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-GNP1TQ 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-GNP1TQ. 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 GA1 and GA3 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.
細胞分裂素和赤霉素（GAs）在生殖分生組織活性的調(diào)控中發(fā)揮拮抗作用。細胞分裂素會正向影響分生組織活性和維持。在花分生組織發(fā)育中，通過KNOX介導(dǎo)的通路激活細胞分裂素的合成。細胞分裂素活性增加會導(dǎo)致更高的籽粒數(shù)目，而GAs則會負向影響分生組織活性。GA合成基因GA20oxs受到KNOX蛋白的負向調(diào)控。KNOX作為調(diào)控因子發(fā)揮功能，平衡分生組織中細胞分裂素和GA活性。但是細胞分裂素和GA調(diào)控因子連同KNOX蛋白間的相互作用以及KNOX如何介導(dǎo)激素活性功能的動態(tài)平衡仍知之甚少。通過QTLs的圖位克隆，我們克隆了一個GA生物合成基因，GNP1，它編碼水稻GA20ox1。NIL-GNP1TQ的籽粒數(shù)目和產(chǎn)量顯著高于同基因?qū)φ眨↙emont）。啟動子區(qū)域的序列變化會增加GNP1的轉(zhuǎn)錄水平，它會在NIL-GNP1TQ的花序分生組織的頂尖區(qū)域積累。我們認為，細胞分裂素活性的增加是由于KNOX介導(dǎo)的轉(zhuǎn)錄反饋環(huán)，這是由GNP1較高轉(zhuǎn)錄水平引起的，進而導(dǎo)致GA代謝基因GA2oxs表達水平增加以及GA1和GA3積累降低。這種重新調(diào)整過程會增加細胞分裂素活性，進而增加水稻籽粒數(shù)目和糧食產(chǎn)量。這些結(jié)果揭示了GAs在水稻花序分生組織發(fā)育中新的且十分重要的作用，為理解發(fā)育過程中細胞分裂素和GA間的相互作用提供了新的觀點。