Improving Growth and Stimulating Natural Defenses in Spearmint with Four Compatible PGPR Strains

Mint cultivation requires heavy use of chemical pesticides to manage pathogens and insect pests. To develop a biological alternative to these pesticides, we investigated the potential of plant growth-promoting rhizobacteria (PGPR) to stimulate the natural defense mechanisms of mint, potentially inducing systemic acquired resistance. In the present study, we utilized a bacterial consortium comprising four PGPR strains, Sphingobacterium suaeda, Pseudomonas sp., Bacillus cereus and Bacillus pumilus, to promote mint growth and activate natural defense mechanisms. The defense mechanisms of mint were analyzed by measuring phenylalanine ammonia lyase (PAL) activity at both the root and aerial levels, a key enzyme in phenolic and lignin metabolism, along with the levels of phenolic compounds and lignin. The results showed that the PGPR consortium substantially boosted mint growth, leading to a 28% increase in the number of leaves, a 25% increase in shoot height, a 34% improvement in aerial dry biomass, and an 80% increase in root biomass. Similarly, the PGPR consortium stimulated PAL activity and increased the levels of phenolic compounds and lignin in both the roots and shoots. The stimulation of these remote defense mechanisms at the shoot level evokes a systemic defense reaction known as the expression of systemic acquired resistance in plants. These results present promising opportunities for improving the bioprotection of mint against pathogens.