β-aminobutyric acid primes lignin-associated resistance to Penicillium digitatum in postharvest lemon fruit involving ClMYB46L-mediated transcriptional activation

Green mold caused by Penicillium digitatum severely limits the postharvest storage of lemon fruit. β-Aminobutyric acid (BABA) is known to enhance disease resistance in harvested fruit, but the transcriptional basis by which BABA reinforces defense responses in infected lemon peel remains insufficiently understood. Here, BABA pretreatment slowed the development of green mold in postharvest lemon fruit, as reflected by smaller lesions, reduced fungal biomass, and less infection-associated tissue damage. RNA-seq analysis showed that BABA did not merely intensify the overall response to P. digitatum , but reshaped a defined subset of pathogen-responsive genes. Among the enriched pathways, phenylpropanoid biosynthesis was particularly prominent, with lignin biosynthesis-related genes showing stronger induction in BABA-pretreated infected fruit than in fruit inoculated with the pathogen alone. Consistently, BABA pretreatment increased lignin accumulation and enhanced phloroglucinol-HCl staining around inoculation sites. A MYB46-like R2R3-MYB transcription factor, ClMYB46L, was identified as a candidate regulator associated with BABA-primed lignin biosynthesis. ClMYB46L expression was induced by P. digitatum challenge and was further increased in BABA-pretreated inoculated fruit. EMSA and Y1H assays showed that ClMYB46L binds SMRE-containing promoter fragments of lignin biosynthesis-related genes, including Cl4CL , ClCCR , ClCCoAOMT , ClCOMT , ClCAD6 and ClCAD9 . Dual-luciferase assays showed that ClMYB46L activates the corresponding full-length promoters. Transient overexpression of ClMYB46L in lemon fruit increased lignin deposition, up-regulated lignin biosynthetic genes, reduced fungal biomass and restricted lesion development. These findings suggest that BABA-enhanced resistance in postharvest lemon fruit is associated with ClMYB46L-mediated activation of lignin biosynthesis and local cell wall reinforcement.