Flowers are the primary reproductive organs of seed plants, playing a critical role in reproduction. In agriculture, normal flower development directly impacts crop yield and quality, making research on the mechanisms of floral organ development particularly important. In Arabidopsis, flowers are derived from the differentiation of the floral meristem (FM), which gives rise to four whorls of floral organs: sepals, petals, stamens, and carpels. During stage 6 of flower development, the FM begins to differentiate into carpel primordia. At this stage, the activity of stem cells within the FM must be gradually terminated; otherwise, excessive stem cell activity will lead to the formation of ectopic floral organs. The C2H2-type zinc finger protein KNUCKLES (KNU) is a key transcriptional repressor that regulates FM termination. During stage 6 of flower development, KNU terminates FM activity by directly disrupting the WUSCHEL (WUS)-CLAVATA3 (CLV3) feedback loop, which is essential for maintaining FM activity.
Auxin and cytokinin are two major plant hormones that play broad regulatory roles in various stages of plant growth and development. During flower development, auxin and cytokinin also participate in regulating FM activity. While KNU, auxin, and cytokinin signaling pathways are all critical factors in FM regulation, whether they interact to jointly control FM termination and the precise molecular mechanisms underlying this process remain to be further elucidated.
The group of Prof. Bo Sun, School of Life Sciences, recently uncovered the molecular mechanism by which the KNU protein regulates the distribution and activity of auxin and cytokinin to ensure the timed termination of the FM in Arabidopsis. The study demonstrates that the dynamic distribution of auxin and cytokinin within the FM during early flower development is critical for its timed termination. Through phenotypic analysis of the knu-2 mutant and related transgenic plants, combined with molecular and cell biology experiments, it was found that KNU directly binds to and represses the expression of the auxin transporter gene PIN-FORMED1 (PIN1) and the cytokinin biosynthesis gene ISOPENTENYLTRANSFERASE7 (IPT7). Further experiments revealed that KNU mediates the repressive histone modification H3K27me3 at these gene loci to regulate auxin and cytokinin activity. Moreover, studies on the transgenic lines pKNU:PIN1 and pKNU:IPT7 showed floral indeterminacy phenotypes similar to the knu-2 mutant, while the double mutants knu-2 ipt7 and knu-2 pin1-1 exhibited a significant rescue of the knu-2 mutant phenotype. These findings confirm that KNU regulates FM determinacy through PIN1 and IPT7. This study provides new perspectives and directions for understanding the regulatory mechanisms of flower development.
In summary, this study reveals a novel mechanism by which KNU mediates the regulation of auxin and cytokinin activity to terminate the FM, further advancing the understanding of how KNU ensures the timed termination of FM activity. This work provides new insights and directions for future research on the regulatory mechanisms of flower development.
This study was published online on November 22, 2024 with the title " KNUCKLES Regulates Floral Meristem Termination by Controlling Auxin Distribution and Cytokinin Activity " in PLANT CELL. Guangling Wang, a PhD student in the School of Life Sciences, is the first author of the paper, and Prof. Bo Sun is the corresponding author. This study was supported by the National Natural Science Foundation of China, and the Fundamental Research Funds for the Central Universities.
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