Cloning And Molecular Evolution Of LABA1Controlling Long,Barbed Awns In Common Wild Rice(Oryza Rufipogon Griff.)

Asian cultivated rice (O. sativa L.) is domesticated from common wild rice (O. rufipogon Griff.). Morphological and physiological traits had been remarkably changed during rice domestication. Wild rice not only harbors a long awn at the tip of the seed, but the awn of wild rice is densely covered with numerous sharp barbs of-200μm in length. Long, barbed awns are critical for the survival and propagation of wild rice seeds under natural conditions as they serve to aid in seed dispersal by clinging to animal fur and enabling self-planting, as well as to protect seeds from bird predation. In contrast, cultivated rice is selected to be awnless, or harbors short, glabrous awn which facilitates seed collection, storage and processing by humans and increases rice yield. Thus, the transition from long, barbed awn of wild rice to short, glabrous awn of cultivated rice represents a critical event in rice domestication. In this study, we screened an introgression line9YIL304with long, barbed awns from an advanced backcross population derived from the cross between Yuanjiang common wild rice (O. rufipogon) and an elite indica rice variety93-11, and mapped the gene controlling long, barbed awn of wild rice (LONG AND BARBED AWN1, LABA1) to a～35-kb region on the long arm of chromosome4using a segregating population derived from the cross between9YIL304and93-11. Through sequence comparison between9YIL304and93-11, we found a1-bp deletion in the first exon of a cytokinin activating enzyme that led to premature termination of translation in93-11. The1-bp Indel was highly associated with awn length and the presence or absence of awn barbs in cultivated rice. Complementation test confirmed that the gene encoding cytokinin activating enzyme is the LABA1gene controlling long, barbed awn in wild rice. Recombinant LABAl was able to catalyze cytokinin ribose5’-monophosphate (iPRMP) to generate bio-active cytokinin iP, moreover, endogenous cytokinin concentrations in young panicles of the complementary transgenic plants was significantly greater compared to that of control plants, demonstrating the LABA1function as a cytokinin nucleotide phosphoribohydrolase. LABA1transcripts were intensively localized in the young epidermal cells of awn primordia. Histological analysis and OsHistone H4expression indicated that awn length and awn proportion were positively regulated by LABA1through activating cell division in the awn primordia mediated by cytokinin. Sequence analysis showed low nucleotide diversity at labal and provided evidence of a selective sweep encompassing a-800-kb region around the labal allele in cultivated rice, suggesting that labal locus was a major target of artificial selection in rice domestication. Identification of LABA1provides new insights into the process of rice domestication, also sheds light on the molecular mechanism underlying awn development.