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Quantitative Modulation of Polycomb Silencing Underlies Natural Variation in Vernalization

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Science  03 Aug 2012:
Vol. 337, Issue 6094, pp. 584-587
DOI: 10.1126/science.1221881

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Is Winter Past?

Some plants are adapted to wait through the winter before their genetic programs for flowering are initiated. But the duration of winter varies with location, and thus, to avoid flowering too late into the summer or exposing blossoms to frost, the adaptation needs to hold the flowers off for just the right amount of time. Working with Arabidopsis thaliana, Coustham et al. (p. 584, published online 12 July) identified a quantitative mechanism that “counts” off the duration of winter. FLC, a repressor of flowering, carries sequence polymorphisms that direct epigenetic events. Plants with fewer of these polymorphisms are adapted to short winters, and plants with all four polymorphisms are adapted to long winters.

Abstract

Arabidopsis thaliana accessions have adapted to growth in a wide range of climates. Variation in flowering and alignment of vernalization response with winter length are central to this adaptation. Vernalization involves the epigenetic silencing of the floral repressor FLC via a conserved Polycomb (PRC2) mechanism involving trimethylation of Lys27 on histone H3 (H3K27me3). We found that variation for response to winter length maps to cis polymorphism within FLC. A rare combination of four polymorphisms localized around the nucleation region of a PHD-Polycomb complex determines a need for longer cold. Chromatin immunoprecipitation experiments indicate that these polymorphisms influence the accumulation of H3K27me3 in Arabidopsis accession Lov-1, both at the nucleation site and over the gene body. Quantitative modulation of chromatin silencing through cis variation may be a general mechanism contributing to evolutionary change.

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