A Critical Review of Knowledge Gaps in PGPR-Based Enhancement of Medicinal Plant Production

The therapeutic value of medicinal plants is attributed to the accumulation and constituents of secondary metabolites, such as phenolics, flavonoids, and terpenoids. Plant growth-promoting rhizobacteria (PGPR) provide an efficient approach to enhancing plant growth, development, health, and production. PGPR genera including Bacillus, Streptomyces, and Pseudomonas stimulate plant growth and development through a variety of mechanisms, including increased secondary metabolites, phytohormone production, and phytopathogen biocontrol, and thus promote sustainable agriculture. Africa is abundant in medicinal plants; however, the underrepresentation of PGPR research in Africa and the impact of specific environmental stress factors, such as drought, poor soil fertility, and climate variability, make PGPR-based sustainable cultivation strategies highly relevant. Medicinal plants are therapeutic in nature and, similar to crop plants, are susceptible to environmental stressors. Research optimizing production in both quality and quantity of these plants is essential, and hence, PGPR-mediated mechanisms that may enhance plant growth, stress tolerance, nutrient uptake, and sustainable cultivation practices are of interest. This review seeks to identify the gaps and limitations in recently published articles that focus on the application and influence of PGPR on the production of medicinal plants to inform future research on these rhizobacteria as well as optimize the use of PGPR. To collect relevant data, a variety of databases were used for literature search, such as Scopus, PubMed, ScienceDirect, and Web of Science. Several studies have demonstrated the beneficial effects of PGPR on the biosynthesis and accumulation of secondary metabolites in medicinal plants. The review of these studies revealed major limitations, including limited research conducted within Africa as well as a lack of validation of the bioactivity of the identified secondary metabolites. To address the limitations mentioned, it is important to conduct more continent-based research on medicinal plants with a focus on the application of PGPR in Africa.