Biotic stress-associated microRNAs: identification, detection, regulation, and functional analysis.

The methods described herein first highlight the strategies that were used to discover a biotic stress-associated miRNA. This involved (1) the selection of transcripts that were more abundant in transgenic plants expressing viral-derived suppressors of RNA silencing and transcripts that were repressed in wild-type seedlings treated with a biotic stress, (2) a 5' RACE-derived assay to map miRNA target sites, and (3) a bioinformatic analysis to retrieve specific miRNA loci from the Arabidopsis genome. We then describe methods used to monitor (1) the levels of primary miRNA transcripts (pri-miRNAs)/mature miRNAs and (2) the transcriptional activity of miRNAs in response to a biotic stress and bacterial challenge. Furthermore, we present a strategy to identify additional biotic stress-responsive miRNA genes and get insight into their regulation. This involves (1) a microarray approach that allows detection of pri-miRNAs, coupled with (2) a promoter analysis of co-regulated miRNA genes. Finally, we describe strategies that can be used to functionally characterize individual biotic stress-associated miRNAs, or the miRNA pathway, in disease resistance.