Bactérias diazotróficas endofíticas na cultura do sorgo sacarino [Sorghum bicolor (L.) Moench]: estratégias de isolamento, promoção do crescimento e mitigação de estresses de salinidade e nitrogênio

Abstract

Diazotrophic bacteria convert atmospheric N2 into ammonia through biological fixation, presenting other attributes related to plant growth promoting (PGP), such as indoleacetic acid production (IAA), phosphate solubilization, 1-aminocyclopropane-1-carboxylate activity (ACC), which contribute to the attenuation of environmental stresses. Therefore, they serve as a promising, sustainable, and low-cost alternative for the increase of plant productivity and expansion of sorghum (Sorghum bicolor) in the Northeast, Brazil. In the first stage, the PGP activity and abiotic stress attenuation in sorghum were evaluated by isolates of Achromobacter xylosoxidans and Enterobacter bugandensis, from sugarcane stalks in the Zona da Mata of the state of Paraíba. Simple inocula were effective (Duncan's test; p≤0.05) under nitrogen stress condition (2 mM). Co-inoculation were not more effective than simple inoculums under N stress, however, they presented a PGP competence under double stress condition (2 mM N + 75 mM NaCl). In the second stage, were isolated and characterized 103 new diazotrophs (from stem and leaves) of three sorghum cultivars (BRS 506; IPA 467-4-2; SF-15) in three regions (Zona da Mata, Agreste, Sertão) in the state of Pernambuco, using five isolation media (JNFb; NFb; LGI; LGI-P; JMV). Fourteen species were obtained, belonging to the genera Pseudomonas (16 isolates), Pantoea (54), Agrobacterium (9), Sphingomonas (12), Herbaspirillum (10), Acinetobacter (1) and Serratia (1), being Pantoea the most widespread. Non-metric multidimensional scaling showed that region was the most important factor (PERMANOVA: p=0.004) shaping the diazotrophic communities. The Sertão communities presented lower structural diversity, with predominance of Pantoea. In the Zona da Mata and Agreste communities, Pseudomonas, Pantoea and Agrobacterium predominated. Although a conserved core has been observed for the production of IAA and halotolerance, Dunn's test (p≤0.05) showed that: a) Sertão communities present high IAA productions (~3.11 μM·h-1 ) and are predominantly halotolerant and halophytic, whereas communities Zona da Mata and Agreste were specialized in the metabolization of ACC; b) JMV and LGI-P media accessed communities with, respectively, high production of IAA (3.04 μM·h-1 ) and phosphate solubilization (IS = 2.96); c) communities of BRS 506 cultivar were superior in the production of IAA (2.68 μM·h-1

) and solubilization of phosphate (IS = 2.14), being predominantly halotolerant and halophytes. Spearman’s correlations indicated parameters related to soil salinity (exchangeable Na+ and K + , CEC, S value, PES, EC) that influence the abundance of some species. In the third stage, PGP sorghum trials (cv. Wray) showed that isolates from Sertão and Agreste were more promising, which induced greater efficiency of vegetal water consumption. Increased root (BR; 85.8%) and shoot (BS; 38.6%) biomass production in the condition of simple stress (3 mM N) were induced by Agrobacterium pusense ICS2 and TCI2, respectively. In the condition of double stress (3 mM N + 75 mM NaCl), the highest production in BR (108.1%) and BS (75.4%) were induced by Pseudomonas oryzihabitans CFS2 and Pantoea stewartii subsp. indologenes TFB1. The results indicate that the use of JMV and LGI-P for isolation, BRS 506 as a host plant, can be a good alternative for the access of putative endophytes from Sertão soils with high PGP potential, particularly for sorghum.