Streptomyces spp. promove o crescimento vegetativo de plantas de milho (Zea mays L.) sob estresse salino

Abstract

Plant growth promoting rhizobacteria (PGPR) are microorganisms recognized by their positive effect on plants, directly affecting plant metabolism by production of hormones, ACC deaminase enzyme, and siderophores, as well as phosphate solubilization and nitrogen fixation. Indirectly, they act in the antagonism to phytopathogenic microorganisms (producing antibiotics and phenazines) promoting tolerance to biotic and abiotic stresses and inducing systemic resistance. Saline stress is one of the abiotic stresses that most compromises crop yields, such as maize. The excess of salts in the soil results in osmotic and ionic stress, which causes nutritional and metabolic damages to the plant. PGPR may reduce plant saline stress by mechanisms of induced systemic tolerance, which implies hormonal changes, antioxidant defense stimulus, osmotic adjustment and the expression of stress response genes. The objective of this work was to characterize the isolates of rhizobacteria from the genus Streptomyces spp. as PGPR and to evaluate the role that these bacteria play in the growth and in the attenuation of the effects of the saline stress in maize. The characterization of the isolates was performed by the quantification of the metabolites (indole compounds, siderophores and phenazines), the tolerance to salinity and the promotion of maize vegetative growth. For the growth experiments, maize seeds (Zea mays L.) were bacterized with the Streptomyces spp. and cultivated in a greenhouse. Growth was evaluated through the parameters of shoot and root length and dry matter, 45 days after sowing (stage V5). The tolerance of maize plants to salinity was evaluated by determining growth at the concentrations of 0, 50, 100, 150, 200 and 300 mM of NaCl. Sub lethal concentrations of NaCl were determined and used in the following experiments. The tolerance of Streptomyces spp. isolates to the salt stress was analyzed by the growth of the bacterial suspensions in the presence of different concentrations of NaCl. Four isolates of Streptomyces spp. exhibiting PGPR traits and salinity tolerance were selected and their action in attenuating the effects of saline stress on maize plants was evaluated in plants from bacterized seeds and submitted to 100 and 300 mM of NaCl. The growth parameters were evaluated 20 days after the onset of salinity stress. The activity of antioxidant ascorbate peroxidase and catalase was analyzed at 0, 6, 12 and 24 hours after treatment with NaCl. Results showed that all Streptomyces spp. isolates were capable to produce of indolic compounds and siderophores, with CLV178 being the most productive of these two compounds. Phenazines were found in CLV186 and CLV194. The isolates were tolerant to salinity, growing at concentrations up to 300 mM. Growth of maize plant was favored by the interaction with rhizobacteria Streptomyces. CLV 95 promoted root and shoot growth, while seed treatment with CLV179 resulted in plants with a significant dry matter increase, as 40% root and 120% shoot when compared to non-bacterized seeds. Treating seeds with CLV179 resulted in significant promotion of growth under salt stress, regardless the concentration of NaCl used. An increase of 61.3% and 73% in shoot growth of the maize plants was recorded at 100 and 300 mM NaCl, respectively. Overall, APX activity was more intense in roots than in shoots of maize plants. The contrary result was observed with CAT. Specifically, CAT activity was modulated in the roots by CLV97, CLV178 and CLV179 at 100 mM NaCl, whereas the effect on APX activity was more expressive in the shoots at the same concentration.