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Cytokinins and gibberellins (GAs) play antagonistic roles in regulating reproductive meristem activity. Cytokinins have positive effects on meristem activity and maintenance. During inflorescence meristem development, cytokinin biosynthesis is activated via a KNOX-mediated pathway. Increased cytokinin activity leads to higher grain number, whereas GAs negatively affect meristem activity. The GA biosynthesis genes  GA20oxs  are negatively regulated by KNOX proteins. KNOX proteins function as modulators, balancing cytokinin and GA activity in the meristem. However, little is known about the crosstalk among cytokinin and GA regulators together with KNOX proteins and how KNOX-mediated dynamic balancing of hormonal activity functions. Through map-based cloning of QTLs, we cloned a GA biosynthesis gene,  Grain Number per Panicle1 (GNP1) , which encodes rice GA20ox1. The grain number and yield of NIL- GNP1  TQ  were significantly higher than those of isogenic control (Lemont). Sequence variations in its promoter region increased the levels of  GNP1  transcripts, which were enriched in the apical regions of inflorescence meristems in NIL- GNP1  TQ . We propose that cytokinin activity increased due to a KNOX-mediated transcriptional feedback loop resulting from the higher  GNP1  transcript levels, in turn leading to increased expression of the GA catabolism genes  GA2oxs  and reduced GA 1  and GA 3  accumulation. This rebalancing process increased cytokinin activity, thereby increasing grain number and grain yield in rice. These findings uncover important, novel roles of GAs in rice florescence meristem development and provide new insights into the crosstalk between cytokinin and GA underlying development process.

The QTL GNP1 Encodes GA20ox1, Which Increases Grain Number and Yield by Increasing Cytokinin Activity in Rice Panicle Meristems