Biopriming vegetable seeds with metal-tolerant biocontrol agents to enhance tolerance to biotic and abiotic stress

Metal-tolerant biocontrol agents (BCAs) are able to confer tolerance towards pathogen and metal stress. In this study, the metal tolerant characteristics of Trichoderma asperellum and Pseudomonas fluorescens were first established via metal tolerance assay. The pak choy (Brassica rapa) and chilli (Capsicum annum) seeds were bioprimed with both isolates, sown into soils containing various concentrations of Zn, Cu, Cr and Pb; and inoculated with Fusarium solani and Pythium ultimum. Results from the metal tolerance assay revealed that T. asperellum demonstrated higher adaptability to all metals (TI = 0.89–1.00) (except towards Cd) compared to P. fluorescens (TI = 0.56–1.00). In pot trials, biopriming with BCAs maintained growth of seedlings (pak choy: 9.46–12.88 mm root length; 20.48–32.55 mm shoot length) (chilli: 20.45–26.94 mm root length; 40.59–45.57 mm shoot length) cultivated in soils with metal stress, irrespective of the absence or presence of pathogen (F. solani, P. ultimum). Pak choy seedlings bioprimed with P. fluorescens was more effective in tolerating metal stress and suppressing infection by P. ultimum, marked by lower proline level (0.58–0.76 µmol/g ft. wt.), total phenolic content (TPC, 57.94 mg GAE/100 g ft. wt.) and malondialdehyde level (MDA, 0.45 µmol/g ft. wt.) compared to seedlings bioprimed with T. asperellum. Improved seed tolerance may be correlated to the ability of T. asperellum and P. fluorescens in immobilizing metals that protects seeds from metal toxicity. In contrast, bioprimed chilli did not suppress disease infection by P. ultimum under metal stress. Our study demonstrated the potential of metal-tolerant BCAs in rendering tolerance to pak choy against biotic and abiotic stresses.