RESUMO
Water stress is a major limiting factor for agricultural production under current and projected climate change scenarios. As a sustainable strategy, plant growth-promoting bacterial consortia have been used to reduce plant water stress. However, few studies have examined the effects of stress on multi-trait efficiency and interactivity of bacterial species. In this study, we used several in-vitro experiments, plant assays and greenhouse trials to investigate the effects of stress and bacterial consortia on 1-aminocyclopropane-1-carboxylic acid deaminase (ACCD) activities, indole-3-acetic acid (IAA) production and plant growth-promoting traits (Phosphate-solubilization, starch hydrolysis, siderophores and ammonium production). We further assessed biofilm formation and the chemotactic behaviour in response to ACC. A total of fifteen ACCD rhizobacteria with multiple growth-promoting traits from the dominant plant species from the hyperseasonal Aripo Savannas were screened in this study. Five of the isolates were further analyzed based on their ACCD activities and were tested in single and dual consortium to assess their abilities in promoting growth under simulated drought stress (-0.35 MPa) and chemically induced ACC conditions (0.03 mM). Our findings showed that bacteria which produce high concentrations of IAA affected the isolates' ability to promote growth under stress, irrespective of microbial combination with ACCD activity above the minimal threshold of 20 nmol α-ketobutyrate mg-1 h-1. Biofilm production with co-culture interaction varied greatly across treatments, however, the general trend showed an increase in biofilm under stress induce conditions. The best performing co-culture, UWIGT-83 and UWIGT-120 (Burkholderia sp.) showed enhanced growth in germination assays and in greenhouse trials with Capsicum chinense (Moruga red hot peppers) under drought stress, when compared to non-inoculated treatments. The findings highlight the importance of testing interactivity of bacterial species with multiple growth promoting traits under stress conditions; and proposed the use of ACC growth media as a novel biofilm screening method for selecting potential stress plant growth-promoting bacteria. Better screening strategies for appropriate plant growth-promoting bacteria may narrow the inconsistency observed between laboratory and field trials.