Induction of chitinases in tobacco plants systematically protected against blue mold by Peronospora tabacina or tobacco mosaic virus.

Direct detection of chitinases after polyacrylamide gel electrophoresis (PAGE) run with an anodic buffer system revealed eight chitinase isozymes in tobacco, designated as C1-C8. Inoculation of three to four lower leaves of tobacco plants with tobacco mosaic virus (TMV) or stem injection of tobacco with sporangiospores of Peronospora tabacina systemically protected plants against blue mold caused by P. tabacina and systemically elevated the activities of C5 and C6 in the protected plants to about seven times higher than those in controls. On subsequent challenge with P. tabacina, the activities of C5 and C6 in the systemically protected plants were further enhanced and were about 10 times higher than those in the controls at 2 days after challenge. The activities of C5 and C6 were markedly elevated in the control plants when symptoms started to appear four days after challenge. However, the activities of C1-C4, C7, and C8 were constant and similar in both the protected and control plants before and after challenge with P. tabacina. Experiments with protoplasts and intercellular washing fluid indicated that C1-C3, C5, and C6 were predominantly located in intercellular spaces, C4 was present in both intracellular and intercellular spaces, and C7 and C8 were uniformly distributed inside and outside of tobacco cells. The data support the suggestion that the induction of chitinases may be considered as one of the general metabolic responses participating in resistance, each of which is nonselectively induced and effective against many but not all pathogens. The coordinate expression of many different resistance mechanisms can restrict development of diverse pathogens and appears to be responsible for both induced resistance of susceptible plants and resistance of noninduced resistant plants. Both kinds of resistance are determined by the regulation of the rapidity and magnitude of gene expression for resistance mechanisms. Although susceptible plants lack gene(s) responsible for the rapid recognition of specific pathogens, they have genes for resistance mechanisms and gene(s) for their regulation. Thus resistance can be induced in susceptible plants by affecting the gene(s) regulating the genes for resistance mechanisms.