The role of endophytes in secondary metabolites accumulation in medicinal plants under abiotic stress

Endophytes are microbial symbionts of plants which dwell within the internal tissues of healthy plants without the manifestation of any infection symptoms on their hosts. In addition to being vital to the growth and development of their host, endophytes also assist their medicinal plant host species in accumulating some highly coveted plant secondary metabolites under various abiotic stress conditions. The type and quantity of secondary metabolites accumulated by medicinal plants under abiotic stress rely on the age and developmental phase of the host as well as the taxonomic groupings of both symbionts. Medicinal plants during their life cycle become vulnerable to different types of tissue-damaging abiotic stress factors that hamper their survival and productivity. These stressors are connected to an appreciable build-up of reactive oxygen species (ROS) in plant tissues. ROS are a reactive group of chemicals that interact with several metabolites and cellular molecules, thus prompting irreversible metabolic anomalies and cell necrosis. Reactive oxygen species also serve as stress signals to medicinal plants and to ensure their survival; these plants produce antioxidants (both enzymatic and non-enzymatic) to maintain the integrity of their system. Some of the antioxidants are, however, produced as a result of the symbiotic relationship between medicinal plants and their endophytes. In this review, we aim to examine the interaction between endophytes and their medicinal plant host under abiotic stress.