Arbuscular mycorrhizal colonization alters biochemical, molecular defense responses and root exudate composition against Phytophthora capsici infection in black pepper

Mycorrhizal symbiosis benefits the host plants in several aspects. Among the soil microbiota, AM fungi have established a substantial potential to decrease crop damage from plant pathogens in several crops, yet the mechanism of its action in black pepper has not yet been studied. In this study, we investigated the effect of mycorrhizal colonization on biochemical, molecular defense responses, and root exudates composition in the Phytophthora capsici -black pepper host-pathosystem. Four biochemical parameters such as total phenols, orthodihydroxy (OD) phenols, peroxidase activity, and lignin were studied to delineate the effect of mycorrhizal colonization upon pathogen inoculation. Total phenol, OD phenol, and lignin content increased consistently upon challenge inoculation with P. capsici as well as in AM alone inoculated plants in both leaves as well as roots. Similar peroxidase activity was noticed in AM alone and challenged inoculated plants. The quantitative polymerase chain reaction analysis showed that AM pre-inoculation led to up regulation of pathogenesis related genes viz., cAPX, Osmotin and β-1,3-glucanase, phenylalanine ammonia-lyase and NPR 1 in black pepper leaves and roots upon of P. capsici inoculation. Sole inoculation of P. capsici also positively influenced the copies of most genes studied. Root exudates of AM colonized plants showed higher fatty acid/fatty acyl compounds viz., tetradecanoic acid, n-hexadecanoic acid, octadecanoic acid, and nonacos-1-ene compared to P. capsici inoculated plants. On the other hand, most of the alkane hydrocarbons such as (z)-3-tetradecene, octylcyclohexane, 4-cyclohexylundecane, and decylcyclohexane were observed majorly in the root exudates of P. capsici inoculated plants. Multivariate analysis of the root exudate data revealed that AM colonization in black pepper roots normalized the content of root exudates during Phytophthora infection comparable to control plants. Our findings indicates that the AM colonization enhances the resistance of P. capsici in black pepper plants by alterations in defense enzymes and gene expression and root exudates composition.