Oxidative stress in Agrobacterium-induced tumors on Kalanchoe plants

Abstract

The induction of tumor formation by wild-type Agrobacterium tumefaciens was found to be associated with the apparent rate of production of superoxide anions (O2·-) 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 induction support this. A. tumefaciens strain with mutations in the cytokinin biosynthetic gene produced delayed, smaller and abnormal tumors which underwent significantly lower levels of superoxide stress compared with normal tumors, produced by the wild-type strain. These suppressed tumors developed into normal tumors with the concurrent production of O2·- when supplemented in vitro with exogenous phytohormones. Phytohormone-induced callus from non-infected stem tissues maintained in vitro in the presence of phytohormones produced O2·- in the same order 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 O2·- production compared with that of the non-transgenic plants when challenged with wild-type Agrobacterium. These results suggest the possible role of phytohormones in creating superoxide stress which, in turn, may cause deregulation of cell division, oncogenicity and ultimately tumor formation.