Quantitative resistance to Rhizoctonia solani AG-4 develops independently in leaves and roots of Brachypodium distachyon.

Rhizoctonia solani is a soil-borne phytopathogen with a broad host range, classified into multiple anastomosis groups (AGs). While the rice sheath blight pathogen AG-1 IA is well-studied, defense mechanisms against AG-4, which infects diverse monocots and dicots, remain unclear. Here, we screened 153 Brachypodium distachyon accessions for resistance to AG-4 HG-I+II using both leaf and soil inoculation assays. The accessions exhibited a quantitative resistance spectrum in both assays. Some accessions were resistant to both methods, while others showed resistance in only one, indicating that leaf and root defenses operate independently. Unlike the salicylic acid (SA)-dependent resistance observed in several accessions against AG-1 IA, AG-4 leaf resistance appears to be largely SA-independent. Among 22 resistant accessions, only eight induced the SA marker BdWRKY38, whereas twelve induced the jasmonic acid (JA) marker BdAOS. Additionally, resistance in Bd3-1 was unaffected by expression of the SA hydroxylase NahG. These results are consistent with previous findings that exogenous application of SA, JA, or ethylene fails to confer AG-4 resistance. Instead, most AG-4-resistant accessions showed common upregulation of cell wall biosynthesis-related genes following leaf infection, suggesting that cell wall fortification may contribute to defense against AG-4. These findings establish B. distachyon as a valuable model for dissecting resistance mechanisms against R. solani AG-4.