Abstract
Seed coat-imposed dormancy is part of the seed survival strategy of many species. The seed coat (testa) exerts its germination-restrictive action most of the time by being impermeable to water and/or oxygen or by its mechanical resistance to radicle protrusion. These properties have been positively correlated with seed coat color due to phenolic compounds in diverse species. The brown pigments of the seeds are mainly condensed tannins of the procyanidin type, which are end-products of the flavonoid biosynthetic pathway. The seeds of Arabidopsis transparent testa (tt) mutants which have defect in flavonoid metabolism have been reported to reduce dormancy at various levels. We have isolated tt7 allele (tt7-4) as one of the high temperature resistant germination mutants, and found that tt4-1, tt5-1, tt6-1, tt7-1 and fis1, all of which are deficient in quercetin accumulation, were high temperature resistant but tt3-1 and banyuls, which are deficient in proanthocyanidins, were not. We confirmed that quercetin contents in wild type and tt mutant seeds showed good anti-correlation with germination levels at high temperatures. These results suggest that quercetin, quercetin glycosides or its derivatives are responsible for coat-imposed dormancy and thermoinhibition of Arabidopsis seeds. We are now isolating flavonol glycosidase mutants and are planning to see germination phenotypes of the knockout mutant seeds.
