Bacillus amyloliquefaciens var. plantarum strain D747: directly active and resistance inducer towards bacterial leaf spot of tomato.

The strain D747 of Bacillus amyloliquefaciens var. plantarum (Bam), the active principle of the bio-fungicide Amylo-X, was applied in vitro, to evaluate its ability in directly inhibiting the growth of Xanthomonas vesicatoria (Xv, strain IPV-BO 2684), the causal agent of bacterial leaf spot of tomato. Under climatic chamber conditions, Bam, as water suspensions of spores (SPS; 2 g/L of Amylo-X) and 24 h-old living cells (LCS; ca. 107 CFU/mL), was applied at leaves and roots of tomato plants 'VF10'; after 48 h, the plants were inoculated with the Xv strain. Sterile distilled water (SDW), streptomycin sulphate and acibenzolar-S-methyl were used as negative and positive controls, respectively. Moreover, the transcription abundance of selected genes related to the plant defence was investigated in response to LCS treatment at the root apparatus. The strain D747 was directly active in vitro by significantly inhibiting the Xv growth. Under high infection pressure, the tomato plants treated at leaves with Amylo-X (SPS) showed a significantly lower disease severity compared to SDW-treated plants, with a relative protection of ca. 53%; also the tomato plants treated at the roots with LCS showed a significant reduction of the disease severity, providing a relative protection of ca. 45%. The transcripts abundance of several genes (i.e. erf1, pr-1, pr-4), related to the host defence response, resulted significantly higher than that of control plants from 1 to 48 h after the root treatment, highlighting that Bam was able to induce a plant mediated response against Xv infection.