INDUCTION OF HYPERSENSITIVE CELL-DEATH BY A FUNGAL PROTEIN IN CULTURES OF TOBACCO CELLS

Treatment of suspension-cultured tobacco (Nicotiana tabacum cv. Xanthi ) cells (Line XD6S) with fungal proteinaceous elicitors, namely, xylan ase (EC 3.2.1.8) from Trichoderma viride (TvX) and xylanase from T. re esei (TrX), induced shrinkage of the cytoplasm, condensation of the nu cleus, and, finally, cell death, which were accompanied by typical def ense responses that included an oxidative burst and expression of defe nse genes. A Ca2+ channel blocker, Gd3+, inhibited the Epical response of XD6S cells to TvX, which resembled the hypersensitive reaction (HR ). These results suggested that the influx of Ca2+ ions plays an impor tant role as a secondary signal. The HR was not observed in TvX-treate d tobacco cells (line BY-2) derived from cv. Bright Yellow 2. This res ult suggests that key features of cultivar-specific interaction can be observed in cultures of tobacco cells. Xylanase from Bacillus circula ns (BcX) and B. subtilis (BsX), which has enzymatic properties similar to those of TvX but an amino acid sequence different from that of TvX , did not induce the HR-like response in XD6S cells. These results sug gest that the elicitor action of TvX is not due to its ability to hydr olyze cell walls but requires the TvX-specific recognition factors in plant cells. Thus, TvX-induced cell death was not due to some general toxic effect, but seems to be mediated by the activation of a specific cellular signal-transduction cascade that converges with a pathway th at activates the intracellular cell death program.