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Abstract
Plant glutathione transferases (GSTs) are classified as enzymes of secondary metabolism,
but while their roles in catalysing the conjugation and detoxification of herbicides
are well known, their endogenous functions are largely obscure. Thus, while the presence
of GST-derived S-glutathionylated xenobiotics have been described in many plants,
there is little direct evidence for the accumulation of similarly conjugated natural
products, despite the presence of a complex and dichotomous metabolic pathway which
processes these reaction products. The conservation in glutathione conjugating and
processing pathways, the co-regulation of GSTs with inducible plant secondary metabolism
and biochemical studies showing the potential of these enzymes to conjugate reactive
natural products are all suggestive of important endogenous functions. As a framework
for addressing these enigmatic functions we postulate that either: (a) the natural
reaction products of GSTs are unstable and undergo reversible S-glutathionylation;
(b) the conjugation products of GSTs are very rapidly processed to derived metabolites;
(c) GSTs do not catalyse conventional conjugation reactions but instead use glutathione
as a cofactor rather than co-substrate; or (d) GSTs are non-catalytic and function
as transporter proteins for secondary metabolites and their unstable intermediates.
In this review, we describe how enzyme biochemistry and informatics are providing
clues as to GST function allowing for the critical evaluation of each of these hypotheses.
We also present evidence for the involvement of GSTs in the synthesis of sulfur-containing
secondary metabolites such as volatiles and glucosinolates, and the conjugation, transport
and storage of reactive oxylipins, phenolics and flavonoids.
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