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Phytoalexins are small antimicrobial molecules synthesized and accumulated by plants upon exposure to pathogens. Camalexin is an indole-derived phytoalexin, which is accumulated in plants including Arabidopsis thaliana, and other Brassicaceae, which plays a major role in disease resistance against fungal pathogens. The productivity of Brassica crops is adversely affected by Alternaria blight disease, which is caused by Alternaria brassicae. In Arabidopsis thaliana, MAP kinase signalling cascade is known to be involved in synthesis of camalexin, which contributes to disease resistance against a necrtrophic fungal pathogen, Botrytis cinerea. In the present study, MAPK signalling cascade leading to biosynthesis of camalexin that triggers defense responses in B. rapa upon exposure to the most devastating nectrophic fungus, Alternaria brassicae has been elucidated with the help of previously reported MAPK cascade in Arabidopsis thaliana, Molecular modelling, docking, and protein-protein interaction analysis of MAP kinases retrieved from Brassica rapa genome have been carried out to reveal the above cascade. The tertiary structure prediction of MAPKs obtained through molecular modelling revealed that all the protein models fulfil the criteria of being the stable structures. The molecular docking of predicted models for elucidating potential partners of MAPKs revealed strong interactions between MKK1, MKK4, MKK5, MAPK3 and MAPK6 with MKK9. The MAPK signalling cascade also shows different genes that express and play major role in camalexin biosynthesis in B. rapa during defense response to A. brassicae. The understanding of MAPK defense signaling pathway in B. rapa against devastating fungal pathogen Alternaria brassicae would help in devising strategies to develop disease resistance in Brassica crops.

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