Proteinase inhibitor I accumulation in tomato suspension cultures : induction by plant and fungal cell wall fragments and an extracellular polysaccharide secreted into the medium.

Suspension-cultured cells of tomato accumulate proteinase Inhibitor I as the sucrose is depleted from 1% to less than 0.1% in the culture medium. Inhibitor I can be prematurely induced to accumulate in the cells by the addition to the medium of the proteinase inhibitor inducing factor, trigalacturonic acid, ethylene glycol chitin, or chitosan. In cultures grown in 0.6% initial sucrose with no inducers added, a uronic acid-rich extracellular polysaccharide appears in the medium during growth of the cells. This extracellular polysaccharide apparently contains an ;endogenous inducer' of Inhibitor I synthesis. When the partially purified polysaccharide is added to the culture medium, Inhibitor I accumulation is induced. Proteinase inhibitors also accumulate in tobacco and alfalfa suspension-cultured cells as the cell cultures age. As with the tomato cultures, a uronic acid-rich component(s) appears in the media prior to inhibitor accumulation. These data suggest that an endogenous inducer may be activating proteinase inhibitor genes through a similar mechanism in all three types of cells.