Host-Pathogen interactions modulated by small RNAs

Abstract

Biological processes such as defense mechanisms and microbial offense strategies are regulated through RNA induced interference in eukaryotes. Genetic mutations are modulated through biogenesis of small RNAs which directly impacts upon host development. Plant defense mechanisms are regulated and supported by a diversified group of small RNAs which are involved in streamlining several RNA interference pathways leading toward the initiation of pathogen gene silencing mechanisms. In the similar context, pathogens also utilize the support of small RNAs to launch their offensive attacks. Also there are strong evidences about the active involvement of these RNAs in symbiotic associations. Interestingly, small RNAs are not limited to the individuals in whom they are produced; they also show cross kingdom influences through variable interactions with other species thus leading toward the inter-organismic gene silencing. The phenomenon is understandable in the microbes which utilize these mechanisms to overcome host defense line. Understanding the mechanism of triggering host defense strategies can be a valuable step toward the generation of disease resistant host plants. We think that the cross kingdom trafficking of small RNA is an interesting insight that is needed to be explored for its vitality.

Keywords: Development; RNAi; cross kingdom; miRNA; mutualism; pathogenecity; virulence.

Figure 1.

sRNAs involved in regulation of various leaf development phases in plants.

Figure 2.

Endogenous sRNAs contribute toward plant defense via using various pathways. The biogenesis of these RNAs is regulated by 4 Dicer-like endonucleases (DCLs) inside the cell (a) RNA Pol IV exhibits short interfering RNAs (siRNAs) regulated by DCL3 leading toward the generation of RNA-induced silencing complex (RISC) with Argonaute 4 (AGO4). (b) RNA Pol II transcribes plant primary-microRNA (pri-miRNA) which is processed into miRNA or miRNA duplex regulated by DCL1. The particular duplex is shifted into cytoplasm from nucleus by Hasty (animal exportin-5 homolog); At this point mature miRNA interacts with Argonaute 1 (AGO1) to form a miRISC (miRNA-induced silencing complex) leading toward the facilitated targeted mRNAs degradation. (c) dsRNA are generated by RNA dependent RNA polymerase 6 (RDR6) from miRNA-mediated cleavage products of TAS gene transcripts which are further processed into trans-acting siRNAs (tasiRNAs) by DCL4 leading toward the suppression of mRNA through RISC and AGO2 complex. (d) RDR6 utilizes the invading RNAs to exhibit dsRNAs, which further produce siRNAs by DCL2. These siRNAs undergo degradation either via RISCs, AGO1 or AGO2.

Figure 3.

Nodule formation is regulated through sRNAs in legumes. Different steps in nodule formation are shown along with the miRNAs predicted to be involved at specific steps. (A) First coordination in between nitrogen seeking legume roots and rhizobial bacteria take place via exchange of chemical signals i.e., flavonoids are secreted by plants and lipochito oligosaccharides are generated as bacterial response. (B) As a result of positive response to these signals, bacterial cells attach themselves to root hairs. (C) This scenario changes the ionic balance in roots which triggers the transcription of nodulation-specific genes. Further progress reveals the engulfing of these bacteria by roots via infection peg like structure. (D) Nodule formation completes with 12–18 d after post inoculation. sRNAs that take part in various steps of nodule formation at various steps have been mentioned accordingly (Lelandais-Briére et al. 2009, 2016; Subramanian et al. 2008; Wang et al. 2009; Wang and Chua, 2014; Wu et al., 2016).

Figure 4.

Cross kingdom transport of sRNAs have been depicted through selection of few organisms from each kingdom. Where; IAV (Influenza A-Virus), CMV (Cocumber Mosaic Virus) and Bc (B. cineraria).