In vitro growth and tuber formation by transgenic potato plants harboring rolC or rolB genes under control of the patatin promoter

Growth and tuber formation by transgenic potato (Solanum tuberosum L., cv. Desiree) plants bearing bacterial rolC or rolB genes under the control of t uber-specific class 1 patatin promoter (B33 promoter) were studied in vitro under conditions favorable (5-8% sucrose in the medium) or unfavorable (2% sucrose in the medium) for tuberization. Wild-type plants grown under simi lar conditions and plants bearing only a 35S-GUS marker construction served as controls. Under unfavorable conditions for tuber formation, both types of transformants (B33-rolC and B33-rolB) differed little from control plant s. Under conditions promoting tuber formation, B33-rolC plants differed fro m control plants by active shoot and root growth, suppressed apical dominan ce, enhanced branching, delayed tuberization, and the formation of abnormal ly elongated tubers with numerous buds. B33-rolB plants showed no marked ch anges in growth and tillering activities, but they began to form tubers ear lier. These tubers were elongated in shape when they were formed in the lig ht, but not in darkness. We concluded that the expression of inserted gene constructions B33-rolC and B33-rolB affected some stages of tuber formation , namely tuber initiation and shape determination. The products of the rolC and rolB genes seem to affect tuber formation by changing the sensitivity of cells in the subapical zone of the stolen and those of developing tubers toward endogenous cytokinins and/or auxins.