Root Exudates Mediate <i>Bacillus velezensis</i> FZB42’s Colonization-Independent Biocontrol in Maize

<i>Bacillus</i> 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 <i>Bacillus</i> colonization. This study analyzed the rhizosphere bacterial community and root exudates of maize in response to <i>Bacillus velezensis</i> FZB42 and the colonization-defective mutant FZB42Δ<i>cheA</i>. Both treatments significantly altered the root exudate composition. Notably, hydrocinnamic acid and tryptophol, which were upregulated more than 50-fold in both FZB42 and Δ<i>cheA</i> treatments, were confirmed to enhance maize resistance against <i>Dickeya zeae</i>. These compounds significantly reduced the pathogenicity of <i>D. zeae</i> with minimal effects on bacterial survival. Moreover, inoculation with FZB42 and the Δ<i>cheA</i> mutant reduced bacterial community diversity while increasing the abundance of beneficial bacteria, such as <i>Cyanobacteria</i> and <i>Azospirillum</i>, thereby indirectly enhancing maize resistance. This study offers a new understanding of the colonization-independent biocontrol mechanisms of <i>Bacillus</i> spp.