Diversity of panicle branching patterns in wild relatives of rice

Panicle branchiness is one of the key characters specifying rice yield. To understand the potential of panicle branchiness, the diversity of panicle branching patterns was investigated in 43 accessions of 19 wild Oryza species and 27 accessions of O. rufipogon, which is the wild ancestor of the Asian cultivated species O. sativa. All these accessions were selected from a core collection of wild relatives of rice maintained at the National Institute of Genetics (NIG), Mishima, Japan. We found that panicles of wild Oryza species closely related to O. sativa generated primary branches with homogeneously large numbers of secondary branching organs, resulting in a truncated conical type of branching pattern. In contrast, panicles of distantly related wild species had primary branches with acropetally declining numbers of secondary organs toward the distal end of panicles, resulting in a conical type of branching pattern. The main axis meristem, which was aborted in closely related species, was converted into a terminal spikelet in distantly related species. Our data suggested that the panicle branching pattern evolved from conical to truncated conical in the genus Oryza. The possible advantage of a truncated-conical type of branching pattern for rice yield will be discussed.