Elicitor signal transduction leading to production of plant secondary metabolites.

The flavonoid pathway produces a diverse array of plant compounds with functions in UV protection, as antioxidants, pigments, auxin transport regulators, defence compounds against pathogens and during signalling in symbiosis This review highlights some of the known function of flavonoids in the rhizosphere, in particular for the interaction of roots with microorganisms Depending on their structure, flavonoids have been shown to stimulate or inhibit rhizobial nod gene expression, cause chemoattraction of rhizobia towards the root, inhibit root pathogens, stimulate mycorrhizal spore germination and hyphal branching, mediate allelopathic interactions between plants, affect quorum sensing, and chelate soil nutrients Therefore, the manipulation of the flavonoid pathway to synthesize specifically certain products has been suggested as an avenue to improve root‐rhizosphere interactions Possible strategies to alter flavonoid exudation to the rhizosphere are discussed Possible challenges in that endeavour include limited knowledge of the mechanisms that regulate flavonoid transport and exudation, unforeseen effects of altering parts of the flavonoid synthesis pathway on fluxes elsewhere in the pathway, spatial heterogeneity of flavonoid exudation along the root, as well as alteration of flavonoid products by microorganisms in the soil In addition, the overlapping functions of many flavonoids as stimulators of functions in one organism and inhibitors of another suggests caution in attempts to manipulate flavonoid rhizosphere signals

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The role of flavonoids in root–rhizosphere signalling: opportunities and challenges for improving plant–microbe interactions

Phenylalanine ammonia lyase (PAL; E.C.4.3.1.5), which catalyses the biotransformation of l-phenylalanine to trans-cinnamic acid and ammonia, was first described in 1961 by Koukol and Conn. Since its discovery, much knowledge has been gathered with reference to the enzyme’s catabolic role in microorganisms and its importance in the phenyl propanoid pathway of plants. The 3-dimensional structure of the enzyme has been characterized using X-ray crystallography. This has led to a greater understanding of the mechanism of PAL-catalyzed reactions, including the discovery of a recently described cofactor, 3,5-dihydro-5-methyldiene-4H-imidazol-4-one. In the past 3 decades, PAL has gained considerable significance in several clinical, industrial, and biotechnological applications. The reversal of the normal physiological reaction can be effectively employed in the production of optically pure l-phenylalanine, which is a precursor of the noncalorific sweetener aspartame (l-phenylalanyl-l-aspartyl methyl ester). The...

A modern view of phenylalanine ammonia lyase

Flavonoids are biologically active low molecular weight secondary metabolites that are produced by plants, with over 10,000 structural variants now reported. Due to their physical and biochemical properties, they interact with many diverse targets in subcellular locations to elicit various activities in microbes, plants, and animals. In plants, flavonoids play important roles in transport of auxin, root and shoot development, pollination, modulation of reactive oxygen species, and signalling of symbiotic bacteria in the legume Rhizobium symbiosis. In addition, they possess antibacterial, antifungal, antiviral, and anticancer activities. In the plant, flavonoids are transported within and between plant tissues and cells, and are specifically released into the rhizosphere by roots where they are involved in plant/plant interactions or allelopathy. Released by root exudation or tissue degradation over time, both aglycones and glycosides of flavonoids are found in soil solutions and root exudates. Although the relative role of flavonoids in allelopathic interference has been less well-characterized than that of some secondary metabolites, we present classic examples of their involvement in autotoxicity and allelopathy. We also describe their activity and fate in the soil rhizosphere in selected examples involving pasture legumes, cereal crops, and ferns. Potential research directions for further elucidation of the specific role of flavonoids in soil rhizosphere interactions are considered.

https://www.springer.com/cda/content/document/cda_downloaddocument/Flavonoids.pdf?SGWID=0-0-45-1382603-0

Flavonoids: Their Structure, Biosynthesis and Role in the Rhizosphere, Including Allelopathy

Plants release a multitude of organic compounds into the rhizosphere, some of which are flavonoids. These products of secondary metabolism are mainly studied for their antioxidant properties and for their role in the establishment of rhizobium-legume symbiosis; however, it has been recently demonstrated that flavonoids can also affect nutrient availability through soil chemical changes. This review will give an overview of the types and amounts of flavonoids released by roots of different plant species, as well as summarize the available knowledge on root exudation mechanisms. Subsequently, factors influencing their release will be reported, and the methodological approaches used in the literature will be critically described. Finally, the direct contribution of plant-borne flavonoids on the nitrogen, phosphorous and iron availability into the rhizosphere will be discussed.

Release of plant-borne flavonoids into the rhizosphere and their role in plant nutrition

Release of phytoalexins and related isoflavonoids from intact chickpea seedlings elicited with reduced glutathione at root level

Purification and properties of phenylalanine ammonia-lyase from sunflower (Helianthus annuus L.) hypocotyls

Light, excision and sucrose increased extractable phenylalanine ammonia-lyase (PAL) activity from hypocotyl tissue of sunflower (Helianthus annuus L. cv. Peredovik) to 2–6 times the basal level. Intact sunflower seedlings or whole hypocotyls incubated in water or 0.1 M sucrose exhibited, in continuous light, a pattern in which PAL peaked 4 and 28 h after the beginning of the illumination. When 0.5 cm long hypocotyl segments were incubated in water or 0.1 M sucrose, they exhibited, both in continuous light and in the dark, a pattern in which PAL rose during an initial period of 10 h (assay in sucrose and light) to 48 h (assay in water and dark) and then remained nearly constant at a high value for at least the next 10 h. When whole hypocotyls were incubated in 0.1 M sucrose, a third pattern in PAL activity was found in which PAL peaked after 28 h and subsequently declined. In all the above systems the increase in PAL activity was significantly reduced by cycloheximide. Furthermore, the subsequent decay of PAL activity following illumination was prevented by delayed transfer to cycloheximide. It is suggested that the results can be explained on the basis of a turnover mechanism involving continued de novo enzyme synthesis and subsequent synthesis of a PAL-inactivating system.

Induction of phenylalanine ammonia-lyase in hypocotyls of sunflower seedlings by light, excision and sucrose

L-phenylalanine ammonia-lyase from sunflower hypocotyls : modulation by cinnamic acids

Contribution of different plant organs to the accumulation and secretion of phenolic compounds in glutathione-treated chickpea (cicer arietinum) seedlings

Rafael López-Valbuena

Papers

Release of phytoalexins and related isoflavonoids from intact chickpea seedlings elicited with reduced glutathione at root level

Purification and properties of phenylalanine ammonia-lyase from sunflower (Helianthus annuus L.) hypocotyls

Induction of phenylalanine ammonia-lyase in hypocotyls of sunflower seedlings by light, excision and sucrose

L-phenylalanine ammonia-lyase from sunflower hypocotyls : modulation by cinnamic acids

Contribution of different plant organs to the accumulation and secretion of phenolic compounds in glutathione-treated chickpea (cicer arietinum) seedlings