RING finger ubiquitin E3 ligase gene TaSDIR1-4A contributes to determination of grain size in common wheat

Salt and drought-induced RING finger1 (SDIR1) is a RING-type E3 ubiquitin ligase that plays a key role in ABA-mediated responses to salinity and drought stress via the ubiquitination pathway in some plant species. However, its function in wheat (Triticum aestivum) is unknown. Here, we isolated a SDIR1 member in wheat, TaSDIR1-4A, and characterized its E3 ubiquitin ligase activity. DNA polymorphism assays showed the presence of two nucleotide variation sites in the promoter region of TaSDIR1-4A, leading to the detection of the haplotypes Hap-4A-1 and Hap-4A-2 in wheat populations. Association analysis showed that TaSDIR1-4A haplotypes were associated with 1000-grain weight (TGW) across a variety of different environments, including well-watered and heat-stress conditions. Genotypes with Hap-4A-2 had higher TGW than those with Hap-4A-1. Phenotypes in both gene-silenced wheat and transgenic Arabidopsis showed that TaSDIR1-4A was a negative regulator of grain size. Gene expression assays indicated that TaSDIR1-4A was most highly expressed in flag leaves, and expression was higher in Hap-4A-1 accessions than in Hap-4A-2 accessions. The difference might be attributable to the fact that TaERF3 (ethylene response factor) can act as a transcriptional repressor of TaSDIR1-4A in Hap-4A-2 but not in Hap-4A-1. Examination of modern wheat varieties shows that the favorable haplotype has been positively selected in breeding programs in China. The functional marker for TaSDIR1-4A developed in this study should be helpful for future wheat breeding.