Investigating the plant growth promoting and biocontrol properties of actinobacteria associated with indigenous plants

Abstract

Actinobacteria are important microorganisms that maintain microecological balances and facilitate nutrient cycling within the rhizosphere. This study aimed to determine whether actinobacteria isolated from two indigenous medicinal plants (Aloe ferox and Sutherlandia frutescens) could be utilised to enhance plant growth and serve as a bio-control agent against harmful phytopathogenic fungi. Using 16S rRNA gene sequencing and whole-genome sequencing, sixteen isolates were identified across multiple genera, with eleven classified as Streptomyces species and others representing Actinomadura, Amycolatopsis, and Micromonospora. Five isolates showed < 98.65% sequence similarity, potentially representing novel species. Isolate A81 demonstrated excellent nitrogen fixation ability with the highest growth (314.2 mm²) on no-nitrogen medium, while simultaneously producing superior ammonia levels (4 μM/ml). Beyond A81's metabolic capabilities, it also exhibited strong antifungal activity against wheat Fusarium pathogens and successfully enhanced bean plant growth in small-scale plant experiments. Similarly, isolate S25 demonstrated high versatility across multiple functional domains, combining strong nitrogen fixation abilities (283.5 mm²) with efficient nutrient mobilisation through robust phosphate solubilisation and exceptional siderophore production. These growth-promoting attributes were complemented by effective biocontrol properties, as S25 achieved the highest antifungal activity (scale 1-2) in microcosm experiments and exhibited remarkable chitin degradation potential (258 mm² zones). Isolate S3 proved equally multifaceted, combining strong nitrogen-fixation, exceptional biosurfactant production (73.5% emulsification) while also producing potent antimicrobials and displayed the largest inhibition zones against both E. coli (904.8 mm²) and S. aureus (1123.9 mm²). Similarly, isolate S41 displayed outstanding auxin biosynthesis, high levels of siderophore activity, as well as its remarkable antifungal efficacy in reducing wheat Fusarium growth to just 0.8-1.4 cm. Completing this group of top-performing isolates (hereafter denoted as "champion" isolates), A37 demonstrated effective siderophore production and achieved outstanding antifungal activity with consistent pathogen growth of 72-80% reduction and reliable inhibition across spore-based assays.