MiR825-5p targets the TIR-NBS-LRR gene MIST1 and down-regulates basal immunity against Pseudomonas syringae in Arabidopsis.

Diego López-Márquez,Eduardo R Bejarano,Ángel Del-Espino,Edgar A Rodríguez-Negrete,Carmen R Beuzón,Ignacio Rubio-Somoza,Nieves López-Pagán,Javier Ruiz-Albert,James Murray

doi:10.1093/jxb/erab354

Abstract

Plants encode numerous intracellular receptors known as nucleotide-binding leucine-rich repeat receptors (NLRs) that recognize pathogen-derived effectors or their activity to activate defenses. miRNAs regulate NLR genes in many species, often triggering the production of phased siRNAs (phasiRNAs). Most such examples involve genes encoding NLRs carrying coiled-coil domains, although a few include genes encoding NLRs carrying a Toll/interleukin-1 domain (TNL). Here, we characterize the role of miR825-5p in Arabidopsis, using a combination of bioinformatics, transgenic plants with altered miRNA levels and/or reporters, small RNAs, and virulence assays. We demonstrate that miR825-5p down-regulates the TNL MIST1 by targeting for endonucleolytic cleavage the sequence coding for TIR2, a highly conserved amino acid motif, linked to a catalytic residue essential for immune function. miR825-5p acts as a negative regulator of basal resistance against Pseudomonas syringae. miR825-5p triggers the production from MIST1 of a large number of phasiRNAs that can mediate cleavage of both MIST1 and additional TNL gene transcripts, potentially acting as a regulatory hub. miR825-5p is expressed in unchallenged leaves and transcriptionally down-regulated in response to pathogen-associated molecular patterns (PAMPs). Our results show that miR825-5p, which is required for full expression of PAMP-triggered immunity, establishes a link between PAMP perception and expression of uncharacterized TNL genes.

Highlights

Plants possess complex immune systems that effectively protect them from the majority of pathogens within the environment
We demonstrate that miR825-5p down-regulates the Toll/interleukin-1 domain (TNL) MIST1 by targeting for endonucleolytic cleavage the sequence coding for TIR2, a highly conserved amino acid motif, linked to a catalytic residue essential for immune function. miR825-5p acts as a negative regulator of basal resistance against Pseudomonas syringae. miR825-5p triggers the production from MIST1 of a large number of phasiRNAs that can mediate cleavage of both MIST1 and additional TNL gene transcripts, potentially acting as a regulatory hub. miR825-5p is expressed in unchallenged leaves and transcriptionally downregulated in response to pathogen-associated molecular patterns (PAMPs)
While analyzing the overlap between transcriptomic responses to P. syringae and gene silencing in Arabidopsis, serendipity led us to look into the MIR825 locus

Summary

Introduction

Plants possess complex immune systems that effectively protect them from the majority of pathogens within the environment. The functioning of these systems relies on a battery of cell surface and intracellular receptors that alert plants of incoming threats, leading to the activation of defense responses capable of hindering disease development (Cui et al, 2015; Couto and Zipfel, 2016; Zhang et al, 2017). The NLR protein structure usually includes: a variable N-terminal domain, a nucleotide-binding domain (NB-ARC), and a leucine-rich repeat domain (LRR) (Meyers et al, 2003; Cui et al, 2015; Zhang et al, 2017). TNLs and CNLs typically engage the ETI machinery through different regulators of plant defense (Feys et al, 2001;Wiermer et al, 2005; Rietz et al, 2011; Wagner et al, 2013; Cui et al, 2017)

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Conclusion