High-level expression of a tobacco chitinase gene in Nicotiana sylvestris. Susceptibility of transgenic plants to Cercospora nicotianae infection.
Endochitinases are believed to be important in the biochemical defence of plants against chitin-containing fungal pathogens. A chimaeric gene containing the class I (basic) tobacco chitinase gene (constructed from genomic and cDNA clones) fused to the CaMV 35S promoter was introduced into N. sylvestris by Agrobacterium tumefaciens-mediated transformation. The gene was expressed to give mature, enzymatically active chitinase targeted to the intracellular compartment of leaves. Most transformants accumulated extremely high levels of chitinase, up to 120-fold that of non-transformed plants in comparable tissues. Unexpectedly, some transformants exhibited chitinase levels lower than in non-transformed plants, suggesting that the chimaeric gene inhibited expression of the homologous host gene. Progeny tests indicated that this effect was not permanent. High levels of chitinase in transformants did not substantially increase resistance to the chitin-containing fungus C. nicotianae, suggesting that class I chitinase is not a limiting factor in the defence reaction to this pathogen.