| Abstract |
In many plant species, seed dormancy is broken by cold stratification, a pre-chilling treatment of fully imbibed seeds. Although the ecological importance of seed response to cold temperature is well appreciated, the mechanisms underlying the physiological changes during cold stratification is unknown. Here we show that the GATA zinc finger protein expressed in Arabidopsis seeds during cold stratification plays a critical role in germination. Characterization of an enhancer-trap population identified multiple lines that exhibited beta-glucuronidase (GUS) expression in the micropylar end of the seed (named Blue Micropylar End, BME lines). One of these lines, BME3, had a T-DNA insertion site in the 5' upstream region of a GATA-type zinc finger transcription factor gene (termed BME3-ZF). The BME3-ZF mRNA accumulated in seeds during cold stratification. Characterization of the BME3-ZF promoter indicated that this gene was activated specifically in the embryonic axis, which was still enclosed by the endosperm. The zinc finger gene knockout plants produced seeds exhibiting deeper dormancy, which showed reduced response to cold stratification. The ungerminated knockout seeds exhibited testa rupture, but failed to penetrate the endosperm layer. Application of gibberellic acid (GA3) rescued impaired germination of knockout seeds without cold stratification, indicating that the normal GA signal transduction pathway is present in the knockout mutants. Expression of GA20-oxidase and GA3-oxidase genes was greatly reduced in the knockout seeds, suggesting the potential involvement of the zinc finger protein in GA biosynthesis. These results suggest that the GATA zinc finger protein is a positive regulator of seed germination. |