Effects of cold on DNA methylation. Southern blot analysis of genomic DNA extracted from mesocotyl (A, C, E, G) or root tissues (B, D, F, H) untreated (23°C) or treated by cold stress (4°C). The DNA extracts were digested with a restriction endonuclease, HpaII, separated on an agarose gel (10 µg per lane), blotted and probed with a radioactively labeled ZmPK4 cDNA fragment (A, B), ZmMET1 cDNA fragment (C, D), Histone H3 cDNA fragment (E, F) or Ac/Ds cDNA fragment (G, H). The membranes were washed at high stringency. The size of the hybridizing signals is indicated along with the positions of DNA molecular markers. The densitometric trace with black and red lines represents the relative intensity of signals for untreated (23°C) and treated (4°C) plants, respectively. Arrowheads indicate the positions of a 0.8 and a 0.5 kb fragment (G, H).