KNUCKLES Regulates Floral Meristem Termination by Controlling Auxin Distribution and Cytokinin Activity

Abstract The termination of floral meristem (FM) activity is essential for the normal development of reproductive floral organs. During this process, KNUCKLES (KNU), a C2H2-type zinc finger protein, crucially regulates FM termination by directly repressing the expression of both the stem cell identity gene WUSCHEL (WUS) and the stem cell marker gene CLAVATA3 (CLV3) to abolish the WUS-CLV3 feedback loop required for FM maintenance. In addition, phytohormones auxin and cytokinin are involved in FM regulation. However, whether KNU modulates auxin and cytokinin activities for FM determinacy control remains unclear. Here, we show that the auxin distribution and the cytokinin activity mediated by KNU in Arabidopsis (Arabidopsis thaliana) promote the termination of FM during stage 6 of flower development. Mutation of KNU leads to altered distribution of auxin and cytokinin in the FM of a stage 6 floral bud. Moreover, KNU directly represses the auxin transporter gene PIN-FORMED1 (PIN1) and the cytokinin biosynthesis gene ISOPENTENYLTRANSFERASE7 (IPT7) via mediating H3K27me3 deposition on these two loci to regulate auxin and cytokinin activities. Our study presents a molecular regulatory network that elucidates how the transcriptional repressor KNU integrates and modulates the activities of auxin and cytokinin, thus securing the timed FM termination.