Protection against biotic and abiotic stresses in common bean by rhizobacteria

Abstract

Biotic and abiotic stresses may interfere with the performance of plantassociated rhizobacteria. The objectives of this study were: (a) to evaluate the effect of the volatile organic compounds (VOCs) of Bacillus amylolicefaciens ALB629 and B. subtilis UFLA285 in anthracnose (Colletotrichum lindemuthianum - Cl) control; (b) to evaluate the effectiveness of ALB629 in promoting plant growth, nutrient uptake and the contribution of Mg supplementation to photosynthetic rates, CO2 assimilation, chlorophyll content, and bacterial wilt (BW) control caused by Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff) at ALB629 presence; (c) to find out whether exudates from seeds are able to interfere with ALB629 biofilm formation and growth; (d) to check the performance of ALB629 on plant health under drought stress. In vitro and in vivo tests were set up to assess the spore numbers and the mycelial growth of Cl as well as anthracnose control in the presence of ALB629 and UFLA285 VOCs. Additionally, bean plants from seeds treated with ALB629 were assessed for growth promotion-related variables, photosynthetic-related variables, and BW control when plants were grown in soil with different Mg contents (0–50 mg kg-1). Alternatively, ALB629 was tested for biofilm formation and for its effect on the plant-related variables under drought stress. ALB629 reduced spore numbers (31%), while UFLA285 and ALB629 inhibited mycelial growth of Cl (16–18%), respectively. Both bacterial volatiles controlled anthracnose in vivo (79–85%) and were identified as 3-hydroxy-2- butanone, 3-methylbutanoic acid, and 2-methylbutanoic acid. At 25 mg kg-1 Mg, an increased accumulation of CO2 was found in the leaf mesophyll of the ALB629 and control, indicating low CO2 fixation and low Rubisco activity. Higher doses of Mg caused an increase in chlorophyll content and in photosynthetic rates in rhizobacterium-treated plants. Furthermore, at 25 mg kg-1 Mg, there was an increase in chlorophyll content in ALB629 (30%) and a reduction in BW severity (51%). Moreover, photosynthesis was negatively correlated with BW (r = -0.53, p<0.01). Seed exudates increased ALB629 biofilm and the ALB629 cell counts both in culture and on the bean seed surface. Furthermore, seed exudates up-regulated biofilm operons in ALB629 TasA and EpsD by about 2- and 6-fold, respectively. Seedlings from seeds treated with ALB629rif-nal showed a higher concentration of the bacteria when the malic acid was present, showed a promotion in plants growth and imparted drought tolerance. This study showed that UFLA285 and ALB629 play a major role in improving common bean health against biotic and abiotic stresses.