Studies on the Induction of Chitinase and Disease Resistance in Cucumber Plants

Abstract

Necrotizing pathogens induce chitinase and resistance against Colletotrichum lagenarium in cucumber plants (1). This response can be mimicked by chemicals such as K3PO4 (2) and salicylic acid (1,3). Endogenous salicylic acid levels in cucumber plants raise after infection with necrotizing pathogens providing evidence that salicylic acid may act as a signal for the induction of the defense response (1). Signals may also be derived from the cell wall of pathogenic fungi. Plant chitinases are known to inhibit fungal growth (4) but also may release chitin oligosaccharides from fungal cell walls. Chitin fragments induce chitinases in melon plants (S) and yam calli (6). Here we examined the effects of chitin fragments on induction of chitinase and pathogen resistance in cucumber. Chitin oligosaccharides of different chain length were prepared. Samples (10ug–100ug) of chitotriose and a mixture of higher oligosaccharides (predominantly chitopentaose) were injected into the first leaf of 3 week-old cucumber plants. To look for endogenous signals of the resistance response, we sprayed the first leaf with a solution of K3PO4 (100mM), a treatment which induces chitinase and pathogen resistance. After 7 days intercellular fluid (ICF) of the first leaf was extracted. Proteins were precipitated with 80% v/v ET-OH and the supernatant freeze dried. The dried material (corresponding to ICF from one leaf) was dissolved in H2O and injected into the first leaf. One half of the leaf was harvested within 8 days and assayed for chitinase activity. The other was inoculated with C. lagenarium.