Identification of Responsible Genes Involved in Awn Development and Discussion about Rice Domestication Process

Do you know “awn”? Awn is the elongated spine-like organ at the tip of seeds. We can see this structure mainly on monocot plants like barley, wheat and rice. Awn is thought to aid in seed dispersal and to prevent predation by animals. Interestingly cultivated barley and wheat still have awn but cultivated rice lost this organ completely. One of the reason of this, the awns of barley and wheat are having chloroplast which could fill up a seed with starch coming from photosynthesis at this point. Whereas barley’s or wheat's awns contribute to photosynthesis during grain-filling, rice’s awns have no chlorenchyma (the site of photosynthesis). It suggests that removal of awns has limited effect on grain maturation in rice (Tsudamori, 1933) and consequently, cultivated rice varieties may have become awnless to enhance the ease of harvest. The cultivated rice in Asia and Africa, Oryza sativa and O. glaberrima, respectively, evolved from their wild ancestors either O. rufipogon/O. nivara or O. barthii. Through the domestication process many traits from wild rice were lost, including awn phenotype. Here we show that 3 loci involved in awn development. Two loci were identified in chromosome segment substitution lines (CSSLs) derived from each O. barthii and O. nivara as donor parent in the background of O. sativa. The CSSLs carrying either Chr. 4 or Chr. 8 exhibited long awn phenotype. Interestingly, despite the absence of awn in African cultivated rice, O. glaberrima derived Chr. 4 and Chr. 8 CSSLs similarly showed awn structure. This indicates that at least another locus in O. glaberrima is responsible for the awn formation and that the loss of awn in cultivated rice in Asia and Africa occurred through different process. I will report that what gene is responsible for awn generation on Chr.4 and Chr. 8, then show the model of domestication of rice.