OXIDATIVE STRESS IN AGROBACTERIUM-INDUCED TUMORS ON KALANCHOE PLANTS

The induction of tumor formation by wild-type Agrobacterium tumefacien s was found to be associated with the apparent rate of production of s uperoxide anions (O-2(.-)) in the infected tissue of Kalanchoe plants. The transcript levels of MnSOD and Cu/ZnSOD and the specific activity of superoxide dismutase (SOD) during different stages of tumor induct ion support this. A. tumefaciens strain with mutations in the cytokini n biosynthetic gene produced delayed, smaller and abnormal tumors whic h underwent significantly lower levels of superoxide stress compared w ith normal tumors, produced by the wild-type strain. These suppressed tumors developed into normal tumors with the concurrent production of O-2(.-) when supplemented in vitro with exogenous phytohormones. Phyto hormone-induced callus from non-infected stem tissues maintained in vi tro in the presence of phytohormones produced O-2(.-) in the same orde r of magnitude as the tumor cells. Transgenic plants expressing three- to five-fold higher levels of SOD activity exhibited a 55% reduction in the apparent rate of O-2(.-) production compared with that of the n on-transgenic plants when challenged with wild-type Agrobacterium. The se results suggest the possible role of phytohormones in creating supe roxide stress which, in turn, may cause deregulation of cell division, oncogenicity and ultimately tumor formation.