Abstract
Small RNAs represent a class of small but powerful agents that regulate development and abiotic and biotic stress responses during plant adaptation to a constantly challenging environment. Previous findings have revealed the important roles of small RNAs in diverse cellular processes. The recent discovery of bidirectional trafficking of small RNAs between different kingdoms has raised many interesting questions. The subsequent demonstration of exosome-mediated small RNA export provided a possible tool for further investigating how plants use small RNAs as a weapon during the arms race between plant hosts and pathogens. This review will focus on discussing the roles of small RNAs in plant immunity in terms of three aspects: the biogenesis of extracellular small RNAs and the transportation and trafficking small RNA-mediated gene silencing in pathogens.
Highlights
Small RNAs in plants with different lengths (21–24 nucleotide, nt) can be divided into two major groups: microRNAs and small interfering RNAs, which are processed from a single-stranded hairpin RNA precursor or double-stranded RNA, respectively [1,2,3]
Since the discovery that double-stranded RNA (dsRNA) can trigger gene silencing in Caenorhabditis elegans [4], sRNA has become commonly recognized as an important signaling molecule in the regulation of plant development and abiotic and biotic stress responses through transcriptional gene silencing (TGS) or posttranscriptional gene silencing (PTGS) [5,6,7]
The machinery for RNA silencing in plants consists of three core components: RNA-dependent RNA polymerases (RDRs), which are responsible for catalyzing the biosynthesis of dsRNAs from a single-strand RNA template; DICER-LIKE (DCL) proteins, which cleave dsRNA or single-stranded hairpin RNA into sRNAs; and Argonaute (AGO) proteins, which are guided by sRNAs and bind to target mRNAs in a sequence-complementary manner, leading to mRNA cleavage or translation inhibition [8,9,10,11]
Summary
Small RNAs (sRNAs) in plants with different lengths (21–24 nucleotide, nt) can be divided into two major groups: microRNAs (miRNAs) and small interfering RNAs (siRNAs), which are processed from a single-stranded hairpin RNA precursor or double-stranded RNA (dsRNA), respectively [1,2,3]. Plant intracellular nucleotide-binding/leucine-rich-repeat (NLR) receptors and the resistance protein (R protein) can detect pathogen effectors and induce a robust resistance response known as effector-triggered immunity (ETI) [13,14,15,16]. MiRNA-mediated secondary siRNA production is crucial for plant anti-viral immunity through R gene homeostasis [20]. The plant anti-bacterial defense conferred by siRNAs is largely dependent upon differences in the perception of bacterial effectors [21]. In addition to the role of innate sRNA synthesis in conferring plant immunity, the mechanism of cross-kingdom RNAi has been identified in plant-pathogen interactions [24,25]. How is sRNA biogenesis initiated when a plant confronts pathogens? How does a plant transport these sRNAs to pathogen tissues? How do the plant sRNAs function in pathogen cells? These questions will be further discussed in this review
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