Rice osa-miR171c Mediates Phase Change from Vegetative to Reproductive Development and Shoot Apical Meristem Maintenance by Repressing Four OsHAM Transcription Factors

Phase change from vegetative to reproductive development is one of the critical developmental steps in plants, and it is regulated by both environmental and endogenous factors. The maintenance of shoot apical meristem (SAM) identity, miRNAs and flowering integrators are involved in this phase change process. Here, we report that the miRNA osa-miR171c targets four GRAS (GAI-RGA-SCR) plant-specific transcription factors (OsHAM1, OsHAM2, OsHAM3, and OsHAM4) to control the floral transition and maintenance of SAM indeterminacy in rice (Oryza sativa). We characterized a rice T-DNA insertion delayed heading (dh) mutant, where the expression of OsMIR171c gene is up-regulated. This mutant showed pleiotropic phenotypic defects, including especially prolonged vegetative phase, delayed heading date, and bigger shoot apex. Parallel expression analysis showed that osa-miR171c controlled the expression change of four OsHAMs in the shoot apex during floral transition, and responded to light. In the dh mutant, the expression of the juvenile-adult phase change negative regulator osa-miR156 was up-regulated, expression of the flowering integrators Hd3a and RFT1 was inhibited, and expression of FON4 negative regulators involved in the maintenance of SAM indeterminacy was also inhibited. From these data, we propose that the inhibition of osa-miR171c-mediated OsHAM transcription factors regulates the phase transition from vegetative to reproductive development by maintaining SAM indeterminacy and inhibiting flowering integrators.

植物从营养生长向生殖生长的阶段转换是其发育过程中的关键步骤之一，该过程受环境与内源因子共同调控。茎尖分生组织（shoot apical meristem, SAM）的身份维持、微小RNA（microRNA, miRNA）以及成花整合因子均参与这一阶段转换进程。本研究报道，微小RNA osa-miR171c通过靶向4个GRAS（GAI-RGA-SCR）植物特异性转录因子（OsHAM1、OsHAM2、OsHAM3及OsHAM4），调控水稻（Oryza sativa）的成花转变及茎尖分生组织未分化状态的维持。我们鉴定了一个水稻T-DNA插入抽穗延迟（delayed heading, dh）突变体，该突变体中OsMIR171c基因的表达量上调。该突变体表现出多效性表型缺陷，尤以营养生长期延长、抽穗期延迟及茎尖体积增大最为显著。平行表达分析显示，osa-miR171c可在成花转变过程中调控茎尖处4个OsHAM基因的表达变化，并响应光信号。在dh突变体中，幼龄-成龄阶段转换的负调控因子osa-miR156的表达量上调，成花整合因子Hd3a与RFT1的表达受到抑制，同时参与维持茎尖分生组织未分化状态的FON4负调控因子的表达也被抑制。基于上述实验数据，我们提出假说：osa-miR171c介导的OsHAM转录因子的抑制作用，可通过维持茎尖分生组织的未分化状态并抑制成花整合因子的表达，从而调控植物从营养生长向生殖生长的阶段转换。