Gene mapping and candidate gene analysis of multi-grains 1 (Mg1) in rice

As the basic unit of rice (Oryza sativa L.) inflorescence, the spikelet has a profound influence on grain size, weight, and yield. However, the molecular mechanism that underlies the development of spikelets has never been fully elucidated. In this study, we characterized a rice spikelet mutant, multi-grains 1 (mg1), which was derived from treatment of 'Xinong 1B' with ethyl methanesulfonate. In the mg1 mutant, the spikelet developed two florets with an extra lemma-like organ and an increased number of inner floral organs. Notably, the spikelet could produce multi-grains. These phenotypes reflect the considerable potential of using mg1 as a means of increasing rice yield. Genetic analysis revealed that the mg1 phenotype is controlled by a single recessive gene. The MG1 gene was mapped to the interval between simple sequence repeat (SSR) markers RM20754 and RM494 on chromosome 6 within a physical region of 121.57 kb. Sequencing analysis identified a single-nucleotide substitution from G to A within Os06g0717200 (which corresponds to FLORAL ORGAN NUMBER 1 [FON1]), causing an amino acid mutation of Gly-869 to Asp-869 in the mg1 mutant. This finding suggests that MG1 may be a novel allele of FON1. MG1 is involved in the development of spikelet determinacy and floral organs by regulating the expression of genes associated with these characteristics. The present results detail a novel function of MG1, which may facilitate the breeding of rice cultivars with increased numbers of grains per panicle.