Potencial antagônico de isolados de Bacillus Spp. no controle da mancha parda (Bipolaris orizae) em cultivares de arroz no sul do Tocantins - Brasil

Abstract

The fungus Bipolaris oryzae constantly threatens rice crops, causing the disease known as brown spot, which results in reduced grain quality and a significant reduction in yield. Efficient disease control in rice crops is a highly relevant factor in agriculture and involves the identification of antagonistic species. A promising method for controlling these diseases is biological control, which uses bacteria of the genus Bacillus sp.. In some cases, this approach has shown promising results, with reduced incidence of the fungus and improved plant health. However, its efficiency may depend on the type of crop and environmental conditions. This study aims to isolate and evaluate the antagonistic potential of three bacterial strains of the genus Bacillus as biocontrol agents against B. oryzae in two rice varieties (Predileta and Rubeita). Thus, the first part of the study consists of the sanitary analysis of the seeds, with the aim of identifying the fungi associated with this crop, as well as evaluating the pathogenic potential of these microorganisms. Another stage of the research involved the in vitro and in vivo application of the isolates. Molecular studies were also carried out to identify compounds related to antimicrobial activity against pathogenic fungi. Based on the results of the sanitary analysis, a high diversity of fungi associated with rice seeds of the Rubelita and Predileta varieties was observed, both before and after treatment. Among the fungi found, B. oryzae was shown to be highly pathogenic for rice crops, with disease transmission rates of 3 and 5%, respectively. In in vitro tests performed against B. oryzae, the isolates Bacillus sp. OP938255.1 and Bacillus sp. OP938696.1 demonstrated inhibition of mycelial growth from the second day of incubation, remaining constant with 50% inhibition after seven days. In contrast, B. velezensis stood out among the isolates, presenting a significant inhibition rate of 95%. In in vivo tests, the three isolates proved to be highly effective in antagonistic activity against the fungus, resulting in a reduction in the severity of the disease in Rubelita plants, whose rates were below 5%. The Bacillus sp. OP938255 strain was especially effective, significantly reducing disease severity when applied at a concentration of 1.0 × 107, reaching an index lower than 1. In the biochemical characterization, all tests were positive, except for Indole-Acetic Acid (IAA) and potassium solubilization. The B. velezensis species was the only one identified as a protease producer, suggesting its potential as a plant growth promoter. Regarding the molecular characterization of B. velezensis, 6,310,092 paired-end reads were generated for B. velezensis. The genome was estimated at 3,913,362 base pairs, with an average G+C content of 46.51%. Additionally, genes that play roles in the biosynthesis of ribosomal peptides and antifungals, such as surfactin and fengycin, which have the ability to inhibit the formation of conidia of pathogenic fungi, were identified. These findings highlight the viability of B. velezensis as a promising alternative for sustainable disease management in agricultural crops.