Evaluation of seed-borne fungus Alternaria brassicicola effects on production and expression of genes involved in thymol and carvacrol biosynthesis pathway in thyme (Thymus vulgaris L.)

Thyme (Thymus vulgaris L.) is rich in phenolic monoterpenes such as thymol and carvacrol. Seed-borne fungi can reduce the quality and quantity of crop and also change the amount and type of secondary metabolites in plants by affecting seed health. The aim of this study was to isolate and identify the seed-borne fungi from thyme seed samples and to evaluate the effects of seeds infection on the production and expression of genes involved in the biosynthesis pathway of thymol and carvacrol compounds. To identify seed-borne fungi of thyme seed samples collected from fields of Alborz province, sampling was done according to the criteria of the International Seed Testing Association (ISTA). The amount of thymol and carvacrol compounds was measured using high performance liquid chromatography (HPLC) and the expression of TvTPS1, DXR, CYP178, and CYP180 genes was assayed by reverse transcription polymerase chain reaction (RT-PCR). Six Alternaria brassicicola isolates were identified based on morphological and molecular characteristics. Analysis of cell wall degrading enzymes activity revealed that cellulase and pectinase were more important than xylanase and chitinase on the pathogenicity and aggressiveness of isolates. Seeds infection by A. brassicicola significantly affected vigor and seed germination indices. The findings of this study showed that the pathogenicity and aggressiveness of A. brassicicola fungal isolates can change the expression level of different genes encoding thymol and carvacrol compounds in seedlings obtained from seed pretreatments with different isolates through different mechanisms including production of extracellular enzymes resulted in increasing (isolates Ab4 and Ab5) and/or decreasing (isolates Ab1, Ab6, Ab2, and Ab3) the production and accumulation of phenolic monoterpenes in thyme seedlings. This is the first report on the identification of seed-borne fungus A. brassicicola from thyme seed samples in Iran and also the role of seeds infection on the production and expression of genes involved in the thymol and carvacrol biosynthesis pathway.