Genome of Mango Endophyte Bacillus velezensis ML21 Reveals Antimicrobial Arsenal

Deep inside the flesh of a seemingly healthy mango, a microscopic guardian was quietly doing battle. Scientists have now decoded the complete genome of that guardian, a bacterium named Bacillus velezensis ML21, and the results read like an inventory of natural weapons against some of agriculture’s most stubborn plant diseases. The study, carried out by a team at Guangxi University in China and published in the journal 3 Biotech, presents the first complete genome sequence of this endophytic strain, which had already demonstrated remarkable antagonistic activity against both bacterial and fungal pathogens of mango and rice. What the sequence reveals is a bacterium that is, in genomic terms, exceptionally well armed.

The sequencing effort produced a single circular chromosome of 3,929,792 base pairs with a GC content of 46.5 percent, a size and composition typical of members of the Bacillus subtilis species group but notable for its completeness and quality. Annotation of the genome identified 3,781 protein-coding genes, alongside 27 ribosomal RNA genes, 86 transfer RNA genes and 5 non-coding RNA genes, indicating a highly functional translational apparatus consistent with a fast-growing, metabolically versatile organism. Beyond the core gene complement, the researchers detected four genomic islands, four prophages and four CRISPR sequences, features that speak to a dynamic evolutionary history shaped by horizontal gene transfer and viral assault. Prophages can carry genes that influence host physiology and competitiveness, while CRISPR arrays record past encounters with invading genetic elements, together providing a fingerprint of the ecological pressures this bacterium has survived within its plant host.

The most striking discovery, however, lies in the bacterium’s chemical arsenal. ML21’s genome harbors no fewer than twelve secondary metabolite gene clusters, the genetic blueprints for complex molecules that microbes use to compete, defend and communicate. Seven of these clusters show high similarity, either 100 percent or 82 percent, to known clusters responsible for synthesizing some of the most celebrated antimicrobial compounds in the Bacillus repertoire: the nonribosomal lipopeptides surfactin, fengycin and bacilysin, and the polyketides difficidin, macrolactin H and bacillaene. Each of these molecules has an established reputation in the biocontrol literature. Surfactin is a powerful biosurfactant that disrupts bacterial membranes and can induce systemic resistance in plants. Fengycin attacks fungal cell membranes and is considered one of the most potent antifungal lipopeptides produced by Bacillus species. Bacilysin, a simple dipeptide antibiotic, interferes with protein synthesis in competing bacteria. Difficidin and macrolactin are broad-spectrum polyketide antibiotics, while bacillaene inhibits protein synthesis in prokaryotes, quietly suppressing microbial rivals before they can establish infection.

The fact that these clusters are present together in a single genome explains much of ML21’s observed potency in laboratory antagonism assays. Rather than relying on one mechanism, the strain deploys a layered defense: lipopeptides that puncture membranes, polyketides that sabotage protein production, and siderophores that starve competitors of iron. Two siderophore clusters, those for bacillibactin and butirosin A/B, were also identified, although they showed much lower similarity to reference sequences, at only 7 percent, hinting at possible structural variation worth further biochemical investigation. Siderophores are small molecules that chelate iron in the environment, and by sequestering this essential nutrient, biocontrol bacteria can deprive pathogens of a resource they need to grow and cause disease.

Perhaps the most tantalizing aspect of the study is the identification of four completely novel secondary metabolite clusters, predicted to encode two terpenes, one lanthipeptide and one polyketide. Terpenes are a vast and chemically diverse class of natural products, many of which possess antimicrobial or signaling properties. Lanthipeptides are members of the ribosomally synthesized and post-translationally modified peptide family, whose members often exhibit potent antibacterial activity through novel mechanisms. The existence of uncharted biosynthetic machinery in ML21 raises the possibility of entirely new antimicrobial compounds waiting to be isolated and characterized, a prospect that will interest natural product chemists as much as agricultural scientists. In an era when antibiotic discovery has slowed to a trickle and resistance genes are spreading across pathogens of both plants and humans, the genomes of environmental and endophytic bacteria have become one of the most promising hunting grounds for new chemistry.

ML21 is not only a fighter; it is also a gardener. The genome contains genes implicated in promoting plant growth, including trpC, which participates in the biosynthesis of indole-3-acetic acid, the principal auxin hormone that stimulates root development and overall plant vigor, and acoA, involved in producing acetoin, a volatile organic compound known to elicit induced systemic resistance in plants. Acetoin emitted by rhizobacteria has been shown to prime plant immune defenses, effectively vaccinating the host against subsequent pathogen attack. The presence of both direct growth promotion genes and defense-triggering pathways suggests that ML21 may benefit its host through multiple simultaneous routes: improving nutrient uptake and hormonal balance while actively suppressing disease organisms and stimulating the plant’s own immune responses.

The ecological context of ML21 adds further weight to these findings. As an endophyte isolated from mango fruit, the bacterium lives inside plant tissue, a privileged niche where it can interact closely with its host and with pathogens attempting the same invasion. Mango cultivation suffers from serious bacterial and fungal diseases, including bacterial black spot caused by Xanthomonas citri pv. mangiferaeindicae, a pathogen whose genome has been sequenced in recent years, as well as postharvest rots that cause substantial losses between orchard and market. The authors’ earlier work had shown that ML21 inhibits a variety of rice pathogens as well, suggesting a broad host-relevant activity that could extend beyond a single crop. The new genome sequence now provides the mechanistic explanation: a dense concentration of antimicrobial biosynthetic clusters that few competing pathogens could easily withstand simultaneously.

The broader significance of the study lies in the mounting scientific and commercial interest in Bacillus velezensis as a biocontrol agent. The species has emerged over the past decade as one of the most promising candidates for replacing or reducing synthetic pesticides and chemical fertilizers in sustainable agriculture. Its spore-forming ability allows formulation into stable products with long shelf life, its lipopeptides degrade readily in the environment, and its plant growth-promoting traits offer yield benefits alongside disease suppression. By providing the complete genome of a fruit-derived endophytic strain, the Guangxi team has added a valuable reference resource for comparative genomics, enabling researchers to pinpoint the genetic differences that determine why some strains are exceptional antagonists while close relatives are not. The genome has been deposited in GenBank under accession number NZ_CP150636, making it freely available to the research community.

Looking ahead, the genomic blueprint of ML21 opens several concrete avenues. The four novel biosynthetic clusters invite heterologous expression or fermentation studies to determine what compounds they actually produce and whether those molecules are active against pathogens. The complete set of lipopeptide and polyketide genes supports targeted strain improvement through metabolic engineering, potentially boosting yields of the most effective antimicrobials. And the combination of biocontrol and plant growth-promoting genes strengthens the case for field trials in mango orchards and rice paddies, where the strain’s dual talents could translate into reduced chemical inputs and healthier harvests. In a small chromosome of fewer than four million base pairs, this mango-dwelling microbe has packed an encyclopedia of defensive chemistry, and scientists are only beginning to read it.

Subject of Research: Complete genome sequencing of the biocontrol endophytic bacterium Bacillus velezensis ML21 isolated from mango fruit

Article Title: Complete genome sequencing of Bacillus velezensis ML21, an endophytic strain from mango fruit with strong inhibitory against bacterial and fungal diseases

Article References: Complete genome sequencing of Bacillus velezensis ML21, an endophytic strain from mango fruit with strong inhibitory against bacterial and fungal diseases. (n.d.). https://doi.org/10.1007/s13205-026-04994-7

Image Credits: AI Generated

DOI: 10.1007/s13205-026-04994-7

Keywords: Bacillus velezensis, whole-genome sequencing, secondary metabolite gene clusters, biocontrol, endophytic bacteria, mango, lipopeptides, surfactin, fengycin, plant growth promotion, sustainable agriculture, plant pathogens