Non-canonical RNA-directed DNA methylation participates in maternal and environmental control of seed dormancy.

Mayumi Iwasaki,Lena Hyvärinen,Luis Lopez-Molina,Urszula Piskurewicz

doi:10.7554/elife.37434

Abstract

Seed dormancy is an adaptive trait preventing premature germination out of season. In a previous report (Piskurewicz et al., 2016) we showed that dormancy levels are maternally inherited through the preferential maternal allele expression in the seed endosperm of ALLANTOINASE (ALN), a negative regulator of dormancy. Here we show that suppression of ALN paternal allele expression is imposed by non-canonical RNA-directed DNA methylation (RdDM) of the paternal ALN allele promoter. Dormancy levels are further enhanced by cold during seed development. We show that DNA methylation of the ALN promoter is stimulated by cold in a tissue-specific manner through non-canonical RdDM, involving RDR6 and AGO6. This leads to suppression of ALN expression and further promotion of seed dormancy. Our results suggest that tissue-specific and cold-induced RdDM is superimposed to parental allele imprints to deposit in the seed progeny a transient memory of environmental conditions experienced by the mother plant.

Highlights

Produced seeds exhibit dormancy, a trait whereby germination is blocked upon imbibition under otherwise favorable germination conditions
We show that DNA methylation of the ALN promoter is stimulated by cold in a tissue-specific manner through non-canonical RNA-directed DNA methylation (RdDM), involving RDR6 and AGO6
WT x nrpd1 hybrid seed endosperm retained preferential maternal ALN allele expression (Figure 1A). These results show that ALN imprinting requires non-canonical RdDM for paternal allele silencing rather than MET1 or canonical RdDM as previously reported (Gehring, 2013; Vu et al, 2013; Rodrigues and Zilberman, 2015)

Summary

Introduction

Produced seeds exhibit dormancy, a trait whereby germination is blocked upon imbibition under otherwise favorable germination conditions. Dormancy prevents germination out of season and facilitates final dispersion of the plant embryo. The Arabidopsis seed consists of an embryo surrounded by a single cell layer of endosperm and an external seed coat. The endosperm is essential to repress dormant seed germination by releasing the phytohormone abscisic acid (ABA), which blocks embryonic growth (Lee et al, 2010; Chahtane et al, 2017). Dry seeds gradually lose dormancy over a period of time, referred as ‘after-ripening’, acquiring the capacity to germinate. After-ripening time is a measure of dormancy levels, which vary across Arabidopsis accessions

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Conclusion