ABSTRACT Brazil nut is a very important nontimber forest product in the Amazon region. Propagation of this tree still represents a challenge due to slow and uneven seed germination. In this context, plant growth-promoting bacteria can facilitate the process of propagation. The aims of this study were to isolate and characterize endophytic bacteria from the roots of Brazil nut trees in native terra firme forest and cultivation areas in northern Brazil, and to identify mechanisms by which bacteria act in plant growth promotion. Overall, 90 bacterial isolates were obtained from the roots of Brazil nut trees in monoculture, agroforestry and native forest areas by using different semisolid media. The isolates were characterized by sequencing the 16S rRNA gene. Plant growth-promoting characteristics were evaluated by the presence of the nifH gene, aluminum phosphate solubilization and the production of indole compounds. The isolates were affiliated with 18 genera belonging to 5 different classes (α-Proteobacteria, β-Proteobacteria, γ-Proteobacteria, Bacilli and Actinobacteria). The genus Bacillus was predominant in the forest and monoculture areas. Fourteen isolates presented the nifH gene. Most of the bacteria were able to solubilize aluminum phosphate and synthetize indole compounds. The results indicated high diversity of endophytic bacteria present among the roots of Brazil nut trees, mainly in the agroforestry area, which could be related to soil attributes. Among the 90 isolates, the 22 that presented the best results regarding plant growth promotion traits were good candidates for testing in seedling production of Brazil nut trees.
RESUMO A castanha-do-brasil é um produto florestal não madeireiro muito importante na região amazônica. A propagação desta árvore ainda representa um desafio, devido ao lento e irregular processo de germinação das sementes. Neste contexto, bactérias promotoras do crescimento vegetal podem facilitar o processo de propagação. O objetivo deste estudo foi isolar e caracterizar bactérias endofíticas em raízes de castanha-do-Brasil em floresta de terra firme e em áreas cultivadas no norte do Brasil, e identificar alguns mecanismos de promoção do crescimento vegetal executados por essas bactérias. No total, 90 isolados bacterianos foram obtidos de raízes de castanha-do-Brasil em monocultura, agrofloresta e floresta nativa, usando diferentes meios de cultivo semi-sólidos. Os isolados foram caracterizados pelo sequenciamento do gene 16S rRNA. As características de promoção do crescimento vegetal foram avaliadas através da presença do gene nifH, solubilização de fosfato de alumínio e produção de compostos indólicos. Os isolados foram afiliados a 18 gêneros, pertencentes a cinco diferentes classes (α-Proteobacteria, β-Proteobacteria, γ-Proteobacteria, Bacilli e Actinobacteria). O gênero Bacillus foi predominante, principalmente nas áreas de floresta e monocultura. Quatorze isolados apresentaram o gene nifH. A maioria dos isolados foi capaz de solubilizar fosfato de alumínio e sintetizar compostos indólicos. Os resultados indicam uma elevada diversidade de bactérias endofíticas presente em raízes de castanha-do-Brasil, principalmente em área de agrofloresta, que pode estar relacionado aos atributos do solo. Entre os 90 isolados, 22 apresentaram os melhores resultados relacionados às características de promoção do crescimento vegetal, e são bons candidatos para testes em produção de mudas de castanha-do-Brasil.
ABSTRACT Inga edulis Mart. is a leguminous tree adapted to acidic and low-fertility soils that establishes symbioses with nitrogen (N)-fixing bacteria. The identification of effective bacteria in biological N fixation may bolster the use of I. edulis in degraded or modified areas and agroforestry systems. Therefore, the aims of this study were evaluation of the symbiotic effectiveness of eight strains of the Bradyrhizobium genus native to Roraima in Inga edulis plants, and in vitro evaluation of the ability of the eight strains of Bradyrhizobium to develop plant growth-promoting characteristics. Determination of symbiotic effectiveness was carried out via three experiments: the first in a greenhouse in pots with a sterile substrate; the second in a greenhouse in pots containing non-sterile soil; and the third in a nursery in bags with a non-sterile substrate. Twelve treatments were evaluated: inoculation with eight strains of Bradyrhizobium ingae (ERR 490, ERR 492, ERR 493, ERR 494T, ERR 496, ERR 497, ERR 498, and ERR 569); inoculation with two strains indicated for Inga marginata, BR 6609 and BR 6610 (positive controls); no inoculation but with mineral N; and neither inoculation nor mineral N. All of the experiments were conducted in a completely randomized design with four replicates. The first experiment was conducted for 60 days, and the other experiments were conducted for 100 days. For all of the experiments, the number of nodules, nodule dry matter, root dry matter, shoot dry matter, number of leaflets, plant height, stem diameter, total N in the shoots, root/shoot dry matter ratio, Dickson’s quality index, relative effectiveness, and the Pearson correlation between the variables under study were evaluated. The strains were also evaluated by their ability to solubilize calcium and aluminum phosphates and to produce indolic compounds. The results showed that B. ingae strains were effective in biological N fixation, especially the ERR 493, ERR 498, and ERR 569 strains. These strains increased the production of shoot dry matter and total N and exhibited relative effectiveness higher than 100 % in all of the experiments. The B. ingae strains were also able to solubilize calcium and aluminum phosphates, despite their synthesis of indolic compounds. Thus, the strains of B. ingae can be used for inoculation in the production of I. edulis seedlings.