Abstract
In plants, each pollen mother cell undergoes two rounds of cell divisions to form a mature pollen grain, which contains a vegetative cell (VC) and two sperm cells (SC). As a companion cell, the VC carries the SCs to an ovule by germinating a pollen tube. In-depth sequencing analyses of mature pollen showed that microRNAs (miRNAs) and short interfering RNAs (siRNAs) are present in both the VC and SCs. Additionally, epigenetically-regulated transposable elements (TEs) are reactivated in the VC and these TE mRNAs are further processed into 21-nt epigenetically reactivated siRNA (easiRNA) in SCs, which prevent 24-nt siRNA accumulation and sequester miRNA loading. Small RNAs are thought to move from the VC to SCs, where they regulate gene expression and reinforce TE silencing. Here, we summarize current knowledge of the biogenesis and function of miRNAs, siRNAs, and easiRNAs in pollen, emphasizing how these different small RNAs coordinately contribute to sperm cell formation and TE silencing.
Highlights
In plants, each pollen mother cell undergoes two rounds of cell divisions to form a mature pollen grain, which contains a vegetative cell (VC) and two sperm cells (SC)
Based on their mode of biogenesis and their functions, small RNAs can be roughly divided into microRNAs and short interfering RNAs. miRNAs are produced from a single-stranded RNA that base pairs to form a hairpin structure, whereas siRNAs result from the processing of long double-stranded RNAs. miRNA biogenesis depends on Pol II transcription and subsequent processing of stem-loop precursors by Dicer-Like 1 (DCL1), and several other genes have been found to facilitate DCL1 cleavage [8]. 21-nt miRNAs are loaded into Argonaute 1 (AGO1) and act by cleavage of target mRNAs or by translational repression [8]
In this review, based on current knowledge about small RNAs in pollen from several genome-wide studies [12,13,14,15,16,17,18,19], we summarize potential functions of different classes of small RNAs during male gametogenesis, especially focusing on how these different classes of small RNAs coordinately contribute to SC formation and transposable elements (TEs) silencing
Summary
Each pollen mother cell undergoes two rounds of cell divisions to form a mature pollen grain, which contains a vegetative cell (VC) and two sperm cells (SC). In-depth sequencing analyses of mature pollen showed that microRNAs (miRNAs) and short interfering RNAs (siRNAs) are present in both the VC and SCs. epigenetically-regulated transposable elements (TEs) are reactivated in the VC and these TE mRNAs are further processed into 21-nt epigenetically reactivated siRNA (easiRNA) in SCs, which prevent 24-nt siRNA accumulation and sequester miRNA loading.
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