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Abstract:
Plants have evolved intracellular nucleotide-binding leucine rich repeat receptors (NLRs) to induce a superior immune response. Upon activation, coiled-coil (CC) domain containing NLRs (CNLs) oligomerize to form apparent cation channels that promote calcium influx and cell death induction, with the alpha-1 helix of the individual CC domains penetrating membranes. Some members of a monophyletic subclass of CNLs, the ancient and autonomous NLRs (ANLs), are characterized by putative N-myristoylation and S-acylation sites at the N-terminus of their CCG10/GA domain, potentially mediating permanent membrane association. Whether these Potentially Membrane Localized NLRs (PMLs) mediate cell death upon activation in a similar way as reported for other CNLs has been unknown. We integrated phylogenetic, cell-biological, and functional studies to uncover the cell death function of an atypical but conserved Arabidopsis PML, PML5, which has a 113 amino acid deletion in its CCG10/GA domain. Active PML5 oligomers localize in Golgi membranes and the tonoplast, changes vacuolar morphology, and induce cell death, with the short N-terminus being sufficient for cell death. Mutant analysis supports a potential key role of PMLs in plant immunity. Similar deletions as in Arabidopsis PML5 are found in several Brassicales paralogs, pointing to the evolutionary importance of this innovation. PML5 is thus a naturally occurring CNL variant with a minimal signaling domain and its further study should help in understanding the functional importance of this minimal domain for NLR signaling.
