Rizóbios associados à cana-de-açúcar (RB 92579) consorciada com feijão-caupi em solo com longo histórico de exploração canavieira

Abstract

Rhizobia are bacteria known to infect legume roots, forming nodules, where these organisms can fix atmospheric nitrogenmore efficiently than the diazotrophic bacteria that colonize grasses. Several works report the presence of rhizobia inside the roots of grasses, as well as occur in sugarcane. It is not known whether these organisms have beneficial functions for plants, but if they can act as growth-promoting bacteria, the association with grassy rhizobia may represent a great possibility for the development of technologies based on bioinputs, or on more independent management of chemical fertilizers for the culture. The results obtained so far suggest that rhizobia and other non-nodulating bacteria phylogenetically related to them represent a group of microorganisms that should be better studied, regarding their application to cultures of the Poaceae family. The objective of this work was to prospect and characterize populations of rhizobia associated with the most used variety of sugarcane in Northeast Brazil, cultivated in soil with a long history of sugarcane exploitation, and to evaluate the effect of intercropping with nodulating legumes on these populations, through isolation, followed by genetic and functional characterization. An experiment was set up in the municipality of Carpina-PE, Zona da Mata de Pernambuco. The treatments were distributed in randomized blocks with four replications, representing by single crop and the intercropping of sugarcane with different species of grain-producing legumes (cowpea, peanut, and soybean). At 5 months after sugarcane planting, non-lignified roots were collected, which were macerated in saline solution and the suspension product was used as an inoculant in bait plant trials, using the same legumes of the consortia. The isolates obtained from the formed nodules were genetically characterized, through 16S rRNA sequencing, and phenotypically in YMA medium. The isolates were tested for their ability to promote growth and biomass production and to change morphoanatomical parameters of sugarcane roots, in a pre-sprouted seedling inoculation experiment of legume growth in a greenhouse experiment. In the first trial with bait plants, nodules were only formed in cowpea inoculated with the suspension prepared from the roots of sugarcane cultivated in consortium with that legume. Results indicated the lack of rhizobia populations capable of nodulating soybeans and peanuts in sugarcane tissues, as well as demonstrating that cowpea cultivation was necessary to stimulate the presence of their microsymbionts in grass roots, probably by stimulating populations naturally established into the soil. These nodules generated a collection of 20 authenticated rhizobia, with low phenotypic and genetic diversity, with 4 isolates classified as Bradyrhizobium and 7 as Rhizobium. Among all the isolates in the collection, 55% solubilized calcium phosphate and all were capable of producing AIA. The thickness of the canopy, cortex, and vascular system, as well as the density and length of root hairs in the sugarcane root were positively influenced by inoculation.