Rizobactérias alteram o fenótipo das plantas afetando as interações planta-herbívoro?

Abstract

The association of plants with rhizobacteria can contribute to increased growth, reproduction and resistance to insects and pathogens, due to changes in hormones and more efficient absorption of nutrients, mainly the application of nitrogen. This interaction also brings benefits to microorganisms, in addition to a favorable place for the development and formation of colonies, plants provide reduced carbon and other metabolites for these organisms. The effects of this symbiosis can be altered by biotic aspects, such as rhizobacteria species and lineage. Some strains tend to be less competitive and this can reduce effectiveness during colonization. Abiotic aspects must also be considered, as climatic adversities in the field can impair the stability of the colonies. Given this scenario, the objective of this study was to evaluate whether the inoculation of the bean plant (Phaseolus vulgaris) by the rhizobacteria Rhizobium tropici (CIAT 899) causes changes in the growth, reproduction and resistance of the plant to herbivorous insects. In addition, we investigated whether phenols, an important chemical group that acts in plant defenses, would be altered by the association with rhizobacteria. Two plantings were carried out to conduct the experiments, one in the dry season, from June to September, and the other in the rainy season, from September to December. The tests contained plants from the control (without inoculation) and Rhizobium (inoculated) treatments. We evaluated parameters related to growth (root biomass and aerial part, height and height with tendril) and reproduction (date of opening of flowers, number of flowers, number of pods and weight of grains) of the plants, but most of them were not changed due to Rhizobium inoculation in both plantations. Only the number of flowers from inoculated plants was greater than that of plants not inoculated in the dry season. The abundance and composition of the herbivorous insect community in the plants of the two treatments was evaluated and there was no significant difference between them in the two seasons. Despite that, in the dry season, a trend towards greater resistance of the inoculated plants was observed, which were 2 times less visited by herbivores than the non-inoculated plants. The analysis of total constitutive phenols (prior to the transfer of the plants to the field) showed that the inoculated plants had reduced phenol content in relation to the non-inoculated plants. However, after exposure to abiotic and biotic stresses in the field, this difference in total phenol levels was no longer observed. Contrary to our hypothesis, in which the effects of inoculation would be positive on the growth, reproduction and resistance of the bean plant to insects, the results indicate a neutral effect of R. tropici on the growth of the bean plant, but a discreet positive effect on reproduction and resistance that deserves further investigation.