Root Exudates Mediate Bacillus velezensis FZB42’s Colonization-Independent Biocontrol in Maize

Bacillus inoculants often show unstable performance due to poor colonization, making it crucial to explore colonization-independent biocontrol mechanisms. While rhizosphere microbial communities and root exudates influence plant disease resistance, it remains unclear if these changes depend on Bacillus colonization. This study analyzed the rhizosphere bacterial community and root exudates of maize in response to Bacillus velezensis FZB42 and the colonization-defective mutant FZB42ΔcheA. Both treatments significantly altered the root exudate composition. Notably, hydrocinnamic acid and tryptophol, which were upregulated more than 50-fold in both FZB42 and ΔcheA treatments, were confirmed to enhance maize resistance against Dickeya zeae. These compounds significantly reduced the pathogenicity of D. zeae with minimal effects on bacterial survival. Moreover, inoculation with FZB42 and the ΔcheA mutant reduced bacterial community diversity while increasing the abundance of beneficial bacteria, such as Cyanobacteria and Azospirillum, thereby indirectly enhancing maize resistance. This study offers a new understanding of the colonization-independent biocontrol mechanisms of Bacillus spp.