Harvey, Patricia J. ORCID: 0000-0001-7193-4570, Xiang, Mingyan and Palmer, John M. (2002) Extracellular enzymes in the rhizosphere. In: Cost Action 837. Plant biotechnology for the removal of organic pollutants and toxic metals from wastewaters and contaminated sites. InterCOST Workshop on Soil-microbe-root interactions: maximising, phytoremediation/bioremediation (WG1+WG4+COST Action 831, 23-25 May 2002, Grainau, Germany. (Unpublished)

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Abstract

We may distinguish two classes of enzyme in the rhizosphere: (1) enzymes that are cytosolic in origin but appear in association with cell debris; (2) enzymes that have been deliberately secreted by plant roots or microbes to the external environment. The former enzymes (cytosolic origin) are expected to play only a very minor role, if any, in catalysis within the rhizosphere. They are more likely to serve instead as a ready source of carbon, nitrogen and reducing equivalents for the growth of microbial communities. By contrast, enzymes that are deliberately secreted comprise a large collection of different oxidoreductases (laccases, peroxidases) and some esterases. These normally serve either a protective function (oxidoreductases) and oxidise extracellular toxic soluble phenolic metabolites to insoluble polymerized products, or a degradative function and hydrolyse (esterases) or oxidise (peroxidase, laccase) polymeric lignin, humic acids or phenols for metabolic purposes. Since oxidoreductases show an apparent lack of substrate specificity, they are able to transform organic xenobiotics as well and this property has prompted efforts to exploit these enzymes for bioremediation purposes.

Plant peroxidases are typically wall-associated, which affords protection against their destruction in the extracellular milieu, and inducible (Zheng and Shetty, 2000; Hirata et al, 2000), oxidising phenols or anilines to non-toxic polymeric products that are typically incorporated into plant cell walls as bound residues. However, available evidence suggests that some members of the Fabaceae, Graminea and Solanaceae exude sufficient amounts of peroxidases into the rhizosphere to take part in the oxidative degradation of some soil constituents as well (Gramss et al, 1999). Extracellular plant (wall-associated) peroxidases may also exhibit quite diverse functions depending on pH value and the nature of electron donors. For example, a wide range of peroxidases with pI’s between 3-9 have been identified in cell suspension cultures of I. batatas that show varying abilities to oxidise hydroxybenzoic acids to their quinones at pH 7.0, but at pH 3.0 the hydroxybenzoic acids serve as redox mediators enabling the peroxidase-dependent oxidation of secondary substrates to proceed. In the case of NADH as secondary substrate, H2O2 is produced that is capable of participating in Fenton-type chemistry and could play a role analogous to cytochromes P450 i.e. introduction of OH functional groups into xenobiotics as part of the ‘phase 1’ metabolism of xenobiotics (see figure 1). Peroxidases have been intensively investigated for their potential to remove phenols from aqueous solutions and bleach Kraft effluent in both immobilized forms (Duran and Esposito, 2000) and in situ in plant material (Dec and Bollag, 1994). Microbial extracellular oxidases (laccases, peroxidases) have been identified from wooddegrading basidiomycetes; terricolous basidiomycetes; ectomycorrhizal fungi and soil-borne microfungi and actinomycetes and, as with plants, are probably periplasmic in vivo, associated with the cell surface of viable cells for protection in an otherwise hostile environment. They are inducible by humic acids (Dehorte and Blondeau, 1992) and normally serve either in a protective (Mn-peroxidase, laccase) or in a degradative (lignin peroxidase) capacity. Mn-peroxidase and laccase normally oxidise phenols and anilines with a standard redox potential up to 0.8 –1.1 V/SCE to non-toxic polymerised products, whilst lignin peroxidase oxidizes non-phenolic compounds with redox potentials in excess of 1.4 V vs SCE; this latter characteristic confers the ability of white-rot fungi to depolymerise lignin in lignocellulosic materials. Nevertheless, the substrate ranges of all the enzymes may be extended with redox mediators enabling potential exploitation in the degradation of xenobiotics. Mn-peroxidase and lignin peroxidase use chelated Mn2+ and veratryl alcohol respectively to catalyse the degradation of lignin, some soil humic material and aromatic xenobiotics (phenanthrene, fluorine, pyrene, benzo(a) pyrene) by hydroxylatio n, oxidation and cleavage of aromatic ring structures, (Harvey and Thurston, 2001), whilst the range of substrates oxidised by laccase may be extended to include anthracene and benzo [a] pyrene with 2,2’-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) as a co-substrate (Collins et al., 1996).

Item Type:	Conference or Conference Paper (Paper)
Additional Information:	This paper was presented at the Cost Action 837. Plant biotechnology for the removal of organic pollutants and toxic metals from wastewaters and contaminated sites. InterCOST Workshop on Soil-microbe-root interactions: maximising, phytoremediation/bioremediation (WG1+WG4+COST Action 831, WG1) held on 23-25 May 2002 in Grainau, Germany.
Uncontrolled Keywords:	enzymes, rhizosphere, plant peroxidases, organic xenobiotics
Subjects:	Q Science > QK Botany Q Science > QP Physiology
Faculty / School / Research Centre / Research Group:	Faculty of Engineering & Science > School of Science (SCI) Faculty of Education, Health & Human Sciences > School of Human Sciences (HUM)
Related URLs:	Organisation Organisation Organisation Organisation Organisation
Last Modified:	09 Oct 2021 04:46
URI:	http://gala.gre.ac.uk/id/eprint/3328

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