Grain width 2 (GW2) and its interacting proteins regulate seed development in rice (Oryza sativa L.).

Background: Seed size has been extensively studied in crop plants, as it determines crop yield. However, the mechanism of seed development remains elusive. In this study, we explored the mechanism of seed development in rice (Oryza sativa L.), and identified proteins affecting seed size.

Results: Proteomic analysis showed that glyceraldehyde 3-phosphate dehydrogenase, chitinase 14 (CHT14), and phosphoglycerate kinase (PGK) accumulated to high levels in the seeds of the natural japonica rice mutant Oochikara, which carries a loss-of-function mutation in the grain width 2 (GW2) gene; GW2 encodes a RING-type E3 ubiquitin ligase. In vitro pull-down and ubiquitination assays showed that CHT14 and PGK directly interacted with GW2 but were not ubiquitinated by GW2. Immunoblot analysis revealed that protein disulfide isomerase-like 1-1 accumulated to high levels in young developing seeds of the gw2 mutant compared with the wild type. Histochemical β-glucuronidase staining showed strong expression of GW2 in leaf and root tissues but weak expression in leaf sheaths and internodes. In addition, transformation of the green fluorescent protein (GFP) gene under the control of the GW2 promoter in rice revealed GFP expression in the aleurone layer of seeds.

Conclusions: Collectively, these results suggest that GW2 regulates seed size through direct interactions with proteins involved in carbohydrate metabolism by modulating their activity or stability and controlling disulfide bond formation in various proteins during seed development. Additionally, GW2 participates in vegetative as well as reproductive growth, and protects the seed from pathogen attack.