Elicitor signal transduction leading to production of plant secondary metabolites.

SUMMARY
Many secondary metabolites found in plants have a role in defence against herbivores, pests and pathogens. In this review, a few examples are described and discussed, and some of the problems in determining the precise role(s) of such metabolites highlighted. The role of secondary metabolites in defence may involve deterrence/anti-feedant activity, toxicity or acting as precursors to physical defence systems. Many specialist herbivores and pathogens do not merely circumvent the deterrent or toxic effects of secondary metabolites but actually utilize these compounds as either host recognition cues or nutrients (or both). This is true of both cyanogenic glucosides and glucosinolates, which art discussed in detail as examples of defensive compounds. Their biochemistry is compared and contrasted. An enormous variety of secondary metabolites are derived from shikimic acid or aromatic amino acids, many of which have important roles in defence mechanisms. Several classes of secondary products are ‘induced’ by infection, wounding or herbivory, and examples of these are given. Genetic variation in the speed and extent of such induction may account, at least in part, for the difference between resistant and susceptible varieties. Both salicylates and jasmonates have been implicated as signals in such responses and in many other physiological processes, though their prescise roles and interactions in signalling and development are not fully understood.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.1994.tb02968.x

Secondary metabolites in plant defence mechanisms

Antibiotic phenols have been found in all plants investigated to date. Some occur constitutively and are thought to function as preformed inhibitors associated with nonhost resistance (84, 94, 128, 134). Others, which are the subject of this review, are formed in response to the ingress of pathogens, and their appearance is considered as part of an active defense response. Since the first suggestions that phenolic intermediates have a role in the active expres­ sion of resistance, an underlying problem in ascertaining that such secondary metabolites are of primary (rather than secondary) importance has been the localization and timing of the host response. In this review we address the problem of response localization and localization of phenolics relative to the sequential development of stages of disease that lead ultimately to resistance expression. Initial demonstrations that phenols are significant components of the host

Phenolic Compounds and Their Role in Disease Resistance

Flavonoids are widely distributed secondary metabolites with different metabolic functions in plants. The elucidation of the biosynthetic pathways, as well as their regulation by MYB, basic helix-loop-helix (bHLH), and WD40-type transcription factors, has allowed metabolic engineering of plants through the manipulation of the different final products with valuable applications. The present review describes the regulation of flavonoid biosynthesis, as well as the biological functions of flavonoids in plants, such as in defense against UV-B radiation and pathogen infection, nodulation, and pollen fertility. In addition, we discuss different strategies and achievements through the genetic engineering of flavonoid biosynthesis with implication in the industry and the combinatorial biosynthesis in microorganisms by the reconstruction of the pathway to obtain high amounts of specific compounds.

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Flavonoids: biosynthesis, biological functions, and biotechnological applications.

As plants are confined to the place where they grow, they have to develop a broad range of defence responses to cope with pathogenic infections. The oxidative burst, a rapid, transient, production of huge amounts of reactive oxygen species (ROS), is one of the earliest observable aspects of a plant's defence strategy. First this Review describes the chemistry of ROS (superoxide radical, hydrogen peroxide and hydroxyl radical). Secondly, the role of ROS in defence responses is demonstrated, and some important issues are considered, such as: (1) which of the ROS is a major building element of the oxidative burst; (2) the spatial and temporal regulation of the oxidative burst; and (3) differences in the plant's responses to biotic and abiotic elicitation. Thirdly, the relationships between the oxidative burst and other plant defence responses are indicated. These include: (1) an oxygen consumption, (2) the production of phytoalexins, (3) systemic acquired resistance, (4) immobilization of plant cell wall proteins, (5) changes in membrane permeability and ion fluxes and (6) a putative role in hypersensitive cell death. Wherever possible, the comparisons with models applicable to animal systems are presented. Finally, the question of the origin of ROS in the oxidative burst is considered, and two major hypotheses, (1) the action of NADPH oxidase system analogous to that of animal phagocytes, and (2) the pH-dependent generation of hydrogen peroxide by a cell wall peroxidase, are presented. On the basis of this material, a third 'unifying' hypothesis is presented, where transient changes in the pH of the cell wall compartment are indicated as a core phenomenon in evoking ROS production. Additionally, a germin/oxalate oxidase system which generates H2O2 in response to pathogenic infection is also described.

Oxidative burst: an early plant response to pathogen infection

Induced distal defence potentiation against Phytophthora sojae in soybean

Effects of soybean genotype on the glyceollin elicitation competency of cotyledon tissues to Phytophthora sojae glucan elicitors

Identification of soybean elicitation competency factor, CF-1, as the soybean Kunitz trypsin inhibitor

Secondary product pathways in plants are extraordinarily diverse. Although classical chemical and biochemical analyses have gone a long way in delineating the major pathways and metabolites in many plant species, no plant species has been completely characterized for all of the secondary products that it produces or is capable of producing. While examinations of plants at the metabolic level (e.g., through metabolic profiling/metabolomics) is powerful in that it provides a picture of the ultimate net accumulation of endproducts in various cells, tissues and organs, unexpected or new metabolites are often found in a given species when the plant is examined under previously unexamined conditions. A very simple example is the production of large quantities of formononetin by soybean following treatment by the disease resistance-inducing herbicide lactofen (Landini et al. 2002). While this metabolite is found at high levels in some other legumes, such as chickpea, it is normally undetectable or at trace levels in soybean. Thus, it is very difficult by analyses of metabolites per se to know all of the potential metabolites that can be produced by a plant. This inherent limitation is an aspect where genomic analyses will have particular impact in that the presence of a gene for a particular enzyme may be suggested even though the product of its action is not. Quantification of the secondary metabolites in classical biochemical or metabolomics approaches is also a challenge. While quantitative data is readily obtained from various chromatographic and spectroscopic techniques, the actual tissue concentration of individual metabolites requires the use of standards for each metabolite, something that is extremely difficult to achieve across all metabolites and tissues. Moreover, many important biological processes involve changes in metabolites in individual cells or groups of cells, which is beyond the limits of detection of most

M.Y. Graham

Papers

Induced distal defence potentiation against Phytophthora sojae in soybean

Effects of soybean genotype on the glyceollin elicitation competency of cotyledon tissues to Phytophthora sojae glucan elicitors

Identification of soybean elicitation competency factor, CF-1, as the soybean Kunitz trypsin inhibitor

Genomics of Secondary Metabolism in Soybean