Simultaneous detection of miRNA and mRNA at the single‐cell level in plant tissues

Detecting the simultaneous presence of a microRNA (miRNA) and a mRNA in a specific tissue can provide support for the prediction that the miRNA regulates the mRNA. Although two such methods have been developed for mammalian tissues, they have a low signal‐noise ratio and/or poor resolution at the single‐cell level. To overcome these drawbacks, we develop a method that uses sequence‐specific miRNA‐locked nucleic acid (LNA) and mRNA‐LNA probes. Moreover, it augments the detection signal by rolling circle amplification, achieving a high signal‐noise ratio at the single‐cell level. Dot signals are counted for determining the expression levels of mRNA and miRNA molecules in specific cells. We show a high sequence specificity of our miRNA‐LNA probe, revealing that it can discriminate single‐base mismatches. Numerical quantification by our method is tested in transgenic rice lines with different gene expression levels. We conduct several applications. First, the spatial expression profiling of osa‐miR156 and OsSPL12 in rice leaves reveals their specific expression in mesophyll cells. Second, studying rice and its mutant lines with our method reveals opposite expression patterns of miRNA and its target mRNA in tissues. Third, the dynamic expression profiles of ZmGRF8 and zma‐miR396 during maize leaf development provide evidence that zma‐miR396 regulates the preferential spatial expression of ZmGRF8 in bundle sheath cells. Finally, our method can be scaled up to simultaneously detect multiple miRNAs and mRNAs in a tissue. Thus, it is a sensitive and versatile technique for studying miRNA regulation of plant tissue development.