Rice Argonaute protein required for reprogramming of histone H3 modifications on meiotic chromosomes.

Experimental Farm / Nonomura Group

Histone H3 modifications are widely reprogrammed during male meiosis I in rice dependently on MEL1 Argonaute protein

Hua Liu, Ken-Ichi Nonomura

Journal of Cell Science, Published online, 12 August, 2016 DOI:10.1242/jcs.184937

Meiosis is a special type of cell division to halve the chromosome number, achieved by two continuous division not intervened by DNA replication. It is an indispensable mechanism to generate genetic diversity via homologous chromosome pairing and meiotic recombination, in addition to stable transmission of genetic information to the next generation.

We focused on the relation of meiosis and histone modifications (glossary), which are important for control of chromosome structure and gene expression. Generally in plants, dimethylation at the position-9 lysine of histone H3 (H3K9me2) is thought to repress gene expressions and promote the compaction of chromatin structure. In contrast, acethylation at the same position (H3K9ac) activates gene expressions. We found that H3 modifications after the meiotic entry were totally altered from the premeiotic H3 status (Fig. 1A). “Large-scale meiotic chromosome reprogramming (LMR)” named in this paper is thought to be one of the mechanisms promoting meiosis in plants.

Interestingly, LMR was completely disrupted in the mutant of MEL1 (Fig. 1B), that is an Argonaute protein (glossary) specifically expressed in rice germ cells. These results suggest possibilities that MEL1 promotes meiosis via control of LMR, and that the RNA silencing mechanism is important for plant meiosis.

This work was supported by JSPS KAKENHI (25252004), and by NIG postdoc fellowship.

Figure1

A wide reprogramming of histone H3K9me2 during meiosis I is dependent on the rice Argonaute protein MEL1.
(A) In wild-type pollen mother cells (PMCs), the level of H3K9me2 (red) is increased remarkably when cells transit from premeiosis to meiosis. Chromatin DNA is stained with DAPI (blue). PAIR2 (green) is a meiotic gene required for homologous chromosome pairing, and shows these cells undergo meiosis I. Scale bar = 5µm.
(B) mel1 mutant PMCs. PAIR2 signal (green) indicates these cells undergoing meiosis I, but no H3K9me2 reprogramming takes place.

<Glossary>

Histone modification
Chromatin, a structural element of chromosomes in eukaryotes, is composed of repetitive structure of nucleosomes including DNA and histone octamer (H2A, H2B, H3, H4). The N-terminal tail of histones protrudes from the DNA/histone core region. The amino acids of histone tails get various modifications, such as acetylation, methylation and phosphorylation. These modifications affect nucleosome states, resulting in alteration of gene expression and chromatin structure.
Argonaute protein
Argonaute (AGO) family proteins are highly conserved in eukaryotes. AGOs, in association with 20-30 nucleotides non-coding small RNAs, modulate both transcriptional and post-transcriptional gene silencing during various developmental events.

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