School of Biological Sciences
Royal Holloway University of London
Egham, Surrey, TW20 0EX
United Kingdom

Tel: 01784-443543
Fax: 01784-414224
E-mail: d.davies@rhul.ac.uk

Some of the mechanisms by which environmental and internal signals are perceived and processed in plants are starting to be elucidated. Some mechanisms are analogous to those which are established to occur in mammalian systems but plants are sessile organisms and they have developed specialised mechanisms which allow them to cope with a mass of external signals originating from physical influences and internal signals from a variety of chemical influences such as growth regulators, peptides, sugars and polysaccharides, lipids as well as ion-fluxes and electrical signals. Plants react to microbial infection with a broad range of defence responses aimed at restricting the growth of the pathogen and ultimately destroying it.

In this laboratory we are interested in the mechanism by which plants perceive the pathogen and in the chain events which results in the mounting of a set of general defence reactions such as the reinforcement of the cell wall, synthesis of anti-microbial proteins and the production of phytoalexins. Infection by avirulent pathogens is often accompanied by the death of a small number of cells at the site of interaction with the pathogen - the hypersensitive response. In one of our model systems, the treatment of suspension-cultured French bean cells with elicitor macromolecules extracted from the cell walls of the bean pathogen Colletotrichum lindemuthianum results in rapid metabolic changes, within a few minutes including increased oxygen consumption, accumulation of active oxygen species notably H2O2, alkalinization of the cell wall, transient ATP depletion and changes in the cytosolic redox potential. We have observed similar changes using a cultured Arabidopsis cell line  responing to an elicitor derived from Fusarium oxypsorum and our current research is focussed on the nature of  the reductant involved in the generation of H2O2 and on the signalling mechanism linking the perception of the elicitor  and this response.

The hypersensitive response follows a series of transient changes occurring at the cell surface including ion fluxes, changes in membrane potential, changes in the pH of the cell wall, and the generation and release of reactive oxygen species (ROS) such as hydrogen peroxide and the superoxide and hydroxyl radicals. Normally the concentration of these toxic species are kept at a low level by compartmentalised isoenzymes of catalase, superoxide dismutase and peroxidases but under stress conditions this protective mechanism is overridden by the oxidative burst a rapid and transient accumulation of high concentrations of ROS. We are interested in the biochemical mechanism which underlies the generation of ROR in the plant defence response. One possibility is an NADPH oxidase system such as that occurring in mammalian cells such as macrophages but there are other enzymes capable of generating ROS such as amine oxidase and germin-like oxalate oxidase. We are particularly interested in the possibility that a cell-wall peroxidase may be involved in the generation of ROS. This may be shown to occur in vitro with the reaction occurring at neutral or basic pH in the presence of a suitable reductant such as cysteine and we have extracted a reductant from the cell-walls of elicited cells which will sustain hydrogen peroxide production by peroxidase at this range of pH. We are interested in characterising this reductant and identifying and characterising the peroxidase(s) responsible for the ROS burst. We have cloned and sequenced the 46 kDa cell wall peroxidase which we believe to be involved in this mechanism. We are interested in the nature and function of the signal transduction pathways from the initial perception of the pathogenic elicitor to the observed metabolic disturbances. We are investigating the possibility that second messengers are involved and how these may interact with protein kinases during signal transduction. We have evidence that cyclic nucleotides may be involved in modulating the intensity of the oxidative burst, most likely via the regulation of Ca2+ flux into the cytosol but it is unlikely that these are the primary signals which regulate the oxidative burst.

Bindschedler LV, Dewdney J,Blee KA, Stone JM, Asai T, Plotnikov J, Denoux C, Hayes T, Gerrish C, Davies DR, Ausubel FM, Bolwell GP, (2006)  Peroxidase –dependent apoplastic oxidative burst in Arabidopsis required for pathogen resistance. The Plant Journal 47: 851-863

 Davies DR, Bindschedler LV, Strickland TS Bolwell GP (2006)  Production of reactive oxygen species in Arabidopsis thaliana cell suspension cultures in response to an elicitor from Fusarium oxysporum: implications for basal resistance J. Exp. Bot. 57: 1817-1827

 Bolwell GP, Bindschedler LV, Blee KA, Butt VS, Davies DR, Gardner SL, Gerrish C, Minibayeva F (2002) The apoplastic oxidative burst in response to biotic stress in plants: a three-component system. J. Exp. Bot. 53: 1367-1376.

Allwood EG, Davies DR, Gerrish C, Bolwell GP (2002) Regulation of CDPKs, including identification of PAL kinase, in biotically stressed cells of French bean. Plant Mol.Biol. 49: 533-544.

Wheatley ER, Davies DR, Bolwell GP (2002) Characterisation and immunolocation of an 87 kDa polypeptide associated with UDP-glucuronic acid decarboxylase activity from differentiating tobacco cells (Nicotiana tabacum L.). Phytochemistry 61: 771-780.

Blee KA, Jupe SC, Richard G, Zimmerlin A, Davies DR, Bolwell GP (2001) Molecular identification and expression of the peroxidase responsible for the oxidative burst in French bean (Phaseolus vulgaris L.) and related members of the gene family. Plant Mol.Biol. 47: 607-620.

Bindschedler LV, Minibayeva F, Gardner SL, Gerrish C, Davies DR, Bolwell GP (2001) Early signalling events in the apoplastic oxidative burst in suspension cultured French bean cells involve cAMP and Ca2+. New Phytol. 151: 185-194.

Bolwell GP, Blee KA, Butt VS, Davies DR, Gardner SL, Gerrish C, Minibayeva F, Rowntree EG, Wojtaszek P (1999) Recent advances in understanding the origin of the apoplastic oxidative burst in plant cells. Free Radic. Res. 31: S137-S145.

Allwood EG, Davies DR, Gerrish C, Ellis BE, Bolwell GP (1999) Phosphorylation of phenylalanine ammonia-lyase: evidence for a novel protein kinase and identification of the phosphorylated residue. FEBS Lett. 457: 47-52.

Smertenko AP, Jiang CJ, Simmons NJ, Weeds AG, Davies DR, Hussey PJ (1998) Ser6 in the maize actin-depolymerizing factor, ZmADF3, is phosphorylated by a calcium-stimulated protein kinase and is essential for the control of functional activity. Plant J. 14: 187-193.

Bolwell GP, Davies DR, Gerrish C, Auh CK, Murphy TM (1998) Comparative biochemistry of the oxidative burst produced by rose and French bean cells reveals two distinct mechanisms. Plant Physiol. 116: 1379-1385.