Arbuscular mycorrhizal association is beneficial for growth and detoxification of xenobiotics of barley under drought stress

Plant growth and productivity under abiotic stresses such as water shortage or pollution are major problems which currently worry scientists in the field of food production and plant health. Since the intensification of livestock production with its associated increased demand for fodder has encouraged farmers to rely more heavily on chemical fertilizers, very often, municipal waste and wastewater sludge is considered a possible source of plant nutrients, although it might carry a significant amount of anthropogeneous pollutants. The extent to which plants react to drought, as well as how pollutants are taken up or how they act on plants, might depend on rhizosphere processes such as mycorrhizal symbioses. Therefore, it seemed timely to investigate plant defense reactions in the presence of arbuscular mycorrhizal fungi (AMF) toward a possible dangerous sewage sludge pollutant under the influence of drought. This study was done with mycorrhizal plants (+AMF) and non-mycorrhizal plants (-AMF) under conditions of water shortage and certain supplements of pharmaceutics. Plant water relations, activity of plant detoxification enzymes, and phylogeny of root mycorrhization were surveyed to investigate beneficial aspects of AMF symbiosis. Plants were exposed to certain drought phases and watered with tap water containing the pharmaceutical paracetamol (acetaminophen) as a model xenobiotic with environmental relevance. Enzymes of interest for the present study were catalase (CAT) and glutathione S-transferase (GST). Assays followed published methods. GST spectrophotometric assays included model substrates 1-chloro-2,4dinitrobenzene, 1,2-dichloro-4-nitrobenzene, and 1,2-dichloro-4-nitrobenzoyl-chloride. Our observations indicate H2O2 accumulation in roots of AMF plants and concomitant increases of catalase activities. Enzyme activities were lower in AMF plants even after exposure to both drought stress and paracetamol. AMF symbiosis seems a strategy to influence plant water tolerance and stress enzymes. Catalase activity seemed to be exhausted at the end of 56 days of drought treatment and pharmaceutics incubation, as indicated by high H2O2 levels at the end of the experiment. GST reactions differed between drought or paracetamol-stressed plants. A combination of both stressors resulted in lowered GST activities. Both catalase and GST activities decreased in the presence of paracetamol in the AMF roots after drought treatment. When soil water contents declined, AMF symbiosis supported leaf water content and production of biomass. During dry cycles, AMF roots achieved lower activities of cytosolic GST than that as compared to non-mycorrhizal roots. However, under drought and pharmaceutics supplements, water relations of mycorrhizal plants were more balanced than in non-mycorrhizal plants. In addition, roots showed an enhanced mycorrhization rate in those circumstances. We propose that the adjustment of CAT and GST activities in mycorrhizal roots under drought and xenobiotics might be caused by a decreased oxidative damage to rhizosphere because AMF plants had consistently better plant water relations and biomass production. Additional drought avoidance mechanisms indicated by the arbuscular mycorrhizal symbiosis might also contribute to the lower oxidative stress in mycorrhizal plants. Since the role of mycorrhiza in the fate of organic pollutants has not yet been understood, further ecotoxicological studies in mycorrhizosphere needs to be achieved as cooperation between plant-microbe specialists and ecotoxicologists.