Diversity and Evolution of NLR Genes in Citrus Species

NLR genes are crucial components of the effector-triggered immunity (ETI) system, which recognizes pathogens and initiates immune responses. While NLR genes in many plants have been extensively studied, the diversity of NLR genes in citrus remains largely unknown. In this study, we systematically identified 1,585 NLR genes across the genomes of 10 citrus species. Significant variations in NLR gene copy numbers were observed among these citrus species, with gene duplication and recombination identified as major driving forces. Citrus NLR genes were classified into TIR-NBS-LRR (TNL), CC-NBS-LRR (CNL), RPW8-NBS-LRR (RNL), and NL categories. It is believed that TNL, RNL, and CNL genes originated from NL genes through the acquisition of TIR, RPW8 domains and CC motifs, respectively, followed by the random loss of the corresponding domains. Phylogenetic analysis revealed that citrus NLR genes originated with the species and underwent adaptive evolution, potentially playing crucial roles in the global colonization of citrus. This study is importance for comprehensively understanding citrus NLR genes and provides a foundational dataset for research aimed at breeding disease-resistant citrus varieties.