Phytoalexins generation

Phytoalexin production is not an immediate response of plants to attack of pathogens or mechanical injury. Phytoalexins begin to appear about 6 hours after attack by microorganisms [95] and also exposure to light [96] and accumulate to maximum levels 20-30 hours later [96]. In contrast 10 min after treatment with UV radiation phytoalexin production was observed [97], indicating that the phytoalexin generation is dependent on the kind of elicitor and probably on the extend of... 

The 2,6-dioxygenated carbazole alkaloid glycozolidine (170) was isolated first by Chakraborty et al. from the root bark of G. pentaphylla in 1966 [182]. Glyco-zolidal (171) and glycozolidol (175) were obtained from the same natural source by Bhattacharyya et al. [183, 184]. Carbalexin C (179) represents a stress-induced phytoalexin generated in the leaves of G. pentaphylla and Glycosmis parviflora... 

Pedras MSC, Zheng QA, Shatte G, Adio AM (2009) Photochemical dimerization of wasalexins in UV-irradiated Thellungiella halophila and in vitro generates unique cruciferous phytoalexins. Phytochemistry 70 2010-2016... 

A further example of the effectiveness of these reagents is demonstrated in the synthesis of the complex spiroventivane phytoalexin-lubiminol 3 (Scheme 1.2).4 Generation in situ of a zinc homoenolate, Et02C(CH2)2ZnCl,3 allows formation of the functionalized cyclopentenone intermediate, essential for the synthesis. 

Lubimin, a phytoalexin found in several Solanaceae, has been assigned the spiro-structure (57) on n.m.r. and biosynthetic evidence,83 whilst the new sesquiterpenoid skeleton of taylorione (58) from the liverwort Mylia taylorii is thought84 to be biosynthesized via an aromadendrene-type precursor which may also generate the co-occurring myliol (59). 

Immunization of cucumbers by (L lagenarium, C. cucumerinum, P. 1achrymans or TNV generates a systemic increase in peroxidase activities (. TJ, ] 9, 8U) > Like 1 i gni f ic a t ion and phytoalexin induction, peroxidase activities also rise more quickly in response to infection in leaves of immunized plants, even though total activity eventually may be highest in infected susceptible leaves (77). Several other stimuli can induce local (mechanical and chemical injury) or systemic (senescence, ethylene) peroxidase increases that are not accompanied by increased disease resistance. Thus, enhanced peroxidase activity per se may not be a defense mechanism, but may be a necessary adjunct with appropriate chemical substrates for processes important in disease resistance, e.g., lignification, suberization, and me 1anization. 

The compound for which the best biochemical evidence has been reported for sensitization of host plant response to pathogens, rather than direct or indirect fungitoxicity or nonspecific phytoalexin induction, is 2,2-d ich 1 oro-3,3-d ime thy lcyc 1 opropane carboxylic acid (108-110). Neither the compound nor any of its metabolites generated after treatment of rice plants are directly inhibitory to the rice blast fungus, Piricularia oryzae. Constitutive phytoalexin production was not induced in rice by the cyclopropane derivative. However, infection of plants treated with the compound results in rapid localized cell death, me lanization, and production of the phytoalexins, momilactones A and B. 

Chemical factors are also involved in the resistance of plants to disease and in the competitive ability of a plant to survive within a community of plants. Plant stress may also generate a chemical response giving rise to compounds known as the phytoalexins, the nature of which will depend on the chemistry of the host plant (18, 19). Such response to injury or infection is of great Interest because it has stimulated investigations of the nature of the bloregulatory processes involved. 

The reactions leading to the induction and accumulation of phytoalexins with phenolic structures have been studied in molecular detail (4,17,22-24). These studies revealed that plants can detect and react rapidly to environmental problems, such as wounding or infection Within 20 min of elicitation, mRNAs coding for enzymes that catalyze the reactions leading to the respective defense compounds are increasingly generated, leading to the accumulation of the respective enzymes and consequently the production of the secondary metabolites (4,17,22-24). Similar processes are likely for alkaloids, but so far the mechanisms have not been elucidated. 

NQOl is a homodimer with a flavodoxin fold (5). This enzyme does not stabilize the semiquinone state. The obligate two-electron transfer mechanism prevents the generation of quinone radicals and redox cycling, which would result in oxidative stress. The NADPH and quinone substrates occupy the same site, consistent with the observed ping-pong bi-bi mechanism. NQOl is inhibited by many (poly)aromatic compounds including the anticoagulant dicoumarol and the phytoalexin resveratrol (5). 

Development of ethylene [64] starts about 40-60 min after mechanical perturbation, much earlier than generation of salicylic acid [53]. This perturbation is induced, for instance, just by wind [30], explaining the observation that the rice blast disease (induced by pathogens) is suppressed in seasons of strong wind (apparently the perturbation induces expression of ethylene, this stimulates generation of phytoalexins which prevent attack by fungi). 

Acetolactate synthase, 25-26 Active insectiddal ingredient, 292 Active oxygen-generating herbitides, 19r Acylation, 252 Affinity concentration, 358 Aflatoxin contamination kernel moisture, 79-81 role of phytoalexins, 73-81... 

The most studied aspect of phytoalexin elicitation is that generated by fungal pathogens of plants. Karban et al.[22] found induced resistance and interspecific competition between spider mites and a vascular wilt fungus. McIntyre et al. [2A] demonstrated that 7 days after tobacco plants were infected with tobacco mosaic virus, reproduction of the green peach aphid, Myzus persicae, was significantly reduced on these infected plants. Inoculation of plants with tobacco mosaic virus induced resistance to several pathogens, however the mechanism for induced resistance was not characterized. 

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