The elite eating quality alleles Wx^b and ALK^b are regulated by OsDOF18 and coordinately improve head rice yield

Head rice yield (HRY) measures rice milling quality and determines final grain yield and commercial value. Here, we report that two major quantitative trait loci for milling quality in rice, qMq-1 and qMq-2, represent allelic variants of Waxy(lv)/Waxy(b) (hereafter Wx) encoding Granule-Bound Starch Synthase I (GBSSI) and Alkali Spreading Value ALK (c)/ALK(b )encoding Soluble Starch Synthase IIa (SSIIa), respectively. Complementation and overexpression transgenic lines in indica and japonica backgrounds confirmed that Wx and ALK coordinately regulate HRY by affecting amylose content, the number of amylopectin branches, amyloplast size, and thus grain filling and hardness. The transcription factor OsDOF18 acts upstream of Wx and ALK by activating their transcription. Furthermore, rice accessions with Wx(b) and ALK(b) alleles showed improved HRY over those with Wx(lv) and ALK (c). Our study not only reveals the novel molecular mechanism underlying the formation of HRY but also provides a strategy for breeding rice cultivars with improved HRY.