Adaptation of flowering-time by natural and artificial selection in Arabidopsis and rice

The adaptation of plants to natural environments depends on the adaptation of flowering-time control at the appropriate season to set seeds. Possible molecular mechanisms underlying this adaptation have recently been revealed. In Arabidopsis thaliana, a model long-day plant, control of floral transition by vernalization and long-day floral promotion pathways is a key regulator in adaptation to different regions. A floral repressor termed FLC and a floral promoter termed CONSTANS (CO), which control FT, a florigen gene, are key transcriptional regulators of these pathways. Recent analyses of haplotypes in accessions of A. thaliana revealed that FLC regulation by an activator termed FRIGIDA (FRI) had been a target for natural selection. By contrast, in rice (Oryza sativa), a model short-day plant, two independent floral pathways-Heading date 1 (Hd1, a CO orthologue)-dependent and Early heading date 1 (Ehd1)-dependent pathways-control Hd3a (an FT orthologue) and flowering time. Interestingly, there is an antagonistic action between Hd1 and Ehd1 in the control of flowering time under long-day conditions, because Hd1 represses floral transition whereas Ehd1 promotes it. A wild rice species, Oryza rufipogon, has common ancestry with cultivated rice and grows wild in the tropics, yet cultivated rice is grown even in the cold regions of northern latitudes. During domestication, the adaptation of O. sativa to northern regions by artificial selection may have become possible through interactions of the two pathways. These suggest that the domestication process of rice will provide novel insights into the adaptation of plants in evolution.