Characterizing the Mode of Resistance Conferred Against Asian Soybean Rust by CcRpp1, a Novel R Gene from Cajanus cajan (L.) Millsap.

Abstract

Asian soybean rust (ASR), caused by Phakopsora pachyrhizi, poses a significant threat to soybean production, especially in South America. The CcRpp1 gene (Cajanus cajan Resistance against P. pachyrhizi 1) has demonstrated robust resistance to ASR when introduced into soybean. This study explores the underlying mechanisms of CcRpp1-mediated resistance through integrated cytological, transcriptomic, and metabolomic analyses. Cytological observations revealed that CcRpp1-expressing plants rapidly halt fungal infection within 24-36 hours post-inoculation (hpi) by inhibiting hyphal growth and differentiation, as well as sequestering fungal feeding structures inside host cells. Transcriptomic profiling at 36 hpi showed upregulation of genes involved in photosynthesis, primary metabolism, and carbon utilization, while genes associated with protein synthesis were downregulated. This pattern suggests a metabolic shift that prioritizes energy production and defense over growth. Metabolomic analyses further indicated that CcRpp1-mediated resistance is marked by a swift and substantial depletion of amino acids and nitrogen-containing metabolites following infection. This rapid metabolic reprogramming likely restricts nutrient availability to the pathogen, contributing to the observed resistance and highlighting a potential mode-of-action. Overall, these findings provide new insights into the molecular and metabolic basis of CcRpp1-mediated ASR resistance and lays the groundwork for additional in-depth studies that will lead to the development of sustainable and effective strategies for deployment of durable resistance against ASR.