EXPRESSION OF 2 HETEROLOGOUS PROMOTERS, AGROBACTERIUM-RHIZOGENES ROLCAND CAULIFLOWER MOSAIC-VIRUS 35S, IN THE STEM OF TRANSGENIC HYBRID ASPEN PLANTS DURING THE ANNUAL CYCLE OF GROWTH AND DORMANCY

We monitored, for the first time, the activity of two model heterologo us promoters, the Agrobacterium rhizogenes rolC and the cauliflower mo saic virus (CaMV) 35S, throughout the annual cycle of growth and dorma ncy in a perennial species, hybrid aspen. Each promoter was fused to t he uidA beta-glucuronidase (GUS) reporter gene and the constructs were introduced into the hybrid aspen genome by Agrobacterium-mediated tra nsformation. Both wild type and transgenic plants were cultivated unde r different regimes of photoperiod and temperature to induce passage t hrough one growth-dormancy-reactivation cycle, and at intervals GUS st aining was assessed in stem sections. In rolC::uidA transformants, GUS activity in rapidly growing current-year shoots was not only tissue s pecific, being localized to the phloem, but also cell-specific at the shoot base, where it was present only in the companion cells. However, during the onset of dormancy induced by short photoperiod, GUS activi ty shifted laterally from the phloem to include the cortex and pith. A fter subsequent exposure to chilling temperatures to induce the transi tion between the dormancy stages of rest and quiescence, GUS activity almost disappeared from all stem tissues, but regained its original ph loem specificity and intensity after the shoots were reactivated by ex posing them to long photoperiod and high temperatures. In contrast, GU S activity in the stem of 35S::uidA transformants was strong in all ti ssues except for the vascular cambium and xylem, and did not vary in i ntensity during the growth-dormancy-reactivation cycle. The lateral sh ift and increased intensity of GUS activity in the stem of rolC::uidA transformants during dormancy induction was shown to be associated wit h the accumulation of starch, and to be mimicked by incubating stem se ctions in sucrose, as well as glucose and fructose, but not sorbitol, prior to the GUS assay. Our results demonstrate that the activities of the rolC and 35S promoters varied in very different, unpredictable wa ys during the annual cycle of growth and dormancy in a perennial speci es, and indicate that the spatial and temporal variation in rolC promo ter activity that we observed in the stem of transgenic hybrid aspen p lants is attributable to cellular and seasonal changes in sucrose cont ent.