Plant Science

MicroRNAs (miRNA) are small (21–24 nt) non-coding RNAs involved in many biological processes in both plants and animals. The biogenesis of plant miRNAs starts with the transcription of MIRNA (MIR) genes by RNA polymerase II; then, the primary miRNA transcripts are cleaved by Dicer-like proteins into mature miRNAs, which are then loaded into Argonaute (AGO) proteins to form the effector complex, the miRNA-induced silencing complex (miRISC). In Arabidopsis , some MIR genes are expressed in a tissue-specific manner; however, the spatial patterns of MIR gene expression may not be the same as the spatial distribution of miRISCs due to the non-cell autonomous nature of some miRNAs, making it challenging to characterize the spatial profiles of miRNAs. A previous study utilized protoplasting of green fluorescent protein (GFP) marker transgenic lines followed by fluorescence-activated cell sorting (FACS) to isolate cell-type-specific small RNAs. However, the invasiveness of this approach during the protoplasting and cell sorting may stimulate the expression of stress-related miRNAs. To non-invasively profile cell-type-specific miRNAs, we generated transgenic lines in which root cell layer-specific promoters drive the expression of AGO1 and performed immunoprecipitation to non-invasively isolate cell-layer-specific miRISCs. In this protocol, we provide a detailed description of immunoprecipitation of root cell layer-specific GFP-AGO1 using EN7::GFP-AGO1 and ACL5::GFP-AGO1 transgenic plants, followed by small RNA sequencing to profile single-cell-type-specific miRNAs. This protocol is also suitable to profile cell-type-specific miRISCs in other tissues or organs in plants, such as flowers or leaves.

Graphical abstract

Cytidine-to-uridine (C-to-U) RNA editing is one of the most important post-transcriptional RNA processing in plant mitochondria and chloroplasts. Several techniques have been developed to detect the RNA editing efficiency in plant mitochondria and chloroplasts, such as poisoned primer extension (PPE) assays, high-resolution melting (HRM) analysis, and DNA sequencing. Here, we describe a method for the quantitative detection of RNA editing at specific sites by sequencing cDNA from plant leaves to further evaluate the effect of different treatments or plant mutants on the C to U RNA editing in mitochondria and chloroplasts.

Analyzing cellular structures and the relative location of molecules is essential for addressing biological questions. Super-resolution microscopy techniques that bypass the light diffraction limit have become increasingly popular to study cellular molecule dynamics in situ. However, the application of super-resolution imaging techniques to detect small RNAs (sRNAs) is limited by the choice of proper fluorophores, autofluorescence of samples, and failure to multiplex. Here, we describe an sRNA-PAINT protocol for the detection of sRNAs at nanometer resolution. The method combines the specificity of locked nucleic acid probes and the low background, precise quantitation, and multiplexable characteristics of DNA Point Accumulation for Imaging in Nanoscale Topography (DNA-PAINT). Using this method, we successfully located sRNA targets that are important for development in maize anthers at sub-20 nm resolution and quantitated their exact copy numbers.

Graphic abstract:

Multiplexed sRNA-PAINT. Multiple Vetting and Analysis of RNA for In Situ Hybridization (VARNISH) probes with different docking strands (i.e., a, b, …) will be hybridized to samples. The first probe will be imaged with the a* imager. The a* imager will be washed off with buffer C, and then the sample will be imaged with b* imager. The wash and image steps can be repeated sequentially for multiplexing.

The micrografting technique in the model plant Arabidopsis has been widely used in the field of plant science. Grafting experiments have demonstrated that signal transductions are systematically regulated in many plant characteristics, including defense mechanisms and responses to surrounding environments such as soil and light conditions. Hypocotyl micrografting is a powerful tool for the analysis of signal transduction between shoots and roots; however, the requirement for a high level of skill for micrografting, during which small seedlings are microdissected and micromanipulated, has limited its use. Here, we developed a silicone-made microdevice, called a micrografting chip, to perform Arabidopsis micrografting easily and uniformly. The micrografting chip has tandemly arrayed units, each of which consists of a seed pocket for seed germination and a micro-path to hold hypocotyl. All micrografting procedures are performed on the chip. This method using a micrografting chip will avoid the need for training and promote studies of systemic signaling in plants.

Graphic abstract:

A silicone chip for easy grafting

Methods that allow the study of gene expression regulation are continually advancing. Here, we present an in situ hybridization protocol capable of detecting individual mRNA molecules in plant root cells, thus permitting the accurate quantification and localization of mRNA within fixed samples (Duncan et al., 2016; Rosa et al., 2016). This single molecule RNA fluorescence in situ hybridization (smFISH) uses multiple single-labelled oligonucleotide probes to bind target RNAs and generate diffraction-limited signals that can be detected using a wide-field fluorescence microscope. We adapted a recent version of this method that uses 48 fluorescently labeled DNA oligonucleotides (20 mers) to hybridize to different portions of each transcript (Raj et al., 2008). This approach is simple to implement and has the advantage that it can be readily applied to any genetic background.

Export of transcribed mRNAs from nucleoplasm to cytosol is an essential process for the translation of genes into proteins. This process is tightly regulated by nuclear pores, composed of about 30 nucleoporin proteins (Nups). Whether or not the mRNAs are able to be appropriately exported to cytoplasm is of an importance for understanding the role of Nups. Here, we describe a practical protocol to detect the intracellular localization of mRNAs in mesophyll cells of Nicotiana benthamiana (N. benthamiana). This protocol is based on poly (A) in situ hybridization method using an oligo d(T) probe conjugated with Alexa Fluor-488.