Melon plants distinguish Bacillus strains; high dose of B. velezensis suppresses growth

An international research team from the Universidad de Málaga and the Spanish National Research Council reported that melon plants can tell closely related beneficial Bacillus bacteria apart. The study, published on 23 February 2026 in Horticulture Research, showed that Bacillus subtilis NCIB3610 consistently promoted root elongation, starch storage in chloroplasts and drought tolerance regardless of the amount applied. In contrast, Bacillus velezensis FZB42 exhibited a dose‑dependent effect: a low dose (10⁷ CFU mL⁻¹) supported normal root growth, while a high dose (10⁸ CFU mL⁻¹) temporarily suppressed it. The suppression required the combined action of surfactin, produced by both strains, and bacillomycin D, a lipopeptide unique to B. velezensis. High‑dose B. velezensis triggered broad transcriptional activation, including up‑regulation of retrotransposon‑related genes and down‑regulation of allene‑oxide synthase in the jasmonic acid pathway. Both bacterial treatments ultimately enhanced above‑ground stress resilience via different mechanisms—B. subtilis‑primed plants accumulated caffeic and rosmarinic acids to defend against Botrytis cinerea, while B. velezensis‑primed plants modulated jasmonic acid signaling to limit reproduction of the spider mite Tetranychus urticae. “What surprised us most was that even though these two Bac…”, the authors noted.