DNA was extracted from leaves of the seedlings as
described previously (Zheng et al., 1995). Briefly, a 2.0 cm
slice of leaf was homogenized in a microfuge tube using a
polished glass or polypropylene pestle. The resulting paste
was mixed with 0.4 ml of extraction buffer (50 mM Tris-HCl
pH 8.0, 25 mM EDTA, 300 mM NaCl and 1% SDS) and the
tube was incubated at 60°C for 10 minutes with occasional
agitations. The tube was cooled down and the lysate was
extracted with 0.5 ml of a mixture of chloroform and isoamyl
alcohol (24:1). The tube was centrifuged at 10,000xg for 8
minutes and the upper aqueous layer of the supernatant was
transferred to a new tube. Equal volume cold isopropanol was
added to the tube and mixed. DNA was precipitated by