Compatible Consortium of Endophytic Bacillus halotolerans Strains Cal.l.30 and Cal.f.4 Promotes Plant Growth and Induces Systemic Resistance against Botrytis cinerea

SIMPLE SUMMARY: This study evaluated two Bacillus halotolerans-beneficial strains (Cal.l.30 and Cal.f.4) for the levels of their compatibility and their potential to be effective biofertilizers and in parallel biocontrol agents. We investigated their plant growth-promoting effect and ability to induce the expression of defense-related genes in the leaves of young tomato seedling plants, applying them individually or in combination under in vitro and greenhouse conditions using seed biopriming and soil drenching as inoculum delivery systems. The results showed that B. halotolerans strains (singly or in combination) are promising bioproducts for sustainable horticulture as they combine both direct antifungal activity against plant pathogens and the ability to prime plant immunity and enhance plant growth. ABSTRACT: Evaluating microbial-based alternatives to conventional fungicides and biofertilizers enables us to gain a deeper understanding of the biocontrol and plant growth-promoting activities. Two genetically distinct Bacillus halotolerans strains (Cal.l.30, Cal.f.4) were evaluated for the levels of their compatibility. They were applied individually or in combination under in vitro and greenhouse conditions, using seed bio-priming and soil drenching as inoculum delivery systems, for their plant growth-promoting effect. Our data indicate that application of Cal.l.30 and Cal.f.4 as single strains and as a mixture significantly enhanced growth parameters of Arabidopsis and tomato plants. We investigated whether seed and an additional soil treatment with these strains could induce the expression of defense-related genes in leaves of young tomato seedling plants. These treatments mediated a long lasting, bacterial-mediated, systemic-induced resistance as evidenced by the high levels of expression of RP3, ACO1 and ERF1 genes in the leaves of young tomato seedlings. Furthermore, we presented data showing that seed and soil treatment with B. halotolerans strains resulted in an effective inhibition of Botrytis cinerea attack and development on tomato leaves. Our findings highlighted the potential of B. halotolerans strains as they combine both direct antifungal activity against plant pathogens and the ability to prime plant innate immunity and enhance plant growth.