Caracterização de rizobactérias Streptomyces spp. como halotolerantes e sua ação na mitigação do estresse salino em plantas de milho (Zea mays L.)

Abstract

Soil salinization, caused either by natural or anthropogenic factors, has been recognized as a serious environmental problem, resulting in extensive areas of degradation and restricting agricultural productivity worldwide. The excess of salts in the soil affects plants by causing (i) osmotic stress, which causes restriction of water absorption by the roots, (ii) ionic stress, which causes nutritional and metabolic damages, and (iii) oxidative stress, which compromises, mainly, the photosynthetic efficiency. Species with agricultural importance, such as maize, have their development compromised by soil salinization. Therefore, studies on methods capable to mitigate the effects of saline stress, whereas maintaining the crop productivity, have become increasingly important. The use of plant growth promoting rhizobacteria (PGPR) is a sustainable alternative to be used in agriculture. They act as biostimulants, producing phytohormones, siderophores, ACC deaminase, and volatile organic compounds (VOCs), and can induce systemic tolerance in plants, from the synthesis of osmolytes and antioxidant enzymes. They can also mitigate the adverse effects caused by salt stress through the synthesis of extracellular polymeric substances (EPS) and biofilm. In addition, some of these bacteria are halotolerant and can be used in saline soils to help plants on reducing the adverse effects caused by salinity. The objective of this work was to characterize Streptomyces spp. (Stm; CLV95, CLV97, CLV178, and CLV179) as halotolerant bacteria and as inducers of salt stress tolerance in maize (Zea mays L.) plants. In a technological approach, the effect of indole metabolites and semi-purified phenazines obtained from Stm isolates CLV91 and CLV95 were evaluated as antimicrobial agents and growth promoters of maize plants. The growth of Stm isolates was evaluated by colony forming units and cell viability. The characterization of Streptomyces spp. as halotolerant PGPR was performed through the production of indolic compounds (IC), siderophores, VOCs, and the ability to solubilize P by the isolates at different concentrations of NaCl. The production of EPS and biofilm was analyzed under normal and saline conditions. To evaluate the mechanisms of salinity tolerance, maize seeds were bacterized with Stm isolates, cultivated in a greenhouse for 45 days and the plants (stage V5) were treated with 100 and 300 mM NaCl. Plant salinity tolerance was evaluated by determining the content of proline and soluble amino acids, and by the activity of antioxidant enzymes in plant cells. To evaluate the effect of metabolites on plant cultures, IC and phenazine, semi-purified from Stm cultures, were sprayed on maize leaves (stage V3) and vegetative growth was evaluated. The results showed that all isolates were able to grow in saline solutions and maintained the production of their metabolites, being considered halotolerant. CLV179 was the major producer of biofilm and EPS in saline conditions. The tetrazolium assay proved to be efficient to determine cell viability, associated with the quantification of bacterial biomass and demonstrated how saline stress can affect the development of the isolates. The inoculation of most Stm isolates in maize plants promoted an increase in the synthesis of proline and soluble amino acids, and in the activity of the antioxidant enzymes CAT, POX, and APX, with differences between the isolates, characterizing the activation of systemic tolerance induction, since oxidative stress was reduced in both plant roots and leaves. The proposed protocol for fractionation and semi-purification of the supernatant of cultures of Stm (CLV91 and CLV95) was efficient in the extraction of IC and phenazine metabolites, and these, when sprayed on the leaves of maize plants, resulted in changes in vegetative growth. The fraction containing the metabolites of the isolate CLV91 showed phenazines in its composition and antimicrobial action against the phytopathogenic fungus Alternaria solani.