Epigenetic variation in tissue culture and pollen is guided by small RNA
2009
Slotkin, K. | Tanurdzic, M. | Schwab, R. | Vaughn, M. | Doerge, R. W. | Martienssen, R.
Heterochromatin is composed of transposable elements (FE) and related repeats which silence genes located nearby, and play a major role in epigenetic regulation of the genome. Far from being inert, heterochromatin is transcribed and small interfering RNA corresponding to heterochromatic sequences can be detected in plants, animals and fission yeast. In plants, small interfering RNA (siRNA) corresponding to some classes of TE depends on DNA methyl transftrase MET1, the SWIISNF ATPase, DDM1, or both, but not on the histone deacetylase SILlIHDA6. All three genes are required for silencing transposons in the absence of siRNA, and we are exploring the roles of these complementary mechanisms in the inheritance of epigenetic silencing from generation to generation, and in dividing cells during development. In pollen we show that TEs are unexpectedly reactivated and transpose, but only in the vegetative nucleus, which accompanies the sperm cells but does not provide DNA to the fertilized zygote. TE expression coincides with down regulation of DDM1 and of most TE siRNAs. However, 21 nucleotide siRNA from Athila retro transposons is generated in pollen and accumulates in sperm, suggesting that siRNA from TEs activated in the vegetative nucleus can target silencing in gametes. We propose a conserved role for reprogramming in germline companion cells, such as nurse cells in insects and vegetative nuclei in plants, to reveal intact TEs in the genome and regulate their activity in gametes.
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