GC-MS-based metabolomics reveals the impacts of water deficit stress and endophyte inoculation on the accumulation of tentatively identified hypoglycemic compounds in Endostemon obtusifolius (E. Mey. ex Benth.) N. E. Br

Abstract

Endostemon obtusifolius, found in semi-arid areas of southern Africa, are commonly used to manage some human ailments, including diabetes. Interestingly, most of the ethnopharmacological potentials of E. obtusifolius remain largely unverified. Given its distribution, E. obtusifolius is prone to water deficit stress, which affects its growth and phytochemistry. Our previous findings showed that the inoculation of E. obtusifolius with endophytic species Paenibacillus polymyxa and Fusarium oxysporum enhances the growth attributes, stress tolerance and physiological parameters of the plant. Therefore, this study was aimed at evaluating the impact of individual and co-inoculation of P. polymyxa and F. oxysporum on the changes in some hypoglycemic compounds of E. obtusifolius subjected to different watering regimes using untargeted GC-MS metabolomics. Both inoculation (Paenibacillus, PI or Fusarium, FI) and co-inoculation (P + FI) significantly influenced the leaf metabolome of E. obtusifolius. Under the well-watered (WW) regime, P+FIWW treatment upregulated the highest number of hypoglycemic agents (caryophyllene, santalol-cis-⍺ and n-pentadecanol). Compared to the control, FIWW significantly increased benzoic acid, ⍺-pinene, trans-sesquisabine hydrate and aromanderene. Moreover, santalol, thunbergol and phyto-acetate were upregulated in the PIWW and P+FIWW under the well-watered treatments, whereas the co-inoculated (P+FISS) E. obtusifolius under severe water-deficit stress accumulated more α-phellandrene, α-pinene, and aromadendrene. Benzoic acid, on the other hand, was either unaffected or downregulated regardless of endophytic inoculation under the severe water deficit stress. Notably, the mild water deficit stress significantly enhanced the accumulation of hypoglycemic agents, and the effects were more marked in the PIMS and P+FIMS treatments. In conclusion, this study revealed that endophyte inoculation and water deficit stress have the potential to modulate the accumulation of bioactive compounds in medicinal plants.