Evolution and Conservation of Plant NLR Functions

Abstract

In plants and animals, nucleotide-binding domain and leucine-rich repeats (NLR)-containing proteins play pivotal roles in innate immunity. Despite their similar biological functions and protein architecture, comparative genome-wide analyses of NLRs and genes encoding NLR-like proteins suggest that plant and animal NLRs have independently arisen in evolution. Furthermore, the demonstration of interfamily transfer of plant NLR functions from their original species to phylogenetically distant species implies evolutionary conservation of the underlying immune principle across plant taxonomy. In this review we discuss plant NLR evolution and summarize recent insights into plant NLR-signaling mechanisms, which might constitute evolutionarily conserved NLR-mediated immune mechanisms.

Keywords: NB-LRR; NLR; effector-triggered immunity; innate immunity; resistance protein.

Figure 1

Phylogenetic distribution of the NLR family. The distribution of the individual domains constitutive of NLRs (NB-ARC, NACHT, and LRR) and the different groups of NLRs are mapped on a simplified phylogenetic tree. The fusion events between either the NB-ARC or the NACHT domain and the LRR domain presumably occurred as indicated on the phylogenetic tree. The structural properties of the N-termini of plant NLRs in the non-TIR group are indicated if the information is available (CC, coiled-coil; BED, BED-DNA-binding zinc finger; H, α/β-hydrolase; PK, protein kinase; for more detail, see Atypical Domains Found in the NLR Structure). This figure is adapted from Yue et al. (25), combined with data as indicated below. The divergence dates are adapted from Ref. (26) and (27). Species representative of some taxa are indicated on the right. Ma, million years; Ga, billion years. The question mark (?) indicates that the presence of NLRs is not clearly resolved in given taxa due to lack of data. (a) Xue et al. (28), (b) Kim et al. (29), (c) Heller et al. (30), (d) Tarr and Alexander (31), (e) Faris et al. (32).