulated switch that determines whether development is skotomorphogenic or photomorphogenic. In this review, we discuss the effects of COP1 on the abundance and activity of various development-related proteins, including photoreceptors, and summarize the regulatory mechanisms that influence COP1 activity and stability in plants.