Alginate-Based Bioformulation with Four Plant Growth-Promoting Bacteria for Sustainable Biostimulation of Spearmint Growth and Natural Defense

Spearmint is a medicinal and aromatic crop vulnerable to several diseases, and its cultivation relies heavily on chemical inputs, which can alter phytonutrient quality and raise environmental and health concerns. To address this, the current study focused on developing a safe bioformulation to promote growth and natural defense in spearmint. The bioformulation is based on four plant growth-promoting rhizobacteria (PGPR), Bacillus aryabhathai, Brevibacterium frigoritolerans, Bacillus vallismortis, and Pseudomonas frederiksbergensis, encapsulated in 2% natural alginate. The compositional, structural, and morphological properties of alginate powder and beads were characterized. The beads were evaluated for encapsulation efficiency, bacterial survival, release, and degradation in soil. The bioformulation was applied to the rhizosphere of spearmint and was assessed for its effects on plant growth and natural defenses. Extracted alginate exhibited high purity and structural integrity, and encapsulation efficiency reached 99.99%, providing long-term protection and bacterial viability while regulating their release. The encapsulated consortium notably increased the number of branches, height, and biomass in spearmint. Additionally, a potential induction of plant defense response was observed, as indicated by increased phenylalanine ammonia-lyase activity, higher phenolic compound levels, and lignin accumulation in the roots and shoots of treated plants. These findings highlight the biostimulating effects of these four PGPR encapsulated in alginate on spearmint’s defense and growth.