ECTOPIC EXPRESSION OF OAT PHYTOCHROME-A IN HYBRID ASPEN CHANGES CRITICAL DAYLENGTH FOR GROWTH AND PREVENTS COLD ACCLIMATIZATION

Survival of temperate-zone tree species under the normal summer-winter cycle is dependent on proper timing of apical growth cessation and co ld acclimatization. This timing is primarily based on the perception o f daylength, and through evolution many tree species have developed ph otoperiodic ecotypes which are closely adapted to the local light cond itions. The longest photoperiod inducing growth cessation, the critica l photoperiod, is inherited as a quantitative character. The phytochro me pigment family is the probable receptor of daylength, but the exact role of phytochrome and the physiological basis for the different res ponses between photoperiodic ecotypes are not known. This report shows for the first time that overexpression of the oat phytochrome A gene (PHYA) in a tree significantly changes the critical daylength and effe ctively prevents cold acclimatization. While the critical daylength fo r elongation growth in the wild-type of hybrid aspen (Populus tremula x tremuloides) was approximately 15 h, transgenic lines with a strong expression of the oat PHYA gene did not stop growing even under a phot operiod of 6 h. Quantitative analysis of gibberellins (GA) as well as indole-a-acetic acid (IAA) revealed that levels of these were not down -regulated under short days in the transgenic plants expressing high l evels of oat PHYA, as in the wild-type. These results indicate that ph otoperiodic responses in trees might be regulated by the amount of PHY A gene expressed in the plants, and that the amount of phytochrome A ( phyA) affects the metabolism of GAs and IAA.