Arabidopsis E3 SUMO ligase SIZ1 (AtSIZ1) controls vegetative growth and development including responses to nutrient deficiency and environment stresses. Here, we analyzed the effect of AtSIZ1 on the stability and amount of seed proteins. Proteomic analysis showed that the amount of three major nutrient reservoir proteins, CRUCIFERIN (CRU) 1, 2 and 3, were decreased in siz1-2 mutants. However, quantitative real-time RT-PCR showed that transcript levels of CRU1, 2 and 3 genes were rather significantly higher in siz1-2 mutants than wild-type plants. Yeast two hybrid analysis revealed that AtSIZ1 interacts with CRU1, CRU2 and CRU3, strongly suggesting that CRU1, 2 and 3 proteins are sumoylated by AtSIZ1. In addition, the analysis of amino acid composition by HPLC showed that the contents of amino acids were a bit high in siz1-2 mutants. Our data indicate that AtSIZ1 plays an important function for accumulation of seed storage proteins through its ligase activity.

The transition from vegetative growth to flowering is a major developmental switch in the plant cycle and the timing of flowering is very critical for reproduction of plant species. In transition to flowering in plants, Flowering locus C (FLC) is one of the crucial factors. Here, we showed How the stability and activity of FLC are regulated by sumoylation mechanism. By pull-down assay, we showed that FLC interact with E3 SUMO ligase in vitro and vivo. And we showed that FLC is sumoylated in vitro condition with AtSUMO1 protein. In transgenic plants with overexpression of FLC and inducible expression of AtSIZ1, sumo E3 ligase led to increase of FLC protein level and delayed the post-translation degradation of FLC indicating that Arabidopsis E3 sumo ligase AtSIZ1 stabilizes FLC. Also, the plants with overexpression of mutant FLC (K154R, a mutation of the sumoylation site on FLC) flowered considerably earlier than plants with overexpression of FLC but comparable with wild type indicating that sumoylation is a important part for function of FLC. Our data indicate that the sumoylation of FLC is critical for its role in the control of flowering time.