Small RNA Pathways Are Present and Functional in the Angiosperm Male Gametophyte

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Small non-coding RNAs are essential for development of the sporophyte, the somatic diploid phase of flowering plants. They are integral to key cellular processes such as defense, generation of chromatin structure, and regulation of native gene expression. Surprisingly, very little is known of their presence and function in the male haploid phase of plant development (male gametophyte/pollen grain), where dramatic cell fate changes leading to gametogenesis occur over just two mitotic divisions. We show that critical components of small RNA pathways are expressed throughout pollen development, but in a pattern that differs from the sporophyte. We also demonstrate that mature pollen accumulates a range of mature microRNAs, the class of small RNA most frequently involved in post-transcriptional regulation of endogenous gene expression. Significantly, these miRNAs cleave their target transcripts in developing pollen-a process that seemingly contributes to the purging of key regulatory transcripts from the mature pollen grain. Small RNAs are thus likely to make a hitherto unappreciated contribution to male gametophyte gene expression patterns, pollen development, and gametogenesis.