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Biotechnology and Applied Biochemistry (2003) 37, (91–102) (Printed in Great Britain)

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Review

Elicitation of plants and microbial cell systems

Romeo Radman, Teresa Saez, Christopher Bucke and Tajalli Keshavarz¹

Fungal Biotechnology Research Group, Biotechnology Department, University of Westminster, 115 New Cavendish Street, London W1W 6UW, U.K.

Key words: defence mechanism, elicitors, enhancement, plant and microbial culture.

Abbreviations used: ACV, d-(L-a-aminoadipyl)-L-cysteinyl-D-valine; BTH, benzo[1,2,3]thiadiazole-7-carbothioic acid S-methyl ester; CAM, calmodulin; DAG, diacylglycerol; DP, degree of polymerization; FDA, Food and Drug Administration (Rockville, MD, U.S.A.); GOD, glucose oxidase; GST, glutathione S-transferase; MAPK, mitogen-activated protein kinase; MO, mannan oligosaccharides; OG, oligo-guluronate; OM, oligomannuronate; PAL, phenylalanine ammonia-lyase; PKC, protein kinase C; ROS, reactive oxygen species.

Definitions: elicitors and elicitation: an elicitor may be defined as a substance which, when introduced in small concentrations to a living cell system, initiates or improves the biosynthesis of specific compounds; thus elicitation is the induced or enhanced biosynthesis of metabolites due to addition of trace amounts of elicitors; the type and structure of elicitors varies greatly; abiotic elicitors: substances of non-biological origin, predominantly inorganic salts, and physical factors acting as elicitors; they include Cu and Cd ions, Ca²⁺ and high pH; biotic elicitors: elicitors with biological origin; they include polysaccharides derived from plant cell walls (pectin or cellulose) and micro-organisms (chitin or glucans) and glycoproteins; G-protein: intracellular proteins whose functions are coupled to receptors and act by activating or inactivating a number of enzymes or ion channels; phytoalexins: low-molecular-mass antimicrobial secondary metabolites produced by plants in response to physical damage, fungi or bacteria attack; protein kinase: phosphorylates a variety of target proteins that control growth and cellular differentiation; calmodulin: intracellular Ca²⁺-binding proteins consisting of at least two different peptides, with four Ca²⁺ binding sites; calmodulin has no enzyme activity of its own, but acts by binding to other proteins.

¹To whom correspondence should be addressed (e-mail T.Keshavarz@westminster.ac.uk).

Plants show physiological and morphological responses to a range of physical and chemical factors known as 'elicitors'. These responses have been considered as defence reactions 'elicited' by the plants' biochemical factory to ensure their survival, persistence and competitiveness. Recently examples have been cited of elicitation in some fungal and bacterial cultures. Through a chronological survey, this Review considers examples of elicitors and elicitation and describes suggested mechanisms of elicitation in plants and microbial cell cultures. The majority of research in this field has been carried out on the plant systems using complex (undefined) biotic elicitors. Carbohydrates are the main class of compounds used as defined elicitors. This Review focuses on carbohydrates as compounds initiating a defence response in cell cultures. Physiological changes brought about on the plant and microbial cultures include expression of novel metabolites and overproduction of already known products. Recent reports confirming elicitation in microbial cultures are of potential importance, as the relative ease of fermentation and scale-up could open an opportunity for the introduction of useful novel metabolites as well as enhancement of commercially useful bioproducts. In this context, a sound knowledge of the elicitor molecules' structure–function relationships and mechanisms of elicitation is essential.

Received 12 December 2002/20 December 2002; accepted 2 January 2003

Portland Press Ltd © 2003