Speakers - 2026

Title: Grain width 2 (GW2) functions as an E3 ubiquitin ligase for the rice cell wall-loosening protein EXPLA1

Abstract

Rice (Oryza sativa L.) yield is regulated by various factors, including tiller number, panicle number, and seed size. Among these factors, seed size is a major yield-determining trait in cereal crops. Therefore, numerous studies have been conducted to uncover the mechanisms underlying seed development. Although recent progress has been made in the cloning and characterization of seed size-related QTLs and genes, the mechanisms determining seed size remain unclear. Here, we show that rice GRAIN WIDTH 2 (GW2), a RING-type E3 ubiquitin ligase, controls seed development by catalyzing the ubiquitination of EXPANSIN-LIKE 1 (EXPLA1), a cell wall-loosening protein that promotes cell growth. Microscopic examination revealed that the gw2 mutant exhibits a chalky endosperm due to loosely packed, spherical starch granules. Immunoblot analysis revealed that protein disulfide isomerase-like 1-1 (PDIL1-1) regulating the amounts of various seed proteins accumulated to high levels in young developing seeds of the gw2 mutant compared with the wild type. Yeast two-hybrid and in vitro pull-down assays demonstrated a strong interaction between GW2 and EXPLA1. In vitro ubiquitination assays showed that EXPLA1 is ubiquitinated by GW2 at lysine 279 (K279). GW2 and EXPLA1 colocalized in the nucleus when co-expressed. GW2 expression analysis using GUS and GFP reporters showed that GW2 was strongly detected in young panicle and anther but not clearly detected in endosperm. Our data indicate that GW2 negatively regulates seed size by targeting EXPLA1 for degradation through its E3 ubiquitin ligase activity. This work was supported by National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT) (Project No. RS-2025-16066766).