Diversidade de bactérias do gênero Bradyrhizobium como endofíticas e na rizosfera de duas cultivares de cana-de-açúcar

Abstract

The genus Bradyrhizobium is commonly found as endosymbiont in legumes (Fabaceae family), such as soybean (Glycine max), forming nodules on the roots and providing nitrogen from the atmosphere to these plants and to the soil. Recent research has shown the presence of bacteria from this genus inside roots of sugarcane (Saccharum spp.), a non-legume belonging to the Poaceae family. This work aims to know the population density of these bacteria associated with the roots of two sugarcane cultivars and to understand how the plant genotype (cultivar) affects the population composition of these bacteria found in the rhizosphere and the endosphere. Rhizosphere and root samples of two economically relevant sugarcane cultivars (RB867515 and IACSP95-5000) were collected in the experimental field of Embrapa Agrobiology, serially diluted and inoculated on gnotobiotic seedlings of cowpea used as trap plants. The most probable number technique was used to quantify the density of the rhizobia present in the samples and the isolates obtained from the nodules were cultivated to form pure cultures. BOX-PCR technique and sequence analysis of the ribosomal internally transcribed spacer region (ITS), recA housekeeping gene, and the symbiotic nodC gene were applied to better understand the genetic diversity of these rhizobia. Acetylene reduction test was applied to verify the presence of nitrogenase activity under free-living conditions. The presence of at least 15625 rhizobia g root -1 in rhizosphere samples and between 125 and 625 rhizobia g root-1 in the endosphere samples was observed. This shows that the density of rhizobia in the rhizosphere of sugarcane is significantly higher than in the interior of the roots for the two cane cultivars. A total of 34 isolates were obtained from the rhizosphere samples of each cultivar and 15 and 14 respectively from the roots of cultivars IACSP95-5000 and RB867515. Most of the isolates presented morphoculture characteristics of the genus Bradyrhizobium. Phylogenetic analyzes positioned most of the isolates in the superclade Bradyrhizobium elkanii. No correlation was observed between the plant cultivar and the abundance of Bradyrhizobium spp. in the roots and rhizosphere of sugarcane plants.