NAM and CUC3 boundary genes maintain shoot apical meristem viability and suppress the development of axillary shoot in rice seedlings

Cell division and differentiation within the shoot apical meristem (SAM) are essential for the morphogenesis of aboveground plant organs. This study reveals that the boundary genes OsNAM and OsCUC3 collaboratively maintain SAM activity. Loss of function in both OsNAM and OsCUC3 during the fourth leaf stage reduced SAM size, with the osnam oscuc3 mutant exhibiting abnormal leaf number and morphology. Furthermore, OsNAM and OsCUC3 inhibited the growth of axillary shoots. In the osnam oscuc3 mutant, the number of new leaves decreased, while buds in the coleoptile and the axil of the first leaf developed into tillers. Since OsNAM and OsCUC3 are involved in regulating both SAM activity and the growth of lateral shoots, we examined their expression patterns at the base of the main shoot. β-Glucuronidase (GUS) reporter activity and GFP reporter lines demonstrated that OsNAM and OsCUC3 have distinct expression patterns. Specifically, OsNAM was expressed throughout the SAM, whereas OsCUC3 was expressed only at the base of the SAM, with its expression gradually decreasing as seedlings develop. RNA sequencing analysis showed that the expression of genes related to leaf epidermal cell development, cell wall components, and hormonal signal transduction was altered in response to the loss of function of OsNAM and OsCUC3. Therefore, the boundary genes OsNAM and OsCUC3 not only inhibit the growth of axillary shoots but also regulate the development of aboveground organs, including leaf morphology and number, by maintaining the SAM activity in the main shoot.