Control of development by Phytochrome in the garden pea (Pisum sativum L.)

The phytochrome family is a well-known and important group of biliprotein photoreceptors which mediate effects of light on plant development. This thesis comprises an investigation of the physiological roles of different forms of phytochrome in the garden pea (Piswn sativwn L.). The presented work focuses on the isolation and subsequent physiological characterization of various phytochrome-deficient mutants of pea, including mutants deficient in phytochrome A, phytochrome B and phytochrome chromophore synthesis. Ethylmethanesulfonate mutagenesis was employed to induce muta tion in the pea cul tivar Torsdag. M2 seedlings were screened under red (R) or far-red light (FR) conditions to identify potential phytochrome-deficient mutants. All candidate mutants were back-crossed to the parental line to establish the nature of the genetic control of the mutant phenotypes, and complementation testing was performed between mutant lines showing similar alterations to spectral sensitivity. Four distinct loci were defined by mutants isolated in these screens, corresponding to the previously described LV locus and three novel loci designated FUN1, PCDl and PCD2. LV and FUN1 were mapped to linkage groups VI and II, respectively. Apoprotein-deficient mutants. Mutant lv plants are deficient in a phyB apoprotein and show reduced responses to R, while funl mutants are deficient in phytochrome A apoprotein and are unresponsive to FR. Physiological studies suggest that phyB controls seedling phytochrome responses in the LFR mode, whereas phyA controls responses in the VLFR and HIR modes. Double mutants lacking both phyA and phyB reveal (a) that phyA is the only phytochrome controlling responses to continuous FR (b) that both phyA and phyB are required for normal responses to continuous R, and (c) neither phyA nor phyB play a substantial role in inhibition of stem elongation by blue light. Examination of mature plant photoresponses revealed a continuing role for both phyA and phyB in the maintenance of full de-etiolation, and an important role for phyA in photoperiod detection and flower induction. Chromophor e-de ficient mutants. Mutant pcd1 and pcd2 plants show a reduction in both phyA- and phyB-mediated responses at the seedling stage and a severe depletion of spectrally active phytochrome, consistent with deficiency in the common tetrapyrrole chromophore, phytochromobilin (Pq>B). In vitro assembly studies, in vivo feeding of Pq>B precursors, and HPLC-based assay of heme and biliverdin (BV) IXa metabolism in isolated plastids revealed the pcd1 and pcd2 mutants to be deficient in the conversion of heme to BV IXa and BV IXa to Pq>B, respectively.