Phytoalexin response

Induced lignification is blocked by treatments which inhibit protein synthesis (10), and it seems likely therefore that the increases in activities observed for enzymes involved in lignin biosynthesis (7,8) depend on enhanced translation, and probably enhanced transcription of the appropriate genes, as has been shown for the phytoalexin response of some plants (27). However, the molecular biology of the response in wheat awaits investigation. 

Dillon, V.M. et al., Differences in phytoalexin response among rice cultivars resistance to blast. Phytochemistry, 44, 599, 1997. 

Dixon R. A. The phytoalexin response elicitation, signaling and control of host gene expression. Biol Rev 1986 61 239-291. 

Whether acetylenes will be uncovered In other solanaceous plant8 remains to be seen. Tobacco has now been so thoroughly Investigated that It is unlikely that acetylenes have yet to be discovered In Its phytoalexin response. 

The results to date indicate that phytoalexin synthesis is a common mechanism of disease resistance in higher plants Whether all plants will eventually yield positive results in this bloassay remains to be seen. It is our experience that there are still technical difficulties in establishing phytoalexin production in many plants and further development of appropriate methodology is essential The separation of the phytoalexin response from the more general stress response of plants is also not easy and this needs to be considered in future experimental design. 

The phytoalexin response of callus cultures derived from Ladino clover to various substances Is shown In Table III. Most of the ellcltors listed for the callus system have also shown ellcltor activity in other plant phytoalexin-producing systems. However, most of the abiotic and biotic non-ellcltors for the callus system (Table III) are ellcltors In whole plant systems (see 1 and references therein). 

The data summarized here provide preliminary evidence for regulation of phytoalexin accumulation by thiol and/or disulfide groups In the interior of the plasma membrane. A hypothesis Is proposed oxidation state of the membrane SH groups regulating the phytoalexin response Is maintained by biological thiol/ disulfide ratios in the cytoplasm and by accessibility of these compounds to the membrane SH moieties. 

Coevolutionary biochemical interactions of plants with their natural predators could contribute to uncover medicinally useful chemicals such as antioxidants [15]. For example, many flavonoids possess allelochemical properties by warding off microbial or animal pathogens as phytoalexins or as antifeedants, and the phytoalexin response is frequently accompanied by an oxidative burst in the cell [16]. Moreover, it has been demonstrated that root flavonoids, mainly flavones, isoflavones and some... 

Minimal processing of carrots induces phenylalanine ammonia lyase (PAL) activity and phenolics accumulation. In shredded carrots, chlorogenic acid, which is rapidly accumulated, represents 60% of the total phenolics. In addition traces of 3 -caffeoylquinic and 4 -caffeoylquinic acids are biosynthesised, and 3, 4 -dicaffeoylquinic and 3, 5 -dicaffeoyl quinic acids also accumulate [44]. p-Hydroxybenzoic acid derivatives are also biosynthesised but more slowly, and are related to defence against microbial attack (phytoalexin response), the degree and speed of which depends on the cultivar [45],... 

Elicitors of the Phytoalexin Response. The production of phytoalexins can be elicited not only by living organisms, but also by many chemical compounds and stress situations (27,46,47). Hg" (48) and Cu (49) ions, fungicides (50,51), and polyamines (52) are some of the different chemical elicitors of phytoalexins. Physical stresses such as cold (53) and UV light (54,55) also stimulate the phytoalexin response. Even though these different stresses may act at different sites and in different ways, the result is always a dramatic change in metabolism of the susceptible plants. 

To establish that the onset of phytoalexin synthesis is both concentration and time dependent is very important. This Indeed would mimic phytoalexin responses in other plants. 

Table 3. Summary of Phytoalexin Response in Treated Celery (57)...

The observations that the phytoalexins phaseollin (92) and pisastin (93) have injurious effects on plant plasma membranes suggests that wilting observed in some plant diseases could be an indirect result of phytoalexin production. Whether this occurs in the case of 3-glucan-induced wilting of Eucalyptus species is not known since phytoalexins have not been reported from eucalypts. However the dose of polysaccharide needed to produce the maximum phytoalexin response in the soya bean hypocotyl test systems (84) is an order of magnitude lower than that required for wilt-induction in the seedling assays (50). 

However, depolymerisation to an inactive form might have the advantage of localising the phytoalexin response (98) and other biological effects such as the hypersensitive reaction to the immediate region of the attack. Enzymic depolymerisation of wilt-inducing 3,6-3-glucans would presumably be a useful part of the plant s resistance mechanism. 

An interesting question is whether the observed synthesis of a given compound is natural , or whether it occurs in response to variable external challenges. For example, the synthesis of poly acetylenic compounds can be induced in cell cultures of Bidens pilosa by fungal culture filtrates (93). The same effect has also been observed in B. sulphureus and in unspecified Tagetes cultures (105), The extent to which some of the presumed normal thiophene constituents of plants may actually be the result of a phytoalexin response is not known. 

Mansfield, J.W., Porter, A.E.A. and SmaEman, R.V. (1980) Dihydrowyerone derivatives as components of the furanoscetylenic phytoalexin response of tissues of Yicia faba. Phytochemistry, 19,... 

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