Abstract
Many quantitative trait loci (QTLs) have been identified by molecular genetic studies which control grain size by regulating grain width, length, and/or thickness. Grain width 2 (GW2) is one such QTL that codes for a RING-type E3 ubiquitin ligase and increases grain size by regulating grain width through ubiquitin-mediated degradation of unknown substrates. A natural variation (single-nucleotide polymorphism at the 346th position) in the functional domain-coding region of OsGW2 in japonica rice genotypes has been shown to cause an increase in grain width/weight in rice. However, this variation is absent in indica rice genotypes. In this study, we report that reduced expression of OsGW2 can alter grain size, even though natural sequence variation is not responsible for increased grain size in indica rice genotypes. OsGW2 shows high expression in seed development stages and the protein localizes to the nucleus and cytoplasm. Downregulation of OsGW2 by RNAi technology results in wider and heavier grains. Microscopic observation of grain morphology suggests that OsGW2 determines grain size by influencing both cell expansion and cell proliferation in spikelet hull. Using transcriptome analysis, upregulated genes related to grain size regulation have been identified among 1,426 differentially expressed genes in an OsGW2_RNAi transgenic line. These results reveal that OsGW2 is a negative regulator of grain size in indica rice and affects both cell number and cell size in spikelet hull.
Highlights
Rice (Oryza sativa L.) is a staple cereal food crop since it is the primary source of caloric intake and is consumed by more than half of the world’s population (Sasaki, 2008; Zuo and Li, 2014; Azizi et al, 2019)
A natural variation was present in large grain-sized japonica rice genotypes in comparison with small grain-sized genotypes which was responsible for truncation of encoded OsGW2 protein functional domain in the large grain-sized genotypes (Song et al, 2007)
The 30-grain weight at the mature stage of OsGW2_RNAi T3 seeds was 12.94–42.96% higher than that of the wild-type grains (Figure 3H). These results indicated that reduced expression of OsGW2 could significantly increase grain width and weight even in indica rice, where a natural polymorphism was absent
Summary
Rice (Oryza sativa L.) is a staple cereal food crop since it is the primary source of caloric intake and is consumed by more than half of the world’s population (Sasaki, 2008; Zuo and Li, 2014; Azizi et al, 2019). Rice grain size/weight is the key agronomic trait for the improvement of yield. Rice grain yield is a complex trait, which is determined by four typical quantitative component traits, i.e., number of panicles per plant, number of filled grains per panicle, grain size, and grain weight (Xing and Zhang, 2010; Zuo and Li, 2014). Grain size (grain length and width)/weight is the most vital yield-contributing complex quantitative trait in rice (Fan et al, 2006; Mao et al, 2010; Ying et al, 2012). The best approach for enhancing rice yield is to find/develop new high yielding varieties with increased grain size/weight and superior grain nutrient quality (Rosegrant and Cline, 2003).
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