Transgene Expression in Spruce and Poplar: From the Lab to the Field

Abstract

As the development of transformation systems progresses in annual crop plants, one troubling theme has emerged: there is extreme variation in transgene expression, particularly between plants derived from individual transformation events even within the same petri dish. If such variation in transgene expression occurs in annual crops, what variation can be expected in long-lived perennial crops, such as trees? To answer this question, we have studied the expression over time, both in the lab and in the field, of the uidA gene controlled by an enhanced CaMV 35S promoter in transformants from two Populus hybrids and several transformed lines of Picea glauca. Based on MUG assays for ß-glucuronidase (GUS) activity, there is a general trend: the highest and least variable GUS activity occurs in vitro and the most variable GUS activity is in field grown transgenics. Detailed analysis of GUS activity in three different developmental stages of leaves during two growing seasons indicate that uidA expression in young leaves of hybrid poplar NC5339 (P. alba x P. grandidentata) was highly variable and that in older fully expanded leaves, GUS activity was relatively stable during the season and from year to year. In contrast, older leaves in another hybrid poplar, NM6 (P. nigra x P. maximowiczii) had higher GUS activity relative to the younger leaves. In spruce, GUS activity in needles was only detected during needle elongation, yet activity in the stems persists throughout the season. In all transgenics in the field, GUS activity was highest early in the spring just after bud break, then activity rapidly dropped. The suppression of uidA expression has been noted in a limited number of transgenic lines in both spruce and poplar. After three years in the field, uidA expression has not been restored in any transgenic lines that had uidA expression suppressed. Using a wound-inducible promoter (pinII) to control the uidA gene, evidence suggests that year-to-year stability of transgene expression in poplar is not unique to CaMV 35S. It is interesting to note that the overall level of GUS activity in two consecutive seasons was similar within a species and that despite seasonal variability, predictable yearly trends in transgene expression may be possible in woody perennials.