Abstract
BackgroundSmall RNAs (sRNAs) are regulatory molecules impacting on gene expression and transposon activity. MicroRNAs (miRNAs) are responsible for tissue-specific and environmentally-induced gene repression. Short interfering RNAs (siRNA) are constitutively involved in transposon silencing across different type of tissues. The male gametophyte in angiosperms has a unique set of sRNAs compared to vegetative tissues, including phased siRNAs from intergenic or genic regions, or epigenetically activated siRNAs. This is contrasted by a lack of knowledge about the sRNA profile of the male gametophyte of gymnosperms.ResultsHere, we isolated mature pollen from male cones of Norway spruce and investigated its sRNA profiles. While 21-nt sRNAs is the major size class of sRNAs in needles, in pollen 21-nt and 24-nt sRNAs are the most abundant size classes. Although the 24-nt sRNAs were exclusively derived from TEs in pollen, both 21-nt and 24-nt sRNAs were associated with TEs. We also investigated sRNAs from somatic embryonic callus, which has been reported to contain 24-nt sRNAs. Our data show that the 24-nt sRNA profiles are tissue-specific and differ between pollen and cell culture.ConclusionOur data reveal that gymnosperm pollen, like angiosperm pollen, has a unique sRNA profile, differing from vegetative leaf tissue. Thus, our results reveal that angiosperm and gymnosperm pollen produce new size classes not present in vegetative tissues; while in angiosperm pollen 21-nt sRNAs are generated, in the gymnosperm Norway spruce 24-nt sRNAs are generated. The tissue-specific production of distinct TE-derived sRNAs in angiosperms and gymnosperms provides insights into the diversification process of sRNAs in TE silencing pathways between the two groups of seed plants.
Highlights
Small RNAs are regulatory molecules impacting on gene expression and transposon activity
Norway spruce pollen generates 24-nt Small RNA (sRNA) that are exclusively derived from Transposable element (TE) sequences To test whether gymnosperm pollen generates a specific population of TE-derived sRNAs, we harvested pollen from mature male cones of Norway spruce
We conclude that Norway spruce pollen generates tissue-specific 24nt sRNAs
Summary
Small RNAs (sRNAs) are regulatory molecules impacting on gene expression and transposon activity. Short interfering RNAs (siRNA) are constitutively involved in transposon silencing across different type of tissues. In contrast to miRNAs that mainly regulate proteincoding genes, small interfering RNAs (siRNAs) silence. 24-nt siRNAs occupy the predominant fraction of the sRNA population [8,9,10,11,12,13]. This 24-nt sRNA fraction is composed of siRNAs that are derived from TEs [14, 15]. This strong positive association between 24-nt siRNAs and TEs allows us to use 24-nt siRNAs for TE annotations in both monocots and dicots [16]
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