Effect of an Alfalfa Plant-Derived Biostimulant on Sulfur Nutrition in Tomato Plants

Abstract

The excessive use of chemical fertilizers has affected soil and water quality causing the reduction of organic matter content in soils and the increase of nitrates in waters. Organic products known as “biostimulants” could be used in agricultural practices to promote plant growth and mineral nutrient uptake. Previous studies showed that applications of a Medicago sativa L. hydrolysate-based biostimulant (EM) to maize plants stimulated the main metabolic pathways, such as nitrogen assimilation and the tricarboxylic acid cycle, as well as the secondary metabolism associated with the synthesis of phenylpropanoids. In order to evaluate whether EM could also influence sulfur (S) metabolism, the content of S, glutathione (GSH) and the expression of genes involved in S transport were analyzed in tomato plants cv. Micro-Tom treated with the biostimulant. Furthermore, the expression of genes coding for enzymes that use GSH as a substrate in redox reactions (glutathione reductase, GSR2, and glutathione peroxidase, GPX) was assayed.

Plants were cultivated in hydroponics in the presence of EM at the dosages of 0.1 or 1.0 ml l⁻¹. The application of EM to tomato significantly stimulated sulfur accumulation in plants, and in roots the increase was dose-dependent. Interestingly, in roots the level of glutathione concomitantly decreased. qRT-PCR experiments evidenced the up-regulation of genes coding for sulfate transporters (ST1 and ST2) in plants supplied with EM, especially when the biostimulant was furnished at 0.1 ml l⁻¹. The same trend was observed for the GSR2 gene in leaves. The transcript accumulation of GSR2 and GPX in roots was maximal in plants treated with 1 ml l⁻¹ EM. The results obtained suggest a positive role of EM on sulfur transport in tomato plants, and are consistent with previous studies where the enhancement of nitrogen metabolism by EM was reported. The concomitant stimulation of S and N nutrition by EM is likely due to the fact that the pathways of these nutrients in plants are highly inter-related. The increase of GSR2 and GPX transcript level suggests a role for EM to supporting plants to overcome stress by inducing antioxidant enzyme activity.