by Maria C. Albani
Botanical Institute, University of Cologne, Germany.
Max Planck Institute for Plant Breeding Research, Cologne, Germany.
In most temperate environments one can see trees flowering very early in the spring. For most perennials flowering in the spring marks the event of floral emergence instead of the time of the induction of flowering and flower bud initiation as it is for many annual species.
Thus, trees can flower very early because most of them had initiated the flower buds already the previous year during the summer, autumn or winter. To survive the winter, many perennials also cease growth in the autumn and become dormant during the winter. Environmental cues such as photoperiod and cold regulate growth cessation and bud dormancy release. For example, short photoperiods in the autumn are required to induce growth cessation whereas prolonged cold is required to break bud dormancy.
A recent study in hybrid aspen, which is a cross between the European Populus tremula and the American aspen P. tremuliodes, highlights the role of photoperiod in setting the dormant state independently of growth cessation (Tylewicz et al., 2018). Short days block cellular communication through plasmodesmata closure and this process involves the phytohormone ABA. The authors created transgenic aspen with reduced ABA response, overexpression lines of the PDLP1 gene, which impairs trafficking via plasmodesmata, and DsRNAi lines of the chromatin remodelling factor PICKLE (PKL). These transgenics were used to demonstrate the ABA-dependent pathway for plasmodesmata closure and their role in bud dormancy. The authors also used grafting to show that closure of the plasmodesmata regulates the inability of the bud to grow. For this, they grafted scions of wild type and transgenics plants with reduced ABA response grown in short days (so that only scions of the transgenics will have open plasmodesmata) onto rootstocks of lines overexpressing the aspen FLOWERING LOCUS T 1 (FT1) gene. Under these conditions, buds of wild type scions did not reactivate growth whereas buds from scions of the transgenics that had compromised ABA response showed bud outgrowth. These results lead to the conclusion that plasmodesmata closure induced by short days blocks the FT1-derived growth promoting signals to access the meristem. The authors also suggested that re-opening of the plasmodesmata occurs slowly and only after exposure to low temperatures.
The study of Tylewicz et al., 2018 is not about flowering as it has been performed using juvenile/vegetative plants. It however raises interesting questions if one takes into account the flowering patterns in perennials. In P. deltoides trees grown in Starville (Mississippi, USA) it has been demonstrated that flower buds are initiated during the winter when plants are exposed to short day length and low temperatures. In this Populus species, flowering and the return to vegetative development is regulated by two paralogues of FT, FT1 and FT2 (Hsu et al., 2011). FT1 expression was increased during the winter in many tissues including the reproductive buds, whereas FT2 trancripts were only up-regulated in the leaves after the return to warm temperatures. These results suggested that FT1 regulates reproductive onset in response to winter temperatures whereas FT2 promotes vegetative growth after the winter in response to warm temperatures and long days.
In the study of Hsu et al., 2011, although trees were considered to undergo the dormant phase, obviously things still happen during prolonged exposure to cold as flower buds were initiated. Thus it would be interesting to study how the model on bud dormancy in vegetative buds, described by Tylewicz et al., 2018 can be translated to the regulation of flowering in perennials. Is plasmodesmata closure also important in the flowering buds? Does flower bud initiation need open or closed plasmodesmata? Do plasmodesmata also play a role in the outgrowth of these flower buds in the spring? Although these are interesting questions to answer, it is probably technically difficult to address due to the long juvenile phase of trees.
Tylewicz S, Petterle A, Marttila S, Miskolczi P, Azeez A, Singh RK, Immanen J, Mähler N, Hvidsten TR, Eklund DM, Bowman JL, Helariutta Y, Bhalerao RP. 2018. Photoperiodic control of seasonal growth is mediated by ABA acting on cell-cell communication. Science 360(6385): 212-215. doi: 10.1126/science.aan8576.
Hsu CY, Adams JP, Kim H, No K, Ma C, Strauss SH, Drnevich J, Vandervelde L, Ellis JD, Rice BM, Wickett N, Gunter LE, Tuskan GA, Brunner AM, Page GP, Barakat A, Carlson JE, DePamphilis CW, Luthe DS, Yuceer C. 2011. FLOWERING LOCUS T duplication coordinates reproductive and vegetative growth in perennial poplar. Proceedings of the National Academy of Sciences, USA. 108(26):10756-61. doi: 10.1073/pnas.1104713108.