Morphologies of flowers and inflorescences in monocots are distinct from those of flowers and inflorescences in eudicots. Especially in grasses, the inflorescence comprises of unique structural units, the spikelet and the floret. Spikelet structure of rice is unique even in grasses and is composed of the floret, the sterile lemma and the rudimentary glume (Fig. 1B). The rice floret consists of one lemma, one palea, two lodicules, six stamens, and one central pistil. There are many studies to understand how floral organs are specified in rice, and both conserved and diversified functions of genes involved in flower development have been reported so far (Yamaguchi et al. 2004). There are, however, few studies on organs specifically seen in the spikelet, such as the sterile lemma and the rudimentary glumes, except for SUPERNUMERARY BRACT (Lee et al. 2006) that produces a number of extra bract-like structures at the base of the spikelets.
In this study, we focused on a recessive mutant, named aberrant spikelet 1 (asp1). In the vegetative phase, no remarkable phenotypes were observed in asp1. After transition to the reproductive phase, a number of branches and spikelets were generated in wild type. By contrast, the asp1 mutant produced a smaller panicle with reduced numbers of branches and spikelets (Fig. 1A). Wild-type plant has about 120 spikelets per panicle, whereas the asp1 mutant panicle has about 25-30 spikelets in average. These observations suggest that the branch meristem abort at earlier stages of inflorescence development in the asp1 mutant. In all asp1 spikelets examined, the sizes of the rudimentary glume and the sterile lemma were larger than those in wild type. In addition, floral organs exhibited abnormalities; the number of the carpel and the style increased, suggesting that the determinacy of the floral meristem is partially compromised in asp1 (Fig. 1F-1H). These defects caused severe sterility. Taken together, it is likely that ASP1 is involved in the regulation of maintenance in the inflorescence meristem and the floral meristem.
To determine map position of the ASP1 locus, we crossed the asp1 mutant with an indica strain, Kasalath. We selected mutants showing the asp1 phenotype from the F2 plants and used them for mapping. The ASP1 locus was roughly mapped to the short arm of chromosome 8 between two public markers, C61344 and E60275 (Fig. 2). Detailed mapping with STS markers confined the locus to within 200 kb.
Lee D. -Y., J. Lee, S. Moon, S. Y. Park and G. An, 2006. The rice heterochronic gene SUPERNUMERARY BRACT regulates the transition from spikelet meristem to floral meristem.
Plant J. 49: 64-78.
Yamaguchi T., N. Nagasawa, S. Kawasaki, M. Matsuoka, Y. Nagato and H.-Y. Hirano, 2004. The YABBY gene DROOPING LEAF regulates carpel specification and midrib development in Oryza sativa. Plant Cell 16: 500-509.