Characterizing the Mode of Resistance Conferred Against Asian Soybean Rust by <i>CcRpp1</i> , a Novel <i>R</i> Gene from <i>Cajanus cajan</i>

Asian soybean rust (ASR), caused by <i>Phakopsora pachyrhizi</i>, poses a significant threat to soybean production, especially in South America. The <i>CcRpp1</i> gene (<i><u>C</u>ajanus <u>c</u>ajan</i> <u>R</u>esistance against <i><u>P</u>. <u>p</u>achyrhizi</i> <u>1</u>) has demonstrated robust resistance to ASR when introduced into soybean. This study explores the underlying mechanisms of <i>CcRpp1</i>-mediated resistance through integrated cytological, transcriptomic, and metabolomic analyses. Cytological observations revealed that <i>CcRpp1</i>-expressing plants rapidly halt fungal infection within 24 to 36 h postinoculation (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, whereas 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 <i>CcRpp1</i>-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 <i>CcRpp1</i>-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. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.