Protease Involvement and Ethylene Biosynthesis in PCD

In plant organ senescence, protease action has featured prominently for at least two reasons. One is the well-studied nitrogen redistribution that occurs between the source (leaf tissue) and the sink organs (younger leaves, root, fruit). The second is the general protein turnover that allows dynamic degradation of proteins that accumulate during growth concomitant with the synthesis of new, senescence-induced proteins (see Nooden and Leopold, 1988). In several instances, ethylene has been shown to induce senescence-related proteases. Proteases, particularly caspases/metacaspases, play a crucial role in animal PCD. In light of these observations, it is expected that proteases also regulate plant PCD.

What is known about protease action and ethylene during pathogen- and stress-induced PCD? Anderson et al. (1982) showed that a fungal cell wall hydrolytic enzyme, cellulysin, induces ethylene biosynthesis in tobacco leaves. Interestingly, much before plant PCD was described, these investigators showed that phenylmethanesulfonic sulfate (PMSF) and soybean trypsin inhibitor, but not pepstatin A (an inhibitor of carboxyl proteases), markedly inhibited ethylene production and ACC formation in cellulysin-treated tobacco leaf discs (Anderson et al., 1982) and wound-induced increase in ACC synthase activity in tomato fruit slices (Mattoo and Anderson, 1984). These authors proposed that specific proteolytic activity in vivo is associated with the ethylene induction processes, possibly with the activation and/or inactivation of ACC synthase, or with the induction signal itself (Mattoo and Anderson, 1984). Such an activation mechanism was later shown to cause activation of mammalian caspases (Nicholson, 1999; Grutter, 2000). Groover and Jones (1999) also have shown that soybean trypsin inhibitor inhibits protease activity and plant PCD. Like in the case of cellulysin-induced ethylene production, studies with elicitin-induced hypersensitive response (HR) have implicated Ser proteases (Beers et al., 2000). Cysteine protease activity has been implicated in cell death caused by oxidative stress of soybean cells (Solomon et al., 1999).

Both ethylene and proteases have been implicated in plant PCD. Which comes first, ethylene or the protease? From the results presented above it would seem that protease induction is one of the first signaling events followed by other factors, one of which could be mediated by ethylene. Thus, plant PCD induction may require only a small threshold level of ethylene, but it would not take place unless the related protease is activated.

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