"Floral stem cell activity vanishes when WUSHCEL is suppressed and silenced through changes in its chromatin state. What we did not know was how this change begins and how it is sustained," explains NAIST Professor Toshiro Ito, who led the study.

Ito's team looked at the activation and suppression of floral stem cells from Arabidopsis. Stem cell activation was marked by a clear expression ofWUSCHEL, but that changed when the cells also began to express KNUCKLES, which bound to theWUSCHELlocus and led toWUSCHEL's expression almost halving four hours later.

Then at 8-12 hours after the KNUCKLES expression, the group found that theWUSCHEL轨迹显示H3K27me3组蛋白甲基化的迹象, a marker of sustained gene suppression.

The question Ito wanted to answer was what were the events that took place from the KNUCKLES binding to theWUSCHELlocus to the H3K27me3 histone methylation that could terminate the stem cell activation.

"H3K27me3 is catalyzed by Polycomb Group complexes, but nothing is known about how the complexes are recruited to theWUSCHELlocus," says Ito.

The researchers discovered that KNUCKLES binding toWUSCHELjettisoned SPLAYED, a chromatin remodeling protein that activatesWUSCHEL. This effect leads to rapid transcriptional repression ofWUSCHEL, followed by the recruitment of Polycomb Group complex toWUSCHEL, where it formed H3K27me3 marks on the chromatin to suppress gene expression.

"KNUCKLES binding was essential for the rapid removal of active H3K4me3 marks and the following deposition of repressive H3K27me3 marks," explains Ito.

The recruitment was done by KNUCKLES interacting with a specific component of the Polycomb Group complex known as FERTILIZATION-INDEPENDENT ENDOSPERM.

"Our study reveals the temporal steps from KNUCKLES binding to H3K27me marks that silence theWUSCHELchromatin. Understanding how stem cell activation is terminated will assist in new food technologies," says Ito.