Natural variation of the BRD2 allele affects plant height and grain size in rice

The zqdm1 identified from a rice mutant is a novel allele of BRD2 and is responsible for regulating rice plant height, grain size and appearance, which has possibilities on improving rice quality. Plant height is an important agronomic trait related to rice yield, and grain size directly determines grain yield in rice (Oryza sativa L.). With the development of molecular biotechnology and genome sequencing technology, more and more key genes associated with plant height and grain size have been cloned and identified in recent years. This study identified the zqdm1 gene from a mutant with reduced plant height and grain size. The zqdm1 gene was revealed to be a new allele of BRASSINOSTEROID DEFICIENT DWARF 2 (BRD2), encoding a FAD-linked oxidoreductase protein involved in the brassinosteroid (BR) biosynthesis pathway, and regulates plant height by reducing cell number of longitudinal sections of the internode and regulates grain size by altering cell expansion. A 369-bp DNA fragment was found inserted at the first exon, resulting in protein-coding termination. This mutation has not been discovered in previous studies. Complementation tests have confirmed that 369-bp insertion in BRD2 was responsible for the plant height and grain size changing in the zqdm1 mutant. Over-expression of BRD2 driven by different promoters into indica rice variety Jiafuzhan (JFZ) results in slender grains, suggesting its function on regulating grain shape. In summary, the current study has identified a new BRD2 allele, which facilitated the further research on the molecular mechanism of this gene on regulating growth and development.