The Effect of the Antioxidant Quercetin on Morphometry of Broad Beans, Transport of Malate Across the Symbiosome Membrane and Respiration of Root Nodule Bacteroids

The effect of quercetin, a bioflavonoid with antioxidant properties, on the morphometric parameters of plants, as well as on the transport of malate through the symbiosome membrane (SM) and the respiration of bacteroids from the root nodules of Vicia faba L. broad beans was studied. It was found that seed treatment and plant fertilization with quercetin had a positive effect on their growth and mass parameters, on the increase in the biomass of nodules and their number. In addition, quercetin treatment increased the transport of dicarboxylate—malate through the SM, recorded by acidification of the symbiosome space. The rate of ATP-dependent generation of the pH gradient on the SM in the variant with seed treatment with quercetin was also significantly higher than in the variant with control plants. At the same time, quercetin, when added to the symbiosome incubation medium, decreased the membrane potential (Δψ) generated on SM as a result of active work of H+-ATPase. It was shown that in respiration of bacteroids the activity of the phosphorylation-coupled cytochrome electron transport pathway (OXPHOS system) prevailed, which was suppressed by 85–90% by cyanide, a classic inhibitor of cytochrome oxidase. The activity of the alternative SHAM-sensitive oxidase of bacteroids, which is apparently an analogue of the AOX of plant mitochondria, as well as the activity of other oxidases insensitive to the inhibitors used, did not exceed 10–12% of the total rate of oxygen absorption by bacteroids. Treatment with quercetin had a multidirectional effect on the respiration of bacteroids: it somewhat decreased the activity of the cytochrome oxidation pathway, but significantly activated the alternative SHAM-sensitive respiration pathway. It is concluded that quercetin regulates the transport of malate as a C-substrate, as well as the respiration of bacteroids for a more flexible and stress-resistant supply of energy for the nitrogen-fixing activity of bacteroids in root nodules. The data obtained are discussed in terms of the possibility of using quercetin as an environmentally safe means for increasing bean yields and enriching the soil with nitrogen.