Plant glutathione S-transferases (GSTs) are ubiquitous and multifunctional enzymes encoded by large gene families. A characteristic feature of GST genes is their high inducibility by a wide range of stress conditions including biotic stress. Early studies on the role of GSTs in plant biotic stress showed that certain GST genes are specifically up-regulated by microbial infections. Later numerous transcriptome-wide investigations proved that distinct groups of GSTs are markedly induced in the early phase of bacterial, fungal and viral infections. Proteomic investigations also confirmed the accumulation of multiple GST proteins in infected plants. Furthermore, functional studies revealed that overexpression or silencing of specific GSTs can markedly modify disease symptoms and also pathogen multiplication rates. However, very limited information is available about the exact metabolic functions of disease-induced GST isoenzymes and about their endogenous substrates. The already recognized roles of GSTs are the detoxification of toxic substances by their conjugation with glutathione, the attenuation of oxidative stress and the participation in hormone transport. Some GSTs display glutathione peroxidase activity and these GSTs can detoxify toxic lipid hydroperoxides that accumulate during infections. GSTs can also possess ligandin functions and participate in the intracellular transport of auxins. Notably, the expression of multiple GSTs is massively activated by salicylic acid and some GST enzymes were demonstrated to be receptor proteins of salicylic acid. Furthermore, induction of GST genes or elevated GST activities have often been observed in plants treated with beneficial microbes (bacteria and fungi) that induce a systemic resistance response (ISR) to subsequent pathogen infections. Further research is needed to reveal the exact metabolic functions of GST isoenzymes in infected plants and to understand their contribution to disease resistance.

Changes in ascorbate and glutathione levels and in activities of ascorbate peroxidase, catalase, dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione S-transferase (GST), and superoxide dismutase (SOD) were investigated in tobacco mosaic virus (TMV)-inoculated lower leaves and in non-inoculated upper leaves of Nicotiana tabacum L. cv Xanthi-nc. In separate experiments the effects of exogenous salicylic acid (SA) were also studied. Symptom appearance after TMV inoculation was preceded by a slight, transient decline of ascorbate peroxidase, GR, GST, and SOD activities in the inoculated lower leaves, but after the onset of necrosis these activities and the glutathione level substantially increased. Ascorbic acid level and DHAR activity declined and dehydroascorbate accumulated in the inoculated leaves. In upper leaves, the glutathione level and the activities of GR, GST, and SOD increased 10 to 14 d after TMV inoculation of the lower leaves, concomitantly with the development of systemic acquired resistance. From the six distinct SOD isoenzymes found in tobacco leaves, only the activities of Cu,Zn-SOD isoenzymes were affected by TMV. SA injection induced DHAR, GR, GST, and SOD activities. Catalase activities were not modified by TMV infection or SA treatment. It is supposed that stimulated antioxidative processes contribute to the suppression of necrotic symptom development in leaves with systemic acquired resistance.

A wild‐type poplar hybrid and two transgenic clones overexpressing a bacterial γ‐glutamylcysteine synthetase in the cytosol or in the chloroplasts were exposed to the chloroacetanilide herbicides acetochlor and metolachlor dispersed in the soil. The transformed poplars contained higher γ‐glutamylcysteine and glutathione (GSH) levels than wild‐type plants and therefore it was supposed that they would have an elevated tolerance towards these herbicides, which are detoxified in GSH‐dependent reactions. Phenotypically, the transgenic and wild‐type plants did not differ. The growth and the biomass of all poplar lines were markedly reduced by the two chloroacetanilide herbicides. However, the decrease of shoot and root fresh weights caused by the herbicides was significantly smaller in the transgenic than in wild‐type plants. In addition, the growth rate of poplars transformed in the cytosol was reduced to a significantly lesser extent than that of wild‐type plants following herbicide treatments. The effects of the two herbicides were similar. Herbicide exposures markedly increased the levels of γ‐glutamylcysteine and GSH in leaves of each poplar line. The increase in the foliar amounts of these thiols was stronger in the transgenic lines than in the wild type, particularly in the upper leaves. Considerable GST activities were detected in leaves of all poplar plants. Exposure of poplars to chloroacetanilide herbicides resulted in a marked induction of GST activity in upper leaf positions but not in middle and lower leaves. The extent of enzyme induction did not differ significantly between transgenic and wild‐type poplars. Although the results show that the transgenic poplar lines are good candidates for phytoremediation purposes, the further improvement of their detoxification capacity, preferably by transformation using genes encoding herbicide‐specific GST isoenzymes, seems to be the most promising way to obtain plants suitable for practical application.

Global environmental, social and economic challenges drive the need for new and improved solutions for food production and consumption. Food production within a sustainability corridor requires innovations exceeding traditional paradigms, acknowledging the complexity arising from sustainability. However, there is a lack of knowledge about how to direct further activities, to develop technologies as potential solutions for questions related to climate change, loss of soil fertility and biodiversity, scarcity of resources, and shortage of drinking water. One approach that promises to address these problems is controlled environment agriculture. Aquaponics (AP) combines two technologies: recirculation aquaculture systems (RAS) and hydroponics (plant production in water, without soil) in a closed-loop system. One challenge to the development of this technology is the conversion of the toxic ammonium produced by the fish into nitrate, via bacteria in a biofilter, to provide nitrogen to the plants. However, as this Special Issue shows, there are many other challenges that need to be addressed if the goal of the technology is to contribute to more sustainable food production systems.