An atypical EF-hand domain containing resistance protein SRC4 boosts plant immunity in a calcium-enhanced manner.

The typical nucleotide-binding leucine-rich repeat (NLR) proteins, such as TIR-NBS-LRR (TNL) and CC-NBS-LRR (CNL), are known to be engaged in effector-triggered immunity (ETI). However, several atypical resistance (R) proteins with truncated NLR domains or with nonclassical domains are emerging to be key regulators in plant immunity. SMV resistance cluster 4 (SRC4) is an atypical NLR protein in soybean, distinguished by an N-terminal Domain (NTD)-TIR-shortened NBS oligomerization region (SNOR)-EF-hand (EFh) domain architecture. SRC4 lacks both NBS and LRR domains but retains basal antiviral activity through its TIR-SNOR region, with SNOR contributing to TIR-dependent antiviral activity and SRC4 self-association. The EFh domain recognizes coat protein (CP) of soybean mosaic virus (SMV) and serves as a Ca2+-responsive module to distinctly regulate TIR-SNOR-mediated immune response in soybean. SRC4-overexpression (Ox-SRC4) soybean plants show SMV resistance without obvious growth penalty. Furthermore, several SRC4 high-expression soybean varieties showed stronger SMV resistance and Ca2+ responsiveness than the SRC4 low-expression soybean varieties. Our study highlights the essential roles of atypical resistance proteins in plant immunity and growth-defense balance.