Molecular genetic studies in Arabidopsis thaliana and other flowering dicot species have led to the development of the 'ABC' model for determination of floral organ identity, in which three classes of genes, A, B, and C are thought to function together to determine floral organ identity (Coen et al. 1991, Weigel and Meyerowitz 1994). It is unclear whether the model is also applicable to monocot species. Rice is a monocot model plant. Its floret consists of two glumes, two lodicules, six stamens and one pistil. Here we report a rice mutant, stamenless 1, characterized by homeotic conversions in glumes and stamens.

The mutant was found in a backcross population consisting of a male line and a female line which had been utilized for breeding rice cytoplasmic male sterile line. Interestingly, the mutant plants occurred simultaneously in the both lines and the ratio of wild type plants to mutants in the population was 3:1. In the progeny of the heterozygotes the mutant plants accounted for one-fourth of the population (98 out of 389). The F₁ progeny from the cross between the mutant and wild type plant had normal florets like the wild type. Therefore, the mutant phenotype is due to a single recessive gene. The mutant is unique for its floral appearance, and it is impossible to distinguish the mutant from the

wild type until the heading time. After heading, its phenotype is very clear. In a wild type floret the overlapping lemma and palea form glumes with six stamens and a pistil inside. In contrast, a mutant floret has degenerated lemma and palea, so that they fail to enclose the stamens and pistil (Fig. 1A). The mutant does not produce normal stamens and is thus male sterile. So, we designated this mutant as stamenless 1 (sl1).

Rice is a self-pollinating plant. Usually, one floret forms one caryopsis. Although mutant is male sterile, it can produce seeds when pollinated with pollen from wild type plants. In that case one or two naked seeds can be formed in a single floret. So, the mutant is female fertile (Fig. 1B). In general, a mutant floret contains many pistils but no stamens, indicating that the stamens are transformed to pistils. In some mutant florets, pistilloid stamens containing a filament tipped by malformed tissue and deformed stigma or one to three stigmas were observed (Fig. 1C-F). In the cross section of a wild type floret six filaments representing the six stamens and an ovary representing the pistil in the middle can be seen clearly (Fig. 2A), while in a mutant floret, some filaments and some pistilloid tissues, e.g. the ovary-like or malformed tissues are observed (Fig. 2B). The pistilloid tissues are counterparts of the wild type stamens. In the longitudinal section of a mutant floret, three ovaries can be seen clearly (Fig. 2C). Thus, it seems that the mutant phenotype of the homeotic conversions in glumes and stamens is similar to that of B loss-of-function mutants in Arabidopsis and Antirrhinum in which the stamens are transformed into carpels. So, we assume that the sl1 might be a B loss-of-function mutant in rice.

References

Coen, E.S. and E.M. Meyerowitz, 1991. The war of the whorls: genetic interactions controlling flower development. Nature 353: 31-37.

Weigel, D. and E.M. Meyerowitz, 1994. The ABCs of floral homeotic genes. Cell 78: 203-209.