Diversity and Plant Growth-Promoting Potential of Endophytic Actinobacteria Associated with Cleome africana Growing on a Heavy Metal-Contaminated Mining Site in Morocco

Endophytic actinobacteria represent an important component of microbial diversity and constitute a valuable resource for the development of sustainable agricultural bioinoculants. In this study, we explored the diversity and functional traits of culturable endophytic actinobacteria associated with Cleome africana, a pioneer plant naturally growing on the heavy metal-contaminated Kettara mining site in Morocco.

Six endophytic actinobacterial strains were isolated from root and shoot tissues and characterized using morphological, biochemical, and 16S rRNA gene analyses. Phylogenetic identification revealed representatives belonging to three genera: Streptomyces, Amycolatopsis, and Nocardia. The isolates exhibited remarkable adaptation to harsh environmental conditions, including high salinity (up to 2 M NaCl) and drought stress (up to 60% PEG 6000). Functional screening demonstrated multiple plant growth-promoting (PGP) traits, including phosphate solubilization, indole-3-acetic acid (IAA) production, siderophore synthesis, and ACC deaminase activity. Among the isolates, Amycolatopsis sp. A4 showed the highest phosphate-solubilizing capacity (198.03 mg L⁻¹ from tricalcium phosphate and 102.46 mg L⁻¹ from rock phosphate), whereas Streptomyces sp. A3 produced the highest amount of IAA (102.92 μg mL⁻¹).

Greenhouse experiments using Medicago sativa demonstrated that inoculation with selected strains (Streptomyces sp. A3 and Amycolatopsis sp. A4), either individually or as a consortium, significantly enhanced plant growth. Shoot and root dry biomass increased by up to 80% and 88%, respectively, compared with uninoculated controls. Inoculated plants also exhibited improved root development, shoot elongation, and leaf production.

These findings highlight the unexplored diversity of endophytic actinobacteria inhabiting metallophyte plants and demonstrate their potential as microbial resources for culture collections and future applications in sustainable agriculture, particularly in arid and semi-arid environments.