New insights on dormancy in tree buds
For trees in boreal and temperate climates, it is important that buds do not burst precociously, but only when it's spring for real. Therefore, the buds are put in dormancy in the autumn, which means they have to go through a long cold period before they slowly become susceptible to the signals of spring. The mechanism behind this is revealed in a new study led by Rishi Bhalerao from SLU, recently published in the journal Science.
Trees are amongst the longest-living organisms on Earth, and some species can live for thousands of years. One of the key mechanisms that enable such a long life is their synchronization of growth with change in seasons. For example, in temperate and boreal ecosystems, trees stop their growth and establish dormancy prior to the advent of winter. Growth cessation and dormancy establishment is a key adaptive mechanism for winter survival, since failure to cease growth and establish dormancy can result in fatality from extreme low temperatures in the winter.
How trees know when to stop their growth and establish dormancy is a question that has been of interest to researchers since a long time. That growth stops in response to the decrease in daylight during autumn has been well understood. The establishment of dormancy, which means that buds cannot burst before they have experienced a long cold period, and are not awakened by short warm periods during winter, has been more of a mystery. The recent article in Science, however, provides an important insight into how winter dormancy is regulated in perennial trees.
What the researchers could show is that the so-called plasmodesmata, channels that connect different cells with each other, are closed by the deposition of callose, a polysaccharide, in response to the shortening of day length in the autumn. The blockage of the plasmodesmata prevents cells from receiving growth promotive signals, thereby maintaining growth arrest and establishing dormancy in the buds.
The researchers also show that short-day induced dormancy is regulated by the plant hormone abscisic acid which activates (among others) the production of the callose that is used to block the plasmodesmata. Once blocked, a long exposure to low temperatures is needed to slowly re-open the plasmodesmata again, so that the growth-inducing signals can reach the buds and stimulate the growth in the buds in the spring.
"Interestingly, some of the facets of the dormancy regulation mechanism described in our paper have been observed in winter wheat as well as characean algae, suggesting that this mechanism is probably ancient and evolutionarily conserved", says Rishikesh Bhalerao.
The study was conducted using hybrid aspen, which is a model plant in tree research.
The study has been conducted by a research team led by Rishi Bhalerao from SLU's Department of Forest Genetics and Plant Physiology and the Umeå Plant Science Center. The colleagues come from SLU in Alnarp, Uppsala University, University of Helsinki, Cambridge University, Monash University and the University of Environmental and Life Sciences in Norway.
Rishikesh P. Bhalerao, Professor
Department of Forest Genetics and Plant Physiology
Swedish University of Agricultural Sciences, Umeå
+46 (0)90-786 84 88, +46 (0)70-678 37 32, firstname.lastname@example.org
S. Tylewicz, A. Petterle, S. Marttila, P. Miskolczi, A. Azeez, R. K. Singh, J. Immanen, N. Mähler, T. R. Hvidsten, D. M. Eklund, J. L. Bowman, Y. Helariutta, R. P. Bhalerao. 2018. Photoperiodic control of seasonal growth is mediated by ABA acting on cell-cell communication. Science 10.1126/science.aan8576 (2018).
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Birch buds in winter dormancy. Photo: David Stephansson