PHOTOBIOLOGICAL CHARACTERIZATION OF A SPORE GERMINATION MUTANT dkgl WITH REVERSED PHOTOREGULATION IN THE FERN Ceratopteris richardii

Abstract— This paper describes the mutant dkgl in the fern Ceratopteris richardii, which shows rapid germination in darkness but is markedly inhibited by white light. Action spectra plotted at 10 nm intervals from 400 to 800 nm are presented for germination responses of wild-type and mutant spores to photon flux densities of 0.004, 0.04 and 0.4 jtmol/mVs. The action spectra for wild-type spores exhibit a sharp phytochrome-mediated peak at 660 nm, a broad peak from 670 to 740 nm resulting from an apparent high irradiance response and no germination below 560 nm. In the corresponding action spectra for mutant spores, the blue region displays rather complex fine structure with prominent minima at 450 and 470 nm, which suggests that cryptochrome is unaltered in these spores. The region from 550 to 640 nm shows the greatest inhibition of spore germination, but this region exhibits no obvious fine structure, which argues rather strongly against the possibility of a unique photoreceptor being active in mutant spores. The mutant spectra resemble the wild-type spectra in the region from 650 to 800 nm, and thus phytochrome seems normal in the mutant spores. The dkgl mutation appears to act late in the phytochrome transduction pathway where a hypothetical coupling protein may regulate the light-sensitive step in spore germination.