Endophyte Inoculation Enhances Growth, Secondary Metabolites and Biological Activity of Endostemon obtusifolius Grown Under Drought Stress

Abstract

There is a need to cultivate medicinal plants to meet the growing demand. Their cultivation is hampered by extreme environmental conditions such as drought that affect plant growth and its pharmacological potential. Application of stress-tolerant endophytic species may potentially attenuate these negative impacts. This study assessed the effects of individual and co-inoculation of two native endophytic species (bacterium Paenibacillus polymyxa and fungus Fusarium oxysporum) on growth, physiological responses, metabolite accumulation and therapeutic efficacy of Endostemon obtusifolius subjected to varying watering regimes (well watered, mild and severe stress) under greenhouse conditions. Drought stress negatively affected root and shoot biomass, carotenoid content, chlorophyll fluorescence and relative water content in E. obtusifolius. Electrolyte leakage and malondialdehyde and hydrogen peroxide accumulation increased with drought stress. Individual and co-inoculation endophyte treatments significantly improved growth and stress tolerance mechanisms via increased osmolyte production (soluble sugars, proline), up-regulation of the enzymatic antioxidant system (superoxide dismutase) and increased antioxidant metabolite content (total phenolics, flavonoids). Antioxidant (DPPH, FRAP) and in vitro α-glucosidase activity of ethyl acetate leaf extracts were negatively affected by water stress but significantly improved when plantlets were subjected to endophyte inoculation. The most active extracts were from plants subjected to mild water stress with co-inoculation. Thus severe drought stress negatively affected growth and therapeutic efficacy of E. obtusifolius. Inoculation with beneficial endophytes enhanced the biochemical responses, osmoregulatory network and improved the therapeutic efficacy of E. obtusifolius.