INDUCTION OF DEFENSE-RELATED RESPONSES IN CF9 TOMATO CELLS BY THE AVR9 ELICITOR PEPTIDE OF CLADOSPORIUM-FULVUM IS DEVELOPMENTALLY-REGULATED

The AVR9 elicitor from the fungal pathogen Cladosporium fulvum induces defense-related responses, including cell death, specifically in toma to (Lycopersicon esculentum Mill.) plants that carry the Cf-9 resistan ce gene. To study biochemical mechanisms of resistance in detail, susp ension cultures of tomato cells that carry the Cf-9 resistance gene we re initiated. Treatment of cells with various elicitors, except AVR9, induced an oxidative burst, ion fluxes, and expression of defense-rela ted genes. Agrobacterium tumefaciens-mediated transformation of Cf9 to mato leaf discs with Avr9-containing constructs resulted efficiently i n transgenic callus formation. Although transgenic callus tissue showe d normal regeneration capacity, transgenic plants expressing both the Cf-9 and the Avr9 genes were never obtained. Transgenic F-1 seedlings that were generated from crosses between tomato plants expressing the Avr9 gene and wild-type Cf9 plants died within a few weeks. However, c allus cultures that were initiated on cotyledons from these seedlings could be maintained for at least 3 months and developed similarly to c allus cultures that contained only the Cf-9 or the Avr9 gene. It is co ncluded, therefore, that induction of defense responses in Cf9 tomato cells by the AVR9 elicitor is developmentally regulated and is absent in callus tissue and cell-suspension cultures, which consists of undif ferentiated cells. These results are significant for the use of suspen sion-cultured cells to investigate signal transduction cascades.