Functional divergence and gene loss of NRCs led to the division of the NLR network into smaller subnetworks.

(A) Evolutionary history of solanaceous NRC3. The ancestral NRC3 (ancNRC3) was first duplicated into two alleles, NRC3a in Capsicum and Solanum species and ancestral NRC3b/c in Nicotiana species. The ancestral NRC3b/c was further duplicated to generate NRC3b and NRC3c. NRC3b later underwent nonfunctionalization and was lost in N. benthamiana while NRC3c underwent subfunctionalization. As a result, the NRC3 in Nicotiana species lost the ability to function with Rpi-blb2. (B) Evolution of the complex NRC network in plants. The original NLR pair expanded to form an initial NRC network with an ancestral NRC (brown), functioning with multiple sensor NLRs. This ancestral variant duplicated, resulting in two functionally redundant variants: ’b’ and ’c’ (green and blue, respectively). Variant ’b’ lost its sensor NLR compatibility due to gene loss or nonfunctionalization, while variant ’c’ retained partial compatibility, undergoing subfunctionalization. Consequently, variant ’c’ functions with a narrower range of sensor NLRs, forming a distinct NRC subnetwork. Similar events likely occurred to various NRC variants, not limiting to NRC3 dissected in this study.
(JPF)