The Roles of Two FLORAL ORGAN NUMBER Genes, FON1 and FON2, Differ in Axillary Meristem Development

Plant development depends on the activity of stem cells. The shoot branch, which is called a tiller in grasses, is derived from stem cells in the axillary meristem, which continuously emerges in the leaf axil during plant growth. TILLERS ABSENT1 (TAB1) and FLORAL ORGAN NUMBER2 (FON2) play important roles in axillary meristem development in rice. In this process, TAB1 is required for stem cell maintenance, whereas FON2 negatively regulates stem cell fate by restricting TAB1 transcription. FON2 encodes a signaling molecule, which was originally identified as a negative regulator of stem cell maintenance in the floral meristem, and is known to act in the same genetic pathway as FON1 encoding a putative receptor of FON2. Here, we investigated whether FON1 is involved in axillary meristem development. The fon1 mutation did not affect axillary meristem development or tiller formation. Consistent with this result, FON1 expression was not detected throughout axillary meristem development. In addition, the fon1 mutation, unlike fon2, did not rescue the failure in tiller formation in tab1. Therefore, the roles of FON1 and FON2 likely differ from each other in axillary meristem development, and FON1 seems not to be involved in stem cell maintenance. These results also suggest that FON2 might regulate axillary meristem development via a receptor other than FON1.