Pathogenic attack

Insect and plant pathogen attacks are reduced because 1) natural enemies of the pests are increased in the polyculture ecosystem because the array of hosts for the natural enemies is increased, 2) and the movement of pests from one host plant to another is interrupted by the presence of a different host plant growing nearby (36). In this way overall pest damage is substantially reduced. 

Therefore, the ability of certain POs to bind with chitin is a widespread phenomenon and -possibly - connected with the defence reactions of the organisms to pathogen attacks. Since it was shown that some biogenic molecules - such as chitooligosaccharides or salicylic acid -can activate an anionic POs, we might suggest that an application of these compounds optimises the process of anionic PO isolation with a chitin. 

Pathogen attack The induction of LOX genes during plant-pathogen interactions has been reported in several species, suggesting that the function of LOX in the defense against pests seems to be related to the synthesis of a number of different compounds with signaling functions or antimicrobial activity. 

DITTRICH, H., KUTCHAN, T.M., Molecular cloning, expression, and induction of berberine bridge enzyme, an enzyme essential to the formation of benzophenanthridine alkaloids in the response of plants to pathogenic attack, Proc. Natl. Acad. Sci. USA, 1991,88, 9969-9973. 

CHRISTENSEN, A.B., GREGERSEN, P.L., SCHRODER, J., COLLINGE, D.B., A chalcone synthase with an unusual substrate preference is expressed in barley leaves in response to UV light and pathogen attack, Plant Mol. Biol., 1998,37, 849-857. 

At least two specific properties make the immune system vulnerable to chemical or physical insults (1) the immune system develops rather late in life (thymus development lasts at least until puberty), and some bone marrow-dependent immune components are continuously renewed (i.e., granulocytes), and (2) each pathogen attack, as well as immune surveillance, demands a delicate control of the balance between activation, silencing, and regulation of immune reactivity. 

Effect of pathogens on tree carbohydrate concentration and relationship of carbohydrate concentrations to pathogen attack 24b Effect of path< ns on water uptake and transpiration 24c Relation ip between stem and root characteristics and pathogen attack 2Sa Effect of soil moisture and temperature on water uptake and transpiration 2Sb Effect of soil mineral concentration and temperature on mineral nutrition of... 

Pathogen Attack Phytotoxins and Elicitors from Cruciferous Fungi... 

Indole-3-acetaldoxime (30) and 4-hydroxyphenylacetaldoxime were shown to be metabolized by plant and pest fungi to 32 and to other related indole- and 4-hydroxyphenyl carboxylic acids, however, the biochemical transformation differed between the two fungi. These biochemical transformations may be relevant to the ability of certain fungi to cause plant diseases . Compound 30 has been shown to be a key intermediate in the biosynthesis of camalexin, 3-thiazolyl-2 -yl-indole, a member of the family of phytoalexins that are produced in response to pathogen attack. It was demonstrated that CYP71A13 catalyzes the conversion of 30 to indole-3-acetonitrile, which is essential for the biosynthesis of camalexin. Thus, the literature supplies a large body of evidence for the existence of a set of enzymes dedicated to the production of the auxin, 32, from tryptophan via indole-3-acetaldoxime (30). 

Welle, R. et al.. Induced plant responses to pathogen attack. Analysis and heterologous expression of the key enzyme in the biosynthesis of phytoalexins in soybean Glycine max L. Merr. cv. Harosoy 63). Eur. J. Biochem., 196, 423, 1991. 

Shorter-term temporal variation in leaf quality should act to complicate the spatial arrays described above. Thus, not only may a foraging insect have difficulty locating suitable feeding sites in space, but their locations may shift from time to time or continuously, as seasonal changes, induction effects, or even plant pathogen attack (44) alter tissue quality. A suitable tissue at one time may not be suitable later in the day, or later in the insect s life. 

Many aspects of nutrient-allelochemical interactions are probably key factors In the suitability of a given plant species as a host for a particular Insect. At best, this may be a less than optimal situation for the insect, since even what appears to be a susceptible plant Is likely to be fairly well defended against Insect and pathogen attack. If the plant contains the essential nutrients for the Insect, but the utilization of these nutrients Is blocked In some way by allelochemlcs or by too much or too little water, then growth may be slowed. If, due to behavioral modifiers, the Insect will not feed on the plant, then plant nutrients are not available to the Insect. 

However, most of the literature available relates phenolic compounds to resistance against the following types of stress (i) pathogen attack, (ii) wounds, (iii) ultraviolet radiation, (iv) environmental pollution (specially ozone), and finally (v) thermal stress. Due to the importance and incidence of these types of stress for the survival and adaptation of plants, this topic will be explored in more detail in the forthcoming sections. 

Plants have envolved a wide array of chemical defences against pathogens. These include secondary metabolites with anti-microbial properties [57]. Some secondary metabolites are constitutively present in normally developed healthy plants, whereas others are induced by pathogen invasion [58]. The latter include phytoalexins, which are synthesized de novo after a pathogen attack [43], and phytoanticipins [59]. 

Given the scope of the processes in which phenolic compounds are involved (e.g. interactions with plant hormones, resistance to pathogen attack, wounds, UV rays, ozone and thermal stress, implications for agriculture), knowledge of the factors that regulate the metabolism of these compounds could enable the manipulation of their synthesis or degradation, depending on the conditions chosen or the results desired. 

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