Carl Zimmer has a good article in the NY Times entitled “Where Did All the Flowers Come From?” The article summarizes lots of interesting stuff, but I find the speculation regarding the evolution of the endosperm to be particularly though-provoking. Of course, anytime one mentions genome duplication to me, visions of gene silencing and small RNAs begin dancing in my mind. A recent article from David Baulcombe’s group merits mention in this context. This paper describes a developmental study of RNA polymerase IV-derived small interfering RNAs (siRNAs). The remarkable finding in this paper is the observation that the synthesis of many polIV-derived siRNAs is initated at the onset of the development of the maternal gametophyte, and that these siRNAs are in turn derived from the maternal genome(s) in the endosperm. This has ramifications for the expression of the different genomes in the endosperm, for genome imprinting, and likely for the evolution of flowers and seed development in plants.
The abstract from the paper:
“Most eukaryotes produce small RNA (sRNA) mediators of gene silencing that bind to Argonaute proteins and guide them, by base pairing, to an RNA target. MicroRNAs (miRNAs) that normally target messenger RNAs for degradation or translational arrest are the best-understood class of sRNAs. However, in Arabidopsis thaliana flowers, miRNAs account for only 5% of the sRNA mass and less than 0.1% of the sequence complexity. The remaining sRNAs form a complex population of more than 100,000 different small interfering RNAs (siRNAs) transcribed from thousands of loci1, 2, 3, 4, 5. The biogenesis of most of the siRNAs in Arabidopsis are dependent on RNA polymerase IV (PolIV), a homologue of DNA-dependent RNA polymerase II2, 3, 6. A subset of these PolIV-dependent (p4)-siRNAs are involved in stress responses, and others are associated with epigenetic modifications to DNA or chromatin; however, the biological role is not known for most of them. Here we show that the predominant phase of p4-siRNA accumulation is initiated in the maternal gametophyte and continues during seed development. Expression of p4-siRNAs in developing endosperm is specifically from maternal chromosomes. Our results provide the first evidence for a link between genomic imprinting and RNA silencing in plants.”
Mosher RA, Melynk CW, Kelly KA, Dunn RM, Studholme DJ, Baulcombe DC. 2009. Uniparental expression of PolIV-dependent siRNAs in developing endosperm of Arabidopsis. Nature 460, 283-286 (9 July 2009) | doi:10.1038/nature08084.