The application of actinobacterial extracts for plant growth promotion

Abstract

Environmental stress conditions such as drought and salinity are some of the limiting factors affecting crop yield. The frequency of these environmental stress conditions is predicted to increase with the effects of climate change over the years posing a threat to global food security. To overcome these declines, farmers depend on chemical fertilisers which are both expensive and increasingly becoming a major health concern. The use of plant growth-promoting bacteria (PGPB) as bioinoculants is viewed as a promising avenue for sustainable agriculture, especially under environmental stress conditions. In this study, 27 actinobacterial strains isolated from South African medicinal plants Aloe ferox and Sutherlandia frutescens, were evaluated for their potential as candidate bioinoculants under normal and stressed (drought and salinity) conditions. Phenotypic analysis of the isolates using International Streptomyces Project (ISP) media, coupled with molecular identification through 16S rRNA and gyrB gene sequencing revealed that the studied isolates were predominantly Streptomyces strains, alongside two rare actinobacterial strains – Micromonospora and Thermomonosporaceae. Isolates were screened for both direct - (including ammonia production, siderophore production, and IAA activity) and indirect - (including antimicrobial activity and hydrolytic enzyme production such as cellulase and xylanase activity) plant growth-promoting properties. Several isolates had broad activity across all assays, suggesting a multifaceted plant growth-promoting potential. Nine isolates were selected for inclusion in plant growth trials comparing actinobacteria-treated plants to untreated control plants grown under normal, drought, and salinity stress conditions. Notably, some actinobacteria-treated plants were observed to have enhanced plant height, root development, and improved stress tolerance especially under drought conditions. These findings suggested the potential of these isolates in mitigating osmotic stress. Furthermore, genome sequencing coupled with antiSMASH analysis revealed the presence of several of biosynthetic gene clusters associated with stress mitigation and plant growth promotion. Particularly, gene clusters encoding ectoine, siderophores, and naringenin biosynthesis were identified, suggesting potential evidence for the observed drought tolerance in plant growth trials. Overall, the findings of this study revealed that medicinal plants are a potential pool of untapped microbial diversity with promising potential as bioinoculants. Integrating culture-based methods with molecular techniques provided a comprehensive understanding of the functional potential of these actinobacterial isolates as potential bioinoculants, especially under drought stress conditions. This polyphasic approach highlights the potential of actinobacteria associated with medicinal plants as a sustainable alternative for plant growth and stress mitigation.