Stem Cell Termination in Flowers

Sun et al. elucidate the series of events leading to floral stem cell termination.

Plant Cell

By B. Sun and T. Ito

College of Life Sciences, Nanjing Agricultural University, China; Biological Science, Nara Institute of Science and Technology and Temasek Life Sciences Laboratory, 1 Research Link, National University of Singapore

Background: During early flower development, self-renewing stem cells produce enough cells to be consumed for the formation of floral organs, including sepals, petals, stamens and carpels, which are patterned in concentric rings. Floral stem cells are specified by the gene named WUSCHEL (WUS). At a certain stage of flower development, WUS is repressed and silenced; therefore, floral stem cell activity vanishes to ensure proper development of carpels. We previously found that a small protein named KNUCKLES (KNU) functions as an upstream repressor of WUS, with the result that floral stem cell activity is terminated.

Question: We wanted to understand how KNU represses and silences WUS in a detailed manner. Before this, little was known regarding the final events of floral stem cell termination at high resolution.

Findings: We found that KNU integrates initial transcriptional repression with epigenetic stable silencing of WUS. KNU protein directly binds to the WUS promoter and causes eviction of SPLAYED (SYD) protein from the WUS gene. SYD is a known activator of WUS and can act against the repressive chromatin status of WUS. KNU protein also physically interacts with FERTILIZATION-INDEPENDENT ENDOSPERM (FIE), which mediates the silencing of WUS at the chromatin level. These multi-step events lead to the termination of floral stem cells, thus ensuring proper carpel development.

Next steps: We are working on how the robust stem cell activity is fully shut off within a narrow time window to secure the proper timing of carpel differentiation.

Bo Sun, Yingying Zhou, Jie Cai, Erlei Shang, Nobutoshi Yamaguchi, Jun Xiao, Liang-Sheng Looi, Wan-Yi Wee, Xiuying Gao, Doris Wagner and Toshiro Ito. (2019). Integration of Transcriptional Repression and Polycomb-mediated Silencing of WUSCHEL in Floral Meristems. Plant Cell; DOI:

Key words: flower development, stem cells