May 23, 2019 | Updated: 11:35 AM EDT

Suppressing Growth to Make Flowers Grow

May 13, 2019 11:56 AM EDT

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Turning off Growth to Make Flowers Grow
(Photo : Toshiro Ito)

More than just decorating the home, the beautiful colors and smells of flowers serve a much higher purpose. Flowers are where the reproductive organs of a plant are found, and those same colors and smells that make a room beautiful also attract bees and other animals for pollination.

The essential cells for the growth of flower and its organs are the floral stem cells. For the flower to fully develop and set seeds, that growth must eventually terminate. Scientists at Nara Institute of Science and Technology (NAIST) have presented a new study published in The Plant Cells that the transcription factor KNUCKLES is a vital regulator of this stem cell arrest by initiating a series of epigenetic events to repress the stem cell determinant WUSCHEL.

The lead author of the study, and NAIST professor Toshiro Ito said that floral stem cell activity vanishes when WUSCHEL is suppressed and silenced through changes in its chromatin state. And what scientists did not know was how this change begins and how it is sustained.

Ito and team then looked at the activation and suppression of floral stem cells from Arabidopsis. Stem cells activation was marked by a clear expression of WUSCHEL, but that changed when cells also began to express KNUCKLES, which bound to the WUSCHEL locus and led to WUSCHEL's expression almost halving four hours later. Then, at 8 to 12 hours after the KNUCKLES expression, the team discovered that the WUSCHEL locus showed signs of H3K27me3 histone methylation, 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 the WUSCHEL locus to the H3K27me3 histone methylation that could terminate the stem cell activation.

Ito explained that H3K27me3 is catalyzed by Polycomb Group complexes, but nothing is known about how the complexes are recruited to the WUSCHEL locus.

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

Explaining further, Ito said that KNUCKLES binding was essential for the rapid removal of active H3K4me3 marks and the following deposition of repressive H3K27me3 marks. The recruitment was done by KNUCKLES interacting with a specific component of the Polycomb Group complex known as FERTILIZATION-INDEPENDENT ENDOSPERM.

In his conclusion, Ito said that their study shows that the temporal steps from KNUCKLES binding to H3K27me mark that silence the WUSCHEL chromatin. Understanding how stem cell activation is terminated will assist in new food technology.

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