Plant defense reactions

Products released by the action of PL have previously been reported to act as elicitors of plant defense reactions (24,25,26,27). Accordingly, the transgenic plants described in this report provides an excellent mutant collection for the study of factors conferring resistance against Envinia carotovora bacteria. 

Colonization of barley, wheat and maize and rice roots by Glomus intraradices resulted in strong induction of transcript levels of the pivotal enzymes of methylerythritol phosphate pathway of isoprenoid biosynthes i.e., 1 -deoxy-D-xylulose 5-phosphate synthase (DXS) and 1 -deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) (Walter et al., 2000). At the same time six cyclohexenone derivatives were characterized from mycorrhizal wheat and maize roots. DXS2 transcript levels are low in most tissues but are strongly stimulated in roots upon colonization by mycorrhizal fungi, correlated with accumulation of carotenoids and apocarotenoids (Walter et al., 2002). Some reports show that the AM symbiosis may cause an increase, decrease, or no change in the plant defense reactions (Guenoune et al., 2001 Mohr et al., 1998). 

The enzymes most commonly studied In plant defense reactions can be divided Into two major groups (lytic enzymes and enzymes associated with plant phenolic metabolism) and a lesser third group of generally unrelated enzymes (Table T). Discussion of some of the salient Information regarding the roles and action of these enzymes when plants are Infected by pathogens follows. Instances where these enzymes are thought not to play roles In plant defense are also presented. 

As described above, Ja and MeJa act as key compounds of the signal transduction pathway involved in the induction of the secondary metabolite biosynthesis which takes part in plant defense reactions [56, 57]. Thus, the production of secondary metabolites increases when plant cell cultures are elicited with jasmonates [36, 49, 58]. In Vitis, trans-R and stilbene-related production is increased by adding Ja or MeJa [30, 54]. In this respect, Tassoni et al. [54] showed that the addition of 10 pM MeJa was more effective in stimulating endogenous accumulation (24 pg g ... 

On the other hand, SA has long been known to play a central role in plant defense reactions. SA levels increase in plant tissue following pathogen infection, and... 

Horseradish peroxidase (HRP) is an extracellular plant enzyme that acts in regulation of cell growth and differentiation, polymerization of cell wall components, and the oxidation of secondary metabolites essential for important pathogenic defense reactions. Because of these essential functions, and also because of its stability and ready availability, HRP has attracted considerable attention.13 It has been involved in a number of applications, such as diagnostic assays,14 biosensors,15 bioremediation,16 polymer synthesis,17 and other biotechnological processes.18 More applications in which HRP catalysis is translated into an electrochemical signal are likely to be developed in the near future. 

An epidemiological survey of complementary symptoms, reminiscent of the HR of terrestrial resistant plants, such as tip bleaching or accumulation of fluorescent aromatic compounds around the site of penetration of parasite propagules (Figs. 12.2 and 12.3), may facilitate the evaluation of the impact of host-microbe interactions in natural populations (Correa and Sanchez 1996 Ellertsdottir and Peters 1997 Bouarab et al. 2001a). This could allow one to test the importance of oxida-tive-burst-associated defense reactions in various biotic interactions. 

PKSs are characterized by their ability to catalyze the formation of polyketide chains from the sequential condensation of acetate units from malonate thioesters. In plants they produce a range of natural products with varied in vivo and pharmacological properties. PKSs of particular note include acridone synthase, bibenzyl synthase, 2-pyrone synthase, and stilbene synthase (STS). STS forms resveratrol, a plant defense compound of much interest with regard to human health. STS shares high sequence identity with CHS, and is considered to have evolved from CHS more than once. ° Knowledge of the molecular structure of the CHS-like enzymes has allowed direct engineering of CHS and STS to alter their catalytic activities, including the number of condensations carried out (reviewed in Refs. 46, 51, 52). These reviews also present extensive, and superbly illustrated, discussions of CHS enzyme structure and reaction mechanism. 

Farmer, 2001). Even nectar production may be effected by such hormones (Heil et al, 2001). The gaseous hormone ethylene plays an important role in plant development, but also in defense (Mattoo and Suttle, 1991). Upon perception of a pathogen, plants show enhanced ethylene production, which has been shown to be involved in the induction of defense reactions (Boiler, 1991). Wild tobacco plants engineered with an Arabidopsis sp. ethylene-insensitive gene do not show typical leaf development arrestment in the presence of leaves of other tobacco plants, demonstrating the importance of ethylene in plant development (Knoester et al.,... 

Plants synthesize l,3-(3-glucanases that hydrolyze the glycans of fungal cell walls. Synthesis is induced by wounding as a defense reaction (see Box 20-E). These glycanases also function in the removal of callose.146... 

Despite the act that the new compounds have been known and available for years, relatively little is known about their mode of action. Table VI reflects the state of the art. The lack of information is particularly surprising in the case of fosetyl and metalaxyl as far as their indirect effect is concerned, that is, the stimulation of the host plant s defense reactions (e.g. formation of phytoalexins). It is for the first time in the history of fungicides that such effects have been reported. 

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