Suppression of the MicroRNA Pathway by Bacterial Effector Proteins

Abstract

Plants and animals sense pathogen-associated molecular patterns (PAMPs) and in turn differentially regulate a subset of microRNAs (miRNAs). However, the extent to which the miRNA pathway contributes to innate immunity remains unknown. Here, we show that miRNA-deficient mutants of Arabidopsis partly restore growth of a type-three secretion-defective mutant of Pseudomonas syringae. These mutants also sustained growth of non-pathogenic Pseudomonas fluorescens and Escherichia coli strains, implicating miRNAs as key components of plant basal defense. Accordingly, we have identified P. syringae effectors that suppress transcriptional activation of some PAMPresponsive miRNAs, miRNA biogenesis, stability or activity. These results provide compelling evidence that, like viruses, bacteria have evolved to suppress RNA silencing to cause disease.In RNA silencing, double-stranded (ds)RNA is processed into small RNAs through the action of RNase-III-like Dicer enzymes. The small RNAs guide Argonaute (AGO)-containing RNAinduced silencing complexes (RISCs) to inhibit gene expression at the transcriptional or posttranscriptional levels (1). In the Arabidopsis thaliana miRNA pathway, miRNA precursors (pre-miRNAs) are excised from non-coding primary transcripts (pri-miRNAs) and processed into mature miRNA duplexes by Dicer-like 1 (DCL1). Upon HEN1-catalyzed 2′-Omethylation (2), one miRNA strand incorporates an AGO1-containing RISC to direct endonucleolytic cleavage or translational repression of target transcripts (1). DCL4 and DCL2 have major defensive functions by processing viral-derived dsRNA into small interfering (si) RNAs, which, like miRNAs, are loaded into AGO1-RISC. As a counter-defensive strategy, viruses deploy viral suppressors of RNA-silencing, or VSRs (3). RNA-silencing also contributes to resistance against bacterial pathogens (4-7), which elicit an innate immune response upon perception of PAMPs by host-encoded pattern recognition receptors (PRRs). For example, the Arabidopsis miR393 is PAMP-responsive (4,8) and contributes to resistance against virulent P. syringae pv. tomato strain DC3000 (Pto DC3000) (4). Nonetheless, the full extent to which cellular small RNAs, including miRNAs, participate to PAMP-triggered immunity (PTI) in plants remains unknown.To address this issue, Arabidopsis mutants defective for siRNA or miRNA accumulation were challenged with Pto DC3000 hrcC-, a mutant that lacks a functional type-III secretion system required for effector protein delivery into host cells (9). This bacterium elicits, but cannot suppress PTI, and consequently multiplies poorly on wild-type Col-0-and La-er-inoculated leaves (Fig. 1A,S1). However, Pto DC3000 hrcC-growth was specifically enhanced in the

The Role of WRKY Transcription Factors in Plant Immunity

“…SRO5 along with PYRROLINE-5-CARBOXYLATE DEHYDROGENASE (an overlapping gene in the antisense orientation) generate 24-and 21-nucleotide siRNAs, which together are components of a regulatory loop controlling reactive oxygen species production and stress response (Borsani et al, 2005). Similar suppression of the host miRNA machinery by bacterially derived effector proteins has recently been demonstrated in Arabidopsis (Navarro et al, 2008).…”

MicroRNA regulation of plant innate immune receptors

“…This process would facilitate the continuing amplification and diversification of R genes. Both bacteria and viruses have evolved effectors that can suppress miRNA and siRNA pathways (32,33). In addition to contributing to pathogen spread, the temporal shutdown of miRNA and siRNA function by pathogen effectors might also enhance Rgene function by blocking the formation or activity of the R-gene targeting miRNAs and, thus, provide some balance to resistance and susceptibility during host-microbe interactions.…”

Genome-wide profiling of sRNAs in the Verticillium dahliae-infected Arabidopsis roots

“…The mechanism of how effectors modulate plant immunity is largely unclear, especially in RNA interference (RNAi)-involved pathways which usually play pivotal roles in both plant immunity as well as pathogen infection (Bozkurt et al 2012; Qiao et al 2013). Previous study identified Pseudomonas syringae effectors that suppress transcriptional activation of PAMP-responsive micro RNAs (miRNAs) or miRNA biogenesis, stability or activity, and Arabidopsis miRNA-deficient mutants partly restored growth of P. syringae secretion-defectivemutant (Navarro et al 2008). Hundreds of predicted effector genes from oomycete genomes reflect more complex defence–counterdefence crosstalk than that in plant–bacteria interactions (Thines and Kamoun 2010; Qiao et al 2013).…”