The expression of a Vp1-like gene and seed dormancy in Mesembryanthemum crystallinum.

Seeds of the common ice plant (Mesembryanthemum crystallinum) germinate in distinct sub-populations over a time period of more than 4 weeks following imbibition. Distinguishing early (E)- and late (L)-germinating seeds is the expression of a homologue of the transcriptional activator VP1. The deduce d amino acid sequence of ice plant VP1 (MVP1) is 39% identical (50% similar ) to the sequence of the Arabidopsis VP1 homologue, ABI3. The amount of Mvp 1 mRNA, transcribed from a single gene, is different in E and L seeds after water uptake. The levels of the Mvp1 transcripts are very low in immature and mature seeds and they increased during 6 days of imbibition. This expre ssion profile of Mvp1 is different from known Vp1/ABI3-like genes in other plants. Cycloheximide (at 35 muM) abolishes the increase of Mvp1, and L see ds are turned into E seeds, which develop normally when the inhibitor is ap plied for a short time during imbibition. E seeds treated for the same time period are developmentally impaired and show no radicle elongation. We sug gest that the presence and late disappearance of Mvp1 in L seeds is respons ible for dormancy and after-ripening of late-germinating ice plant seeds.