Season dependent growth pause
Trees and other perennial plants stop growing each year when it gets colder. A strategy that helps then to survive the cold winter. But how plants are regulating this is not completely known. A group of Swedish and Chinese researchers found this out for poplars.
Poplars stop growing when, in autumn, the days get shorter. At that moment researchers see that there is less of the growth centre maintaining FT2 present in the growth centre. And in absence of FT2 the cells of the growth centre are no longer dividing. Not surprising plants tightly regulate the amount of FT2. One of the positive regulators of FT2 is LAP1, but absence of LAP1 doesn’t give the same result as absence of FT2. Therefore, there needs to be a second regulator.
To find that one, the researchers looked at parallels in the regulation of growth centres of annual plants. There the module of miR172 and AP2 is regulating the activity of the growth centre. The researchers wondered if that was also the case in poplars.
AP2-like activates FT2
The first thing the researchers did was creating poplars that had miR172 that was more, or less active. After shifting these plants to a shorter day length, 8 hours instead of 18 hours, plants in which miR172 was less active stopped later with growing, while plants in which miR172 was more active stopped earlier with growing. Suggesting that miR172 is indeed involved in the regulation of the growth in the growth centres.
miR172 regulates the expression of AP2-like genes, of which there are six in poplars. To investigate the influence of AP2-like genes on the activity of the growth centres, the researchers created plants that produced more, or less AP2-like protein. After shifting these plants to a shorter day length, plants that had more AP2-like protein stopped earlier with growing, while plants with less AP2-like protein grew longer. The opposite from the effect of miR172 and in line with what was expected.
AP2-like protein turned out to accumulate in both the growth centre as in the leaves. In the growth centre AP2-like protein was needed to regulate the activity of the growth centre. The in the leaves present AP2-like protein also regulate gene expression. Inclusive that of FT2. Is AP2-like present in the leaves, then the plant produces FT2, subsequently FT2 travels to the growth centre where it stimulates growth together with AP2-like.
Age influences the amount of AP2-like
AP2-like is thus needed for the production of FT2. But all this is not saying anything why FT2 reduces when the days get shorter. One of the from annual plants known regulators of the miR172/AP2 module is the module miR156/SPL. Together these modules are responsible for keeping track of the age of the plant.
In poplars regulates the module miR156/SPL also the miR172/AP2-like module. Whereby SPL3/5 is activating miR172, which in turn block the expression of AP2-like. In turn does AP2-like block the expression of SPL3/5 while it stimulates the expression of miR156. And miR156 to complete the circle blocks the expression of SPL3/5. But important to know is that in the middle of all this the plant reduces the expression of miR156 when it gets older. Also, it doesn’t appear to be the case that the miR156/SPL3/5 module is behaving differently in short days than in longer days. There is therefore still a missing daylength regulator of FT2 and or AP2-like.
Literature
J. Wang, X. Liao, Z. Wu, S. Sane, S. Han, Q. Chen, X. Shi, X. Dai, M. Klintenäs, O. Nilsson, & J. Ding, Genetic control of seasonal meristem arrest in trees, Proc. Natl. Acad. Sci. U.S.A. 122 (48) e2505641122, https://doi.org/10.1073/pnas.2505641122 (2025).
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