Far red end-of-day treatment restores wild type-like plant length in hybrid aspen overexpressing phytochrome A.

Shoot elongation in woody plants is modulated by a multitude of light signals, including irradiance, photoperiod and spectral composition, for which the phytochrome system is the probable photoreceptor. In hybrid aspen (Populus tremula x tremuloides) overexpression of the oat phytochrome A (PHYA) prevents growth cessation in response to short photoperiod, and plants exhibit dwarf growth that is related to reduced cell numbers and reduced gibberellin contents. End-of-day far-red treatment significantly enhances internode elongation in PHYA overexpressors as well as in the wild type, and this was found here to be caused by stimulation of cell division and cell extension. In PHYA overexpressors the effects were substantially larger than in the wild type, and resulted in complete restoration of wild type-like plant length as well as cell numbers, and gibberellin content was greatly increased. No clear effect of far-red end-of-day treatment on gibberellin levels could be detected in the wild type. It thus appears that the far-red end-of-day treatment might modify the responsiveness of the tissue to GA rather than the GA levels. The observed effects were completely reversed by a subsequent irradiation with red light. The present data show that dwarfism due to PHYA overexpression can be completely overcome by far red end-of-day treatment, and the observations indicate that effects of far red end-of-day treatments appear to be mediated by phytochrome(s) other than phytochrome A.