Streptomyces spp. enhance vegetative growth of maize plants under saline stress

Saline stress is one of the abiotic stresses that most compromises the yield of crops and can be mitigated by plant growth-promoting rhizobacteria (PGPR). This work characterized rhizobacteria isolates from the genus Streptomyces as PGPR and evaluated their role on growth and alleviation of the effects caused by saline stress in maize (Zea mays L.). Production of indolic compounds (IC), siderophores, ACC deaminase, phenazines, and promotion of plant growth were determined to characterize bacterial isolates. Salinity tolerance was accessed by culturing the Streptomyces isolates under NaCl increasing concentrations (0-300 mM). Four Streptomyces isolates exhibiting PGPR traits and salinity tolerance were selected and their effect on tolerance of maize plants to saline stress was evaluated. Plants obtained from bacterized seeds and submitted to 100 and 300 mM NaCl were used. All Streptomyces spp. produced IC and siderophores, CLV178 being the best producer of these two compounds. ACC deaminase was detected in six of the 10 isolates (CLV95, CLV97, CLV127, CLV179, CLV193, and CLV205), while phenazines were found only in CLV186 and CLV194. All isolates were tolerant to salinity, growing at concentrations up to 300 mM NaCl, with exception of CLV188. Increased concentrations of IC were detected in most of the isolates exposed to salinity. CLV97 and CLV179 significantly promoted growth of roots and leaves of maize plants and attenuated the negative effects of salinity on plant growth. Root colonization by Streptomyces spp. was confirmed in plants cultivated 20 days under saline stress.