Weinthal, D.M., Department of Plant Pathology and Weed Science, ARO Volcani Center, POB 6, Bet Dagan 50250, Israel, Department of Plant Sciences, Faculty of Life Sciences, University of Tel Aviv, Tel Aviv 69978, Israel, Department of Molecular, Cellular and Developmental Biology, The University of Michigan, Ann Arbor, MI 48109, United States
Yablonski, S., Department of Plant Pathology and Weed Science, ARO Volcani Center, POB 6, Bet Dagan 50250, Israel
Singer, S., Department of Plant Pathology and Weed Science, ARO Volcani Center, POB 6, Bet Dagan 50250, Israel
Barash, I., Department of Plant Sciences, Faculty of Life Sciences, University of Tel Aviv, Tel Aviv 69978, Israel
Manulis-Sasson, S., Department of Plant Pathology and Weed Science, ARO Volcani Center, POB 6, Bet Dagan 50250, Israel
Gaba, V., Department of Plant Pathology and Weed Science, ARO Volcani Center, POB 6, Bet Dagan 50250, Israel

Pantoea agglomerans pvs. gypsophilae and betae are related gall-forming bacteria. While P. agglomerans pv. beta initiates gall formation on both beet and gypsophila, the gypsophila pathovar causes gall formation only on gypsophila. PthG is a type III effector determining host range of these pathogens, initiating the hypersensitivity response in beet, but is a virulence factor in gypsophila. The role of PthG and its mode of action in pathogenicity remain unclear. Transgenic Nicotiana tabacum plants expressing pthG were created. PthG over-expression was often lethal, and surviving pthG-bearing lines showed morphological and developmental abnormalities such as leaf deformation and abnormal vascular branching, dwarf stature, loss of apical dominance, seedling apical meristem loss, rapid germination, reduced fertility, plants which cease growth for several weeks later producing a new lateral shoot, and loss of endophyte resistance (bearing unusual saprophyte populations). Some transformants required light for seed germination and showed rapid seedling greening. In in vitro assays PthG expression modified responses to auxin and cytokinin, inhibiting root and shoot production but not callus formation. In vitro differentiation responses to light were modified by PthG expression. This effector may interfere in the plant auxin signalling pathways resulting in higher observed auxin and ethylene levels, and subsequent blockage of root and shoot development. Apparently PthG tunes the host response to high hormone levels, changing the developmental response. Since shoot and root development are delayed, we hypothesize that callus/gall formation is supported by this activity. However, interference by PthG with hormone and light signalling does not explain all the responses observed in pthG-bearing lines. © 2010 KNPV.