Medicarpin confers powdery mildew resistance in Medicago truncatula and activates the salicylic acid signaling pathway

Powdery mildew is a widespread fungal disease that causes significant reduction in total biomass and seed quality of agriculturally important legumes, including pea and mung bean. With the long-term goal of developing an eco-friendly and sustainable strategy for powdery mildew disease management, we studied the role of a class of plant secondary metabolites – isoflavonoids – during legume-powdery mildew interactions. These metabolites are predominantly synthesized in leguminous plants and are known to exhibit antimicrobial activity but their inhibitory role against powdery mildew is ambiguous.

In this study, we demonstrate that the branch of the phenylpropanoid secondary metabolism pathway leading to the synthesis of the isoflavonoid medicarpin plays a significant role in powdery mildew resistance in Medicago truncatula. We show that medicarpin accumulates at powdery mildew infection-sites and obstructs fungal penetration and post-penetration growth in host cells. We further show that a synergism between isoflavonoid and salicylic acid (defense phytohormone) pathways may be crucial for early PM resistance in M. truncatula. Our study highlights the role of medicarpin as an anti-fungal against powdery mildew as well as a potential signal molecule that regulates salicylic acid synthesis and signaling. These findings have important implications for agriculture, as the use of medicarpin can enhance crop protection against powdery mildew and potentially reduce the use of harmful pesticides.

Gupta A#, Awasthi P#, Sharma N, Parveen S, Vats RP, Singh N, Kumar Y, Goel A* & Chandran D* (2022) Medicarpin confers powdery mildew resistance in Medicago truncatula and activates the salicylic acid signalling pathway. Molecular Plant Pathology, 00, 1–18. https://bsppjournals.onlinelibrary.wiley.com/doi/full/10.1111/mpp.13202

Image legend. Left: Image showing localized accumulation of medicarpin (green) at the site of powdery mildew attack (fungal structure in red; a, appressorium; c, conidia), analyzed by fluorescein-tagged medicarpin (left). Right: Proposed model showing the contribution of medicarpin and SA in powdery mildew resistance in M. truncatula. Powdery mildew infection stimulates biosynthesis of medicarpin and salicylic acid (SA). Medicarpin and SA act in concert, perhaps in a regulatory loop, triggering the localized accumulation of medicarpin and hydrogen peroxide (H2O2) at fungal penetration-sites that leads to a hypersensitive response and powdery mildew resistance.

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