Photomorphogenetic mutants of higher plants

In this thesis the hypothesis that different molecular types of phytochrome have different physiological roles has been elaborated and an attempt has been made to assign functions to these phytochrome types. For this purpose a genetic approach was chosen and the following potential phytochrome mutants were used: the tomato aurea ( au ) mutant; the cucumber long-hypocotyl ( lh ) mutant and the Arabidopsis long-hypocotyl ( hy3 ) mutant. Comparing the response of the mutant affected in the abundance or function of a particular phytochrome type with that of its isogenic wild type shows directly the relevance of the phytochrome type concerned. The au mutant is proposed to lack the light-labile phytochrome type (phy A), while normal levels of the lightstable phytochrome type (phy B) are present. This results in a reduction of. (i) the inhibition of the hypocotyl growth; (ii) the anthocyanin synthesis; (iii) the chlorophyll content, but a normal end-of-day far-red (EOD FR) response. In the lh and hy3 mutants the opposite situation appears to be found: normal levels of phy A, but a lack of phy B. Both the lh and hy3 mutant do not show the EOD FR response, while the hypocotyl growth in continuous R and B is less inhibited when compared to the wild type. Therefore it is proposed that phy B plays a role in the EOD FR response, while both phy A and phy B are important for the inhibition of the hypocotyl growth. With the help of other mutants described in the literature a more detailed assignment of functions to the different phytochrome types is proposed.In addition, a potential phytochrome signal transduction mutant, the tomato high- pigment ( hp ) mutant, was characterized. This mutant exhibits exaggerated phytochrome responses, e.g. high anthocyanin synthesis and short hypocotyl length when compared to wild type. It. is proposed that the hp mutation affects an amplification step in the phytochrome transduction chain.