Project

Seed dormancy; Seed coat; Arabidopsis; Abscisic acid (ABA); Gibberellic acid (GA); RGL2; DELLA factors; ABI3 ABI5; gibberellins; Abscisic acid; seed germination; ABI5

Lead
Lay summary
Seeds are of vital importance for life on earth, including that of humans. Indeed, seeds are an essential source of food for animals since plants store in seeds high amounts of nutritive substances (e.g. carbohydrates, oils) to fuel germination. When seeds germinate, they abandon a state highly resistant to heat and drought to develop into a young and fragile young seedling. It is therefore not surprising that seed germination is a tightly controlled process, responding to environmental factors such as water quality (e.g. containing high or low salt) or light quality (e.g. sunlight versus canopy light). Prior to germination, seeds must first lose their dormancy. Indeed, plants generate seeds in a dormant state, i.e. a state where germination will not take place even under favorable germination conditions such as imbibition by water. Dormancy may prevent germination out of season or avoid competition among seeds of the same species by preventing their germination in the same location. The mechanisms preventing the germination of dormant seeds are mostly unknown. A better understanding of seed dormancy is important for agriculture since it prevents precocious germination in the growing plant and thus loss of crop yield.Two key properties characterize dormant seeds: 1) the seed coat is essential to maintain the dormant state since its removal will trigger the growth and greening and the embryo; 2) dormant seeds synthesize high amounts of the phytohormone abscisic acid (ABA) which is essential to block their germination. How the seed coat exerts its inhibitory role and how dormant seeds synthesize ABA is unknown.Our project proposal intends to further explore the role of the seed coat and the molecular genetic mechanisms leading to ABA accumulation. We also wish to investigate whether these two key properties of seeds are related.To address these issues, we have developed a "seed coat bedding" assay that physically separates the seed coat and the embryo while still keeping active the repression of germination by the seed coat. This assay shows that the seed coat is a tissue producing diffusible signals necessary to prevent embryonic growth. Furthermore, the assay allows using seed coats of different genotypic backgrounds to genetically dissect the germination-repressing activities produced by the seed coat. We intend to identify the germination inhibitory compounds released by dormant seed coats and ask whether ABA is one of them. Furthermore, we will address the hypothesis that high ABA production in dormant seeds results from constitutive production of RGL2, a factor previously shown to promote ABA accumulation in non-dormant seeds.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

In Arabidopsis it is well established that repression of seed germination in dormant seeds upon seed imbibition is an active process requiring de novo and constitutive synthesis of the phytohormone abscisic-acid (ABA). In this project proposal, we present preliminary results showing that dormant seeds constitutively express the gibberellic acid (GA)-response factor RGL2, a DELLA factor of the GRAS family recently shown to stimulate endogenous ABA synthesis. Unlike non-dormant seeds, dormant seeds are unable to down-regulate RGL2 expression (mRNA and protein) in response to GA. We propose to explore the hypothesis that de novo ABA synthesis is due to the constitutive accumulation of RGL2. Furthermore, our proposed experiments address the questions of how RGL2 constitutive expression is achieved and how dormancy-breaking treatments (e.g. stratification) down-regulate RGL2 expression.We also present a novel “seed coat bedding assay” that physically separates the seed coat and the embryo while still keeping active the repression of germination by the seed coat. This assay shows that the seed coat is a tissue producing diffusible signals necessary to prevent embryonic growth. Furthermore, the assay allows using seed coats of different genotypic backgrounds to genetically dissect the germination-repressing activities produced by the seed coat. We propose experiments aimed at identifying such diffusible signals as well as to test the hypothesis that the seed coat is an essential organ for ABA production to sustain ABA-dependent repression in dormant embryos.