The clearest evidence for the role of serine PIs in the defense of plants against insect pests is the induced synthesis of these proteins that occurs when many plant species are wounded, which can be caused by insect feeding, or mimicked by mechanical damage. The wound response in plants has been extensively investigated over recent years. Wounding has been shown to result in a variety of changes in the physiological state of the tissue, and can either result in a local reaction when it is restricted to a defined region in the near vicinity of the original wound such as in the case of the production of phytoalexins, or a systemic reaction, when the response occurs in tissues some distance from the wound site.
One of the best studied cases of a systemic reaction induced in higher plants was the demonstration that wounding of the leaves of either potato or tomato plants by adult Colorado potato beetles or their larvae induced a rapid accumulation of serine PIs.16 This accumulation was not confined to just wounded leaves, but was also found to be present in leaves which had not been attacked. Mechanical damage was also found to cause an accumulation of these inducible PIs.17 Interestingly, it has recently been shown that the timing of transcript accumulation of several wound-induced genes in insect-damaged leaves is different to that in mechanically damaged leaves, with transcripts for the inhibitor PI-II accumulating more rapidly in potato leaves which had been damaged by insects than those damaged mechanically. It would appear that insect regurgitant, in this case from Manduca sexta larvae, causes the transcript accumulation profiles to shift to parallel those in insect damaged tissue. On the basis of their findings, Korth and Dixon18 suggested the presence of a heat-stable, insect derived elicitor which functions to induce the rapid accumulation of transcripts that may be involved in plant defense against herbivores.
A factor, or wound hormone, called the PI-inducing factor (PIIF) was found to be released from the damaged leaves and transported to other leaves within 2-3 hours after wounding where it initiates synthesis and accumulation of the two potato serine PIs, PI-I and PI-II.17 Transport appears to take place in the phloem and is directed predomi nantly upwards to younger leaves,19 although evidence for hydraulic signals in the xylem and electrical signals have also been presented. The levels of PIIF released in response to wounding depends upon both the severity and location of the wound. Subsequent work revealed that the PIIF was a highly methylated polysaccharide containing galacturonic acid, rhamnose, galactose, arabinose and fucose, and that it was probably a fragment of the plant cell wall, at least in tomato.20 Within two to three days after attack the inhibitors can account for over 10% of the soluble proteins in leaves throughout the plant, where they can remain for long periods of time stored in the central vacuoles of the cells. , The accumulation requires light and is temperature-dependent.
These wound-induced inhibitors have been well characterized. Both PI-I and PI-II are potent inhibitors of chymotrypsin and subtilisin, and both inhibit trypsin, but less strongly. More recently, a wound-induced trypsin inhibitor has been isolated and characterized from alfalfa leaves. This particular inhibitor was identified as a member of the Bowman-Birk inhibitor family.23 Following these initial reports, genes encoding the wound-inducible inhibitors from both potato24 and tomato25 have been isolated and characterized. Transformation of tobacco plants with a gene encoding potato PI-II resulted in a systemic induction of the transgene expression after wounding,26 showing that the signal inducing PI gene expression was similar in the two species.
Several plant-derived chemicals, including methyl jasmonate, jasmonic acid4 and an 18-amino acid polypeptide called systemin were found to regulate the expression of wound-inducible PI genes. Growth regulators such as abscisic acid and auxin were similarly shown to regulate expression of these genes. A currently accepted model for the expression of wound-inducible PIs is that systemin is released by wounding and activates a membrane-derived lipid signal. This signal is thought to be linolenic acid, which is subsequently converted to jasmonic acid4 via the octadecanoid pathway.27 Further informa-
tion can be found in a recent review by Bergey et al. A signal cascade then leads to the activation of jasmonate-responsive genes (jrgs), and their products are responsible for the physiological response. The involvement of systemin in the wound response and subsequent accumulation of wound-inducible PIs has been elegantly demonstrated by exposing transgenic tomato plants expressing an antisense prosystemin gene to lepidopteran larvae. The plants containing the antisense construct were significantly more attacked than the corresponding control plants, with the insects on the transgenic plants being significantly larger, due to suppression of the normal wounding response.29
An alternative model for jasmonate signalling in barley has been put forward recently, whereby there are several distinct signal-transduction pathways. Contrary to what occurs in tomato and potato, in barley the abscisic acid signal is not mediated through a jasmonate signal cascade, but is independent of it. In the model put forward by Lobler and Lee,30 induction of jrgs 9 (a "wounding-induced" gene) is caused by the presence of products resulting from activation of expression of gene families, some of which are individually induced by either abscisic acid or jasmonate. Jasmonate signalling is mediated through two pathways, one sensing extracellular jasmonate and the other sensing intracellular jasmonate. It has also been suggested that two distinct signal transduction pathways that can distinguish between insect damage and abiotic damage may be involved in the wound-induced responses in potato.18
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