Combined inoculation of rhizobacteria with <i>Mesorhizobium</i> promotes growth, nutrient contents, and protects chickpea against <i>Fusarium redolens</i>

Chickpea (<i>Cicer arietinum</i> L.) is considered a cheap source of plant protein. In Mediterranean regions, and particularly in Tunisia, fungal attacks are likely to further aggravate drought stress and increase the economic vulnerability of chickpea production. Plant growth-promoting rhizobacteria (PGPR) and rhizobia have the potential to enhance plant growth and mitigate the adverse effects of biotic and abiotic stresses. The objective of this study was to isolate non-rhizobial rhizosphere bacteria from the soil and evaluate their ability to enhance plants' growth and symbiotic performance and to control chickpea wilt caused by <i>F. redolens</i>. A total of 26 bacterial isolates from rhizosphere soil samples were subsequently evaluated for their antagonistic properties against five phytopathogenic fungi (<i>Fusarium oxysporum solani, Fusarium oxysporum matthioli</i>, <i>Fusarium oxysporum MN-2</i>, <i>Fusarium oxysporum</i> 184, and <i>Fusarium rdolens</i>). Seven bacterial isolates demonstrated <i>in vitro</i> plant-beneficial characteristics and/or antagonistic activity against 5 <i>Fusarium</i> strains. Two bacterial strains including <i>Streptomyces diastaticus</i> subsp. <i>diastaticus</i> and <i>Bacillus subtilis</i> were chosen for additional investigation because they showed the greatest number of plant growth-promoting (PGP) traits and exhibited an antagonistic effect on pathogens. Assays conducted in pots showed that PGPRs co-inoculated with <i>Mesorhizobium</i> sp. Bj1 protected chickpea plants from <i>F. redolens</i> infection and enhanced plant growth and nutrient uptake. Pot experiments carried out in a greenhouse further demonstrated that the co-inoculation of chickpea plants with the bacterial strains and a <i>Mesorhizobium</i> strain lessened the severity of the <i>F. redolens</i> infection. These results suggest that co-inoculation with <i>S. diastaticus</i> subsp. <i>diastaticus</i> and <i>Mesorhizobium</i> sp. Bj1 may act as a helpful bioformulation to boost chickpea plants' growth and protect them from wilting. Other PGPR candidates included <i>Mesorhizobium</i> spp. and <i>B. subtilis</i> strains. Both <i>Mesorhizobium</i> sp. Bj1 and the uninoculated plants were used as controls. The association of PGPR with other inoculants potentially could substitute for chemical fertilizers, and testing of PGPR under field conditions will further elucidate their effectiveness on grain yields of chickpea.