Analysis of a rice fickle spikelet1 mutant that displays an increase in flower and spikelet organ number with inconstant expressivity.

In rice (Oryza sativa), floral organs develop in the spikelet, an inflorescence unit unique to grass species. The floral organs, such as carpels, stamens and lodicules, are enclosed by two spikelet organs, the palea and lemma. The number of floral organs is genetically regulated. Mutations in the FLORAL ORGAN NUMBER (FON) genes cause an increase in the number of carpels and stamens due to an enlargement of the floral meristem. The spikelet organs, such as lemma and palea, are less affected in the fon mutants. We found a mutant, fickle spikelet1 (fsp1), that displayed an increased number not only of floral organs but also of spikelet organs. Because the fsp1 spikelets showed a pleiotropic phenotype, we classified them into four types. The expressivity of the fsp1 phenotype varied from plant to plant, and also from panicle to panicle within a single plant. In addition, the frequency of each fsp1 spikelet type also varied considerably among plants and among panicles within a plant. When the fsp1 mutants were grown in a growth chamber, an extra abnormality, namely a defect in pollen development, was observed. Furthermore, the expressivity of the mutant phenotype increased dramatically in mutant plants grown in a growth chamber. Thus, the expressivity of the fsp1 phenotype seems to be strongly influenced by environmental conditions.