The LAX1 and FRIZZY PANICLE 2 genes determine the inflorescence architecture of rice by controlling rachis-branch and spikelet development

We have analyzed two mutants that exhibit altered panicle architecture in r ice (Oryza sativa L.). In lax1-2, which is a new and stronger allele of the previously reported lax- mutant, initiation and/or maintenance of rachis b ranches, lateral spikelets, and terminal spikelets was severely prevented. In situ hybridization analysis using OSH1, a rice knottedl (knl) ortholog, confirmed the absence of lateral meristems in lax1-2 panicles. These defect s indicate that the LAX1 gene is required for the initiation/maintenance of axillary meristems in the rice panicle. In addition to its role in forming lateral meristems, the wild-type LAX1 gene acts as a floral meristem ident ity gene which specifies the terminal spikelet meristem. A comparison of th e defects in lax1-1 and lax1-2 plants suggested that the sensitivities to r educed LAX1 activity were not uniform among different types of meristems. I n the fzp2 mutant panicle, the basic branching pattern of the panicle was i ndistinguishable from that of the wild type; however, specification of both terminal and lateral spikelet meristems was blocked, and sequential rounds of branching occurred at the point where the spikelet meristems are initia ted in the wild-type panicle. This resulted in the generation of a panicle composed of excessive ramification of rachis-branches, The lax1-1 fzp2 doub le mutants exhibited a novel, basically additive, phenotype, which suggests that LAX1 and FZP2 function in genetically independent pathways. (C) 2001 Academic Press.