Regulatory and functional interactions of plant growth regulators and plant glutathione S-transferases (GSTs)

Plant glutathioneS-transferases (GSTs) are a heterogeneous superfamily of multifunctional proteins, grouped into six classes. The tau (GSTU) and phi (GSTF) class GSTs are the most represented ones and are plant-specific, whereas the smaller theta (GSTT) and zeta (GSTZ) classes are also found in animals. The lambda GSTs (GSTL) and the dehydroascorbate reductases (DHARs) are more distantly related. Plant GSTs perform a variety of pivotal catalytic and non-enzymatic functions in normal plant development and plant stress responses, roles that are only emerging. Catalytic functions include glutathione (GSH)-conjugation in the metabolic detoxification of herbicides and natural products. GSTs can also catalyze GSH-dependent peroxidase reactions that scavenge toxic organic hydroperoxides and protect from oxidative damage. GSTs can furthermore catalyze GSH-dependent isomerizations in endogenous metabolism, exhibit GSH-dependent thioltransferase safeguarding protein function from oxidative damage and DHAR activity functioning in redox homeostasis. Plant GSTs can also function as ligandins or binding proteins for phytohormones (i.e., auxins and cytokinins) or anthocyanins, thereby facilitating their distribution and transport. Finally, GSTs are also indirectly involved in the regulation of apoptosis and possibly also in stress signaling. Plant GST genes exhibit a diversity of expression patterns during biotic and abiotic stresses. Stress-induced plant growth regulators (i.e., jasmonic acid [JA], salicylic acid [SA], ethylene [ETH], and nitric oxide [NO] differentially activate GST gene expression. It is becoming increasingly evident that unique combinations of multiple, often interactive signaling pathways from various phytohormones and reactive oxygen species or antioxidants render the distinct transcriptional activation patterns of individual GSTs during stress. Underestimated post-transcriptional regulations of individual GSTs are becoming increasingly evident and roles for phytohormones (i.e., ABA and JA) in these processes are being anticipated as well. Finally, indications are emerging that NO may regulate the activity of specific plant GSTs. In this review, the current knowledge on the regulatory and functional interactions of phytohormones and plant GSTs are covered. We refer to a previous extensive review on plant GSTs (Marrs, 1996) for most earlier work. An introduction on the classification and roles of plant GSTs is included here, but these topics are more extensively discussed in other reviews (Dixon et al., 2002a; Edwards et al., 2000; Frova, 2003).