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 summarise recent insights into plant NLR signalling mechanisms, which might constitute evolutionarily conserved NLR-mediated immune mechanisms.

Summary: Pattern recognition receptors (PRRs) and nucleotide‐binding domain and leucine‐rich repeat (LRR)‐containing proteins (NLRs) initiate pattern‐triggered immunity (PTI) and effector‐triggered immunity (ETI), respectively, each associated with the activation of an overlapping set of defence genes. The regulatory mechanism behind this convergence of PTI‐ and ETI‐mediated defence gene induction remains elusive. We generated transgenic Arabidopsis plants that enable conditional NLR activation without pathogen infection to dissect NLR‐ and PRR‐mediated transcriptional signals. A comparative analysis of over 40 transcriptome datasets linked calmodulin‐binding transcription activators (CAMTAs) to the activation of overlapping defence genes in PTI and ETI. We used a dominant camta3 mutant (camta3‐D) to assess CAMTA functions in the corresponding transcriptional regulation. Transcriptional regulation by NLRs, although highly similar to PTI responses, can be established independently of pathogen‐associated molecular pattern (PAMP) perception, defence phytohormones and host cell death. Conditional expression of the N‐terminal coiled‐coil domain of the barley MLA (Mildew resistance locus A) NLR is sufficient to trigger similar transcriptional reprogramming as full‐length NLRs. CAMTA‐binding motifs are overrepresented in the 5′ regulatory regions of the identified primary immune response genes, consistent with their altered expression and disease resistance responses in camta3‐D plants. We propose that CAMTA‐mediated transcriptional regulation defines an early convergence point in NLR‐ and PRR‐mediated signalling.

Significance: Plants produce approximately 300 O -prenylated aromatics, with their O -prenyl moieties often being crucial to their bioactivities. This study identified a gene from grapefruit encoding an aromatic O -prenyltransferase ( O- PT) belonging to the UbiA superfamily. The O -PT was shown responsible for the biosynthesis of pharmaceutically active O -prenylated coumarins that cause grapefruit–drug interactions, an adverse effect disturbing the pharmacokinetics of more than 85 medications. Another UbiA O -PT for coumarins was isolated from Angelica keiskei , an apiaceous medicinal plant. Phylogenetic analysis of the rutaceous and apiaceous O -PTs suggested that aromatic O -prenylation activity emerged in parallel in these distant plant taxa. The molecular evolution of aromatic O -PTs from plant UbiA proteins may aid citrus breeding and a synthetic biology approach to bioactive O -prenylated coumarins.