Impact of Intron and Retransformation on Transgene Expression in Leaf and Fruit Tissues of Field-Grown Pear Trees

Stable and high expression of introduced genes is a prerequisite for using transgenic trees. Transgene stacking enables combining several valuable traits, but repeated transformation increases the risk of unintended effects. This work studied the stability and intron-mediated enhancement of uidA gene expression in leaves and different anatomical parts of pear fruits during field trials over 14 years. The stability of reporter and herbicide resistance transgenes in retransformed pear plants, as well as possible unintended effects using high-throughput phenotyping tools, were also investigated. The activity of β-glucuronidase (GUS) varied depending on the year, but silencing did not occur. The uidA gene was expressed to a maximum in seeds, slightly less in the peel and peduncles, and much less in the pulp of pear fruits. The intron in the uidA gene stably increased expression in leaves and fruits by approximately twofold. Retransformants with the bar gene showed long-term herbicide resistance and exhibited no consistent changes in leaf size and shape. The transgenic pear was used as rootstock and scion, but grafted plants showed no transport of the GUS protein through the graft in the greenhouse and field. This longest field trial of transgenic fruit trees demonstrates stable expression under varying environmental conditions, the expression-enhancing effect of intron and the absence of unintended effects in single- and double-transformed woody plants.