Alteration of PHYA expression change circadian rhythms and timing of bud set in Populus
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In many temperate woody species, dormancy is induced by short photoperiods. Earlier studies have shown that the photoreceptor phytochrome A (phyA) promotes growth. Specifically, Populus plants that over-express the oat PHYA gene (oatPHYAox) show daylength-independent growth and do not become dormant. However, we show that oatPHYAox plants could be induced to set bud and become cold hardy by exposure to a shorter, non-24 h diurnal cycle that significantly alters the relative position between endogenous rhythms and perceived light/dark cycles. Furthermore, we describe studies in which the expression of endogenous Populus tremula × P. tremuloides PHYTOCHROME A (PttPHYA) was reduced in Populus trees by antisense inhibition. The antisense plants showed altered photoperiodic requirements, resulting in earlier growth cessation and bud formation in response to daylength shortening, an effect that was explained by an altered innate period that leads to phase changes of clock-associated genes such as PttCO2. Moreover, gene expression studies following far-red light pulses show a phyA-mediated repression of PttLHY1 and an induction of PttFKF1 and PttFT. We conclude that the level of PttPHYA expression strongly influences seasonally regulated growth in Populus and is central to co-ordination between internal clock-regulated rhythms and external light/dark cycles through its dual effect on the pace of clock rhythms and in light signaling.
KeywordsGrowth cessation Dormancy Circadian clock Photoperiodism Phytochrome Populus
Cauliflower mosaic virus
We are grateful to Ingabritt Carlsson, Gunilla Malmberg, Marie Nygren, Kjell Olofsson, Medisa Hasić, Isabella Pekkari and Sofia Österberg for their excellent technical assistance and to the Wallenberg greenhouse staff for their valuable help. Luis Muniz and Harriet G. McWatters are acknowledged for helpful scientific discussions. We thank Andrew J. Millar for the gift of the promoter:LUC constructs and BRASS software. We are also grateful for valuable help from Yury Shatz regarding the PixelSmart plug-in for ImageJ. This work was supported by grants to MEE from the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS), the Swedish Research Council (VR), the Knut and Alice Wallenberg Foundation and Nils och Dorthi Troëdssons forskningsfond. The research was also supported by the Swedish Natural Science Research Council (NFR), Swedish Research Council for Agricultural Sciences (SJFR), the Swedish Foundation for Strategic Research (SSF), the Kempe Foundation and a Human Frontier Science Program (RG0303) to TM.
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