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

▪ Abstract Glutathione S-transferases (GSTs) play roles in both normal cellular metabolism as well as in the detoxification of a wide variety of xenobiotic compounds, and they have been intensively studied with regard to herbicide detoxification in plants. A newly discovered plant GST subclass has been implicated in numerous stress responses, including those arising from pathogen attack, oxidative stress, and heavy-metal toxicity. In addition, plant GSTs play a role in the cellular response to auxins and during the normal metabolism of plant secondary products like anthocyanins and cinnamic acid. This review presents the current knowledge about the functions of GSTs in regard to both herbicides and endogenous substrates. The catalytic mechanism of GST activity as well as the fate of glutathione S-conjugates are reviewed. Finally, a summary of what is known about the gene structure and regulation of plant GSTs is presented.

The functions and regulation of glutathione s-transferases in plants.

One of the best and longest-studied defense response of plants to infection is the induced accumulation of antimicrobial, low-molecular-weight secondary metabolites known as phytoalexins. Since the phytoalexin hypothesis was first proposed in 1940, a role for these compounds in defense has been revealed through several experimental approaches. Support has come, for example, through studies on the rate of phytoalexins in relation to cessation of pathogen development, quantification of phytoalexins at the infection site, and relationship of pathogen virulence to the phytoalexin tolerance. Evidence in support of phytoalexins in resistance as well some recent advances in phytoalexin biosynthesis are reviewed. Criteria for evaluating a role for phytoalexins in disease resistance are also discussed.

PHYTOALEXINS: What Have We Learned After 60 Years?

Chalcone synthase (CHS, EC 2.3.1.74) is a key enzyme of the flavonoid/isoflavonoid biosynthesis pathway. Besides being part of the plant developmental program the CHS gene expression is induced in plants under stress conditions such as UV light, bacterial or fungal infection. CHS expression causes accumulation of flavonoid and isoflavonoid phytoalexins and is involved in the salicylic acid defense pathway. This review will discuss CHS and its function in plant resistance.

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Chalcone synthase and its functions in plant resistance

Anthocyanins in common foods in the United States, other than fruits and berries, were identified and characterized by high-performance liquid chromatography (HPLC)-electrospray ionization-tandem mass spectrometry coupled with diode array detection. Of all of the 40+ vegetables, nuts, and grains screened, seven vegetables, one nut, and one grain were found to contain anthocyanins; the number of anthocyanins detected varied from two in pistachio nuts to 34 in red radishes. The individual anthocyanins were identified by comparing their mass spectrometric data and retention times with those of standards, published data, and reference food samples. In all of the samples analyzed, except for sorghum, only six common anthocyanidins (delphinidin, cyanidin, pelargonidin, petunidin, peonidin, and malvidin) were found as their glycosides. Anthocyanins in certain vegetables such as red cabbage and red radish were highly conjugated with sugars and acylated groups, and thus, their structures were very complicated. Eight different either aliphatic or aromatic acylated groups (acetoyl, coumaroyl, malonoyl, p-hydroxybenzoyl, feruoyl, caffeoyl, sinapoyl, and oxaloyl) were identified in the anthocyanins. In addition to glucose, six other sugar moieties (galactose, xylose, rhamnose, rutinose, sambubiose, and laminaribiose) were observed. Three varieties of sorghum were found to contain 3-deoxyanthocyanidins and their derivatives as major anthocyanins. A number of new anthocyanins were identified in the foods studied. This paper presents complete HPLC profiles and MS spectrometric data, obtained under the same experimental conditions, for common vegetables, pistachio nuts, and sorghum that contain anthocyanins.

Identification and Characterization of Anthocyanins by High-Performance Liquid Chromatography−Electrospray Ionization−Tandem Mass Spectrometry in Common Foods in the United States: Vegetables, Nuts, and Grains

Infection of the sorghum mesocotyl by Helminthosporium maydis (a nonpathogen) and Colletotrichum graminicola (a pathogen) resulted in the rapid accumulation of a pigment complex by two sorghum cultivars. The components of the complex were fungitoxic. The principal compounds have been identified as the 3-deoxyanthocyanidins apigeninidin and luteolinidin. Apigeninidin accumulated in both sorghum cultivars in response to infection and was the predominant pigment. Luteolinidin accumulated in only one of the cultivars. Because of the speed of synthesis, occurrence only in response to inoculation, and fungitoxicity of the individual components, we propose that synthesis of the pigment complex constitutes a defense response and that the compounds apigeninidin and luteolinidin should be considered as phytoalexins.

https://www.pnas.org/content/pnas/84/16/5520.full.pdf

Sharon S. Kollipara

Papers

Phytoalexin synthesis by the sorghum mesocotyl in response to infection by pathogenic and nonpathogenic fungi