Azelaic acid enhances tomato resistance to Alternaria solani via defense responses and lignin biosynthesis

Tomato (Solanum lycopersicum) yield is drastically lowered by Alternaria solani-induced early blight. This study evaluates azelaic acid as a resistance inducer against A. solani in susceptible tomatoes, with emphasis on defense mechanisms and lignin biosynthesis. Pathogenesis-related (PR) enzyme activities (e.g., PAL, LOX, chitinase, laccase, β-1,3-glucanase), phenolic and flavonoid compounds, and lignin deposition were measured after azelaic acid treatment and pathogen infection under controlled greenhouse conditions. qRT-PCR was used to determine gene expression of defense-related transcription factors (SINAC1, SIWRKY1, SIMAPK3, TPK1b) and defense genes for lignin biosynthesis, and lignin deposition was assessed by histology and spectrophotometry. Azelaic acid significantly suppressed disease severity, with LOX, chitinase, and laccase activities restored during infection, and PAL and laccase activities optimized at 24–48 h post-inoculation (hpi). PPO activity was 11.19 U/mg protein.min⁻¹ in inoculated plants at 96 hpi. Phenolic and flavonoid contents were high at 12 hpi in treated plants, whereas lignin deposition and gene expression (e.g., PAL) were highest in inoculated plants, further enhanced by azelaic acid-induced lignification. Increased lignin deposition, strengthening physical defenses, was also confirmed by histological analysis. These results affirm that tomato resistance against A. solani is induced by azelaic acid through the induction of PR enzymes, regulation of lignin biosynthesis, and defense gene expression. This multi-faceted approach highlights the potential of azelaic acid as an eco-friendly method of managing early blight disease.