Exiguobacterium acetylicum Strain SI17: A Potential Biocontrol Agent against Peronophythora litchii Causing Post-Harvest Litchi Downy Blight

Abstract

Litchi downy blight (LDB) caused by Peronophythora litchii destroys 20–30% of litchi fruit every year and causes significant economic losses. Some Exiguobacterium strains exhibit considerable promise in both agricultural and industrial sectors. E. acetylicum SI17, isolated from the litchi fruit carposphere, demonstrated significant biocontrol activity against LDB through pre-harvest treatment. To elucidate its underlying regulatory mechanisms, the genome of SI17 was sequenced and analyzed, revealing a circular chromosome spanning 3,157,929 bp and containing 3541 protein-coding genes and 101 RNA genes. Notably, 94 genes were implicated in the production of secondary metabolites. Among the 29 Exiguobacterium strains so far sequenced, SI17 possessed the largest genome. In the phylogenomic analysis encompassing the entire genome, SI17 was clustered into Group I. Treating litchi fruit with SI17 before harvesting resulted in a decrease in H2O2 content in the fruit peel and an increase in superoxide dismutase activity, thus enhancing resistance to LDB. Interestingly, SI17 did not display plate antagonism against Peronophythora litchii SC18. It can be inferred that SI17 generates secondary metabolites, which enhance litchi’s resistance to LDB. This study represents the first documentation of an Exiguobacterium strain exhibiting a role in litchi plant disease and showcasing significant potential for the biological control of LDB.