Alleviation of drought stress in the common bean (Phaseolus vulgaris L.) by co-inoculation with Paenibacillus polymyxa and Rhizobium tropici

A greenhouse experiment was performed to evaluate the influence of Rhizobium when co-inoculated with each of two Paenibacillus polymyxa strains, singly and in mixture on growth, nitrogen content, phytohormone levels and nodulation of the common bean (Phaseolus vulgaris L.) under three levels of drought stress. Stress was applied continuously by the control of matric potential (ψm) through a porous cup. Bean plants cv. Tenderlake were grown in pots with Fluvic Neosol eutrophic soil under three different ψm (S1 −7.0; S2 −70.0 and S3 < −85 kPa). The seeds were inoculated with Rhizobium tropici (CIAT 899) and each of P. polymyxa (DSM 36) and P. polymyxa Loutit (L) singly and in mixture (CIAT 899 + DSM36 + Loutit). Co-inoculation of bean with Rhizobium and both Paenibacillus strains resulted increased plant growth, nitrogen content and nodulation compared to inoculation with Rhizobium alone. This was particularly evident at the most negative ψm (S3 < −85 kPa) we used. Drought stress triggered a change in phytohormonal balance, including an increase in leaf abscisic acid (ABA) content, a small decline in indole acetic acid (IAA) and gibberellic acid (GA3) and a sharp fall in zeatin content in bean leaves. The content of endogenous Cks decreased under water stress, possibly amplifying the response of shoots to increasing ABA content. We hypothesize that co-inoculation of bean with R. tropici (CIAT 899) and P. polymyxa strains (DSM 36) and Loutit (L) mitigates some of the negative effects of drought stress on bean.