Boosting disease resistance in Solanum melongena L. (eggplant) against Alternaria solani: the synergistic effect of biocontrol Acinetobacter sp. and indole-3-acetic acid (IAA).

Abstract

Alternaria solani causes early blight disease in eggplants, threatening production and leading to significant economic losses. Fungicides are used to control fungal diseases, but their overuse raises resistance concerns. Finding novel, eco-friendly biocontrol agents is therefore a solution for the future. The coordination between antagonistic bacterial agents and plant growth hormones in defense responses against fungal pathogens are crucial. This study assessed biocontrol potential of Acinetobacter sp. SCR-11 (Accession no. OR751536.1) and indole-3-acetic acid (IAA; 100 µM), singly and in combination, against A. solani in eggplants. Strain SCR-11 produced hydrogen cyanide (HCN; 5.7 µg mL⁻¹), siderophore i.e. salicylic acid (14.7 µg mL⁻¹), 2,3-dihydroxybenzoic acid (23.1 µg mL⁻¹) and various extracellular lytic enzymes. Strain SCR-11 exhibited antagonistic activity by strongly inhibiting (82%) A. solani. Acinetobacter sp. inoculation and IAA treatment enhanced growth, biomass, and leaf pigments of A. solani-diseased eggplants, with effectiveness in order: SCR-11 + IAA > SCR-11 > IAA >. The combined treatments (SCR-11 + IAA) most effectively increased total soluble protein (62.5%), carbohydrate (60%), total soluble sugar (81%), and phenol (74%) in A. solani-infected eggplant. Biocontrol agent and IAA application significantly (p ≤ 0.05) reduced proline and malondialdehyde (MDA) levels, alleviating oxidative stress in A. solani-diseased eggplant. The SCR-11 + IAA treatment significantly reduced the percent disease index (71%) and increased protection (69%) in diseased eggplant. The Acinetobacter sp. and IAA coordination enhanced disease resistance in A. solani-infected eggplants by boosting defense enzyme activities (SOD, POD, PAL, and β-1, 3 glucanase), significantly protecting plants from pathogen attack. At harvest, soil populations of A. solani decreased, while SCR-11 populations increased significantly. Acinetobacter sp. and IAA work synergistically through pathogen suppression, plant growth promotion, and induction of plant defense responses. Thus, applying antagonistic PGPR strain with exogenous IAA enhances eggplant resistance to A. solani, providing an environmentally friendly agricultural solution.