Stress Increases Ecological Risk of Glufosinate-Resistant Transgene Located on Alien Chromosomes in Hybrids Between Transgenic Brassica napus and Wild Brassica juncea

When glufosinate-resistant transgenic <i>Brassica napus</i> (transgene <i>PAT</i> located on C chromosome) were backcrossed with wild <i>Brassica juncea</i>, 50% of the progeny expressed <i>PAT</i> under favourable conditions. However, exposure to stress (drought, salt, flooding, and intraspecific competition) increased the proportion of plants expressing the <i>PAT</i> gene (r-e plants) by approximately 20% compared to those under unstressed conditions. In the self-pollinated progeny of the stressed plants, the proportion of r-e plants increased by a nearly 30% compared to that of the unstressed plants. Composite fitness was comparable between plants developed under drought stress at the seedling stage and those grown under favourable conditions. Abscisic acid (ABA) content and expression of the Repressor of Silencing 1 (<i>ROS1</i>) in leaves increased significantly after stress treatment in the progeny, with r-e plants exhibiting higher levels. Exogenous ABA treatment significantly up-regulated <i>ROS1</i> expression in progeny leaves, and the ABA treatment of seeds increased the survival of progeny exposed to glufosinate by 15%. Results suggest that increasing ABA under stress may enhance the demethylation of <i>PAT</i>'s promoter by promoting <i>ROS1</i> expression, thereby inhibiting transgene silencing of <i>PAT</i>, indicating that transgene located on the C chromosome of transgenic <i>B. napus</i> may pose a higher risk of gene flow to wild <i>B. juncea</i> under stress, especially drought stress.