Although there are extensive studies to implicate gibberellin (GA) biosynthesis and metabolism in the phytochrome-mediated control of different physiological processes (e.g. seed germination and seedling growth), the supporting evidence is sparse. Recently, many genes encoding GA biosynthesis enzymes have been cloned (Hedden et al 1997; Lange 1998; Hedden et al 1999.) This has allowed the study of phytochrome regulation of GA biosynthesis at the molecular level. Research on lettuce seed germination led to the discovery of phytochrome almost half a century ago, and it was later suggested that phytochrome affects germination through GA's (Ikuma et al 1960).
Despite extensive studies on the roles of phytochrome in light-stimulated seed germination (Shinomura et al 1996; Shinomura et al 1994) the molecular mechanisms of hormonal involvement in this process are largely unknown. The cloning of GA biosynthetic enzyme genes in lettuce and in Arabidopsis has enabled the study of these mechanisms. Arabidopsis ga4-1 is a GA deficient semi-dwarf mutant (Ross at al 1997). GA4 gene was then cloned by T-DNA tagging (Hedden et al 1999) and it was shown to encode a GA 3 B-hydroxylase enzyme. Recently, a GA4 homolog ( GA4H ) was isolated and shown to encode a GA 3 B-hydroxylase (Yamaguchi et al 1998). The GA4H gene was found to be predominantly expressed during seed germination. Both GA4 and GA4H genes in imbibed seeds are induced by red light treatment. With the sequencing of the Arabidopsis genome it has been found that at least five different loci in the Arabidopsis genome code for phytochromes. Among these, PHYB (which encodes phyB,) plays a major role in germinating seeds shortly after the start of imbibition (Shinomura et al 1996), and it was suggested that the absolute concentration of the far red-absorbing form of phytochrome is important (Mancinelli 1993). In the phyB-deficient phyB-1 mutant, GA4H expression is not induced by red light, although GA4 expression still is, indicating that the red light-induced GA4 and GA4H expression is mediated by one or more other phytochromes.
Abstract,Introduction,Germination,Dormancy,Temperature,Light,Ecotypes
Ecotype-Storage,Soil preperation,Seed Sowing,Germination-Conditions,Measurement of Results
Cond1,Cond1rep,Cond2,Cond3,Cond4,Cond5,Cond6,Cond6rep,Cond7,Cond8