Leaf rust (Puccinia triticina) mediated RNAi in wheat (Triticum aestivum L.) prompting host susceptibility

Significance of microRNAs in regulating gene expression in higher eukaryotes as well as in pathogens like fungi to suppress host defense is a well-established phenomenon. The present study focuses on leaf rust fungi Puccinia triticina (Pathotype 77-5) mediated RNAi to make wheat (Triticum aestivum L.) more susceptible. To reach such conclusions, we first confirmed the presence of argonaute (AGO) and dicer-like protein (DCL) family sequences in Puccinia. Bioinformatic tools were applied to retrieve the sequences from Puccinia genome followed by cloning and sequencing from P. triticina pathotype 77-5 cDNA to obtain the specific sequences. Their homologs were searched in other 14 Puccinia races to relate them with pathogenesis. Further, precursor sequences for three miRNA-like RNA molecules (milRs) were cloned from P. triticina cDNA. Their target genes like MAP kinase were successfully predicted and validated through degradome mapping and qRT-PCR. Gradual increase in milR2 (milR and milR*) expression over progressive time point of infection and positive expression for all the milRs within 77-5 urediniospores confirmed a complete host- independent RNAi activity by P. triticina.

Differential contributions of plant Dicer-like proteins to antiviral defences against potato virus X in leaves and roots

Members of the plant Dicer-like (DCL) protein family are the critical components of the RNA-silencing pathway that mediates innate antiviral defence. The distinct antiviral role of each individual DCL protein has been established with mostly based on observations of aerial parts of plants. Thus, although the roots are closely associated with the life cycle of many plant viruses, little is known about the antiviral activities of DCL proteins in roots. We observed that antiviral silencing strongly inhibits potato virus X (PVX) replication in roots of some susceptible Solanaceae species. Silencing of the DCL4 homolog in Nicotiana benthamiana partially elevated PVX replication levels in roots. In Arabidopsis thaliana, which was originally considered a non-host plant of PVX, high levels of PVX accumulation in inoculated leaves were achieved by inactivation of DCL4, while in the upper leaves and roots, it required the additional inactivation of DCL2. In transgenic A. thaliana carrying the PVX amplicon with a green fluorescent protein (GFP) gene insertion in the chromosome (AMP243 line), absence of DCL4 enabled high levels of PVX-GFP accumulation in various aerial organs but not in the roots, suggesting that DCL4 is critical for intracellular antiviral silencing in shoots but not in roots, where it can be functionally compensated by other DCL proteins. Together, the high level of functional redundancies among DCL proteins may contribute to the potent antiviral activities against PVX replication in roots.

Different Dicer-like protein components required for intracellular and systemic antiviral silencing in Arabidopsis thaliana

Eukaryotes employ RNA silencing as an innate defense system against invading viruses. Dicer proteins play the most crucial role in initiating this antiviral pathway as they recognize and process incoming viral nucleic acids into small interfering RNAs. Generally, 2 successive infection stages constitute viral infection in plants. First, the virus multiplies in initially infected cells or organs after viral transmission and then the virus subsequently spreads systemically through the vasculature to distal plant tissues or organs. Thus, antiviral silencing in plants must cope with both local and systemic invasion of viruses. In a recent study using 2 sets of different experiments, we clearly demonstrated the differential requirement for Dicer-like 4 (DCL4) and DCL2 proteins in the inhibition of intracellular and systemic infection by potato virus X in Arabidopsis thaliana. Taken together with the results of other studies, here we further discuss the functional specificity of DCL proteins in the antiviral silencing pathway.