THE PHYTOCHROME-DEFICIENT PCD1 MUTANT OF PEA IS UNABLE TO CONVERT HEME TO BILIVERDIN IX-ALPHA

We isolated a new pea mutant that was selected on the basis of pale co lor and elongated internodes in a screen under white light. The mutant was designated pcd1 for phytochrome chromophore deficient. Light-grow n pcd1 plants have yellow-green foliage with a reduced chlorophyll (Ch l) content and an abnormally high Chi a/Chl b ratio. Etiolated pcd1 se edlings are developmentally insensitive to far-red light, show a reduc ed response to red light, and have no spectrophotometrically detectabl e phytochrome. The phytochrome A apoprotein is present at the wild-typ e level in etiolated pcd1 seedlings but is not depleted by red light t reatment. Crude phytochrome preparations from etiolated pcd1 tissue al so lack spectral activity but can be assembled with phycocyanobilin, a n analog of the endogenous phytochrome chromophore phytochromobilin, t o yield a difference spectrum characteristic of an apophytochrome-phyc ocyanobilin adduct. These results indicate that the pcd1-conferred phe notype results from a deficiency in phytochrome chromophore synthesis. Furthermore, etioplast preparations from pcd1 seedlings can metaboliz e biliverdin (BV) IX alpha but not heme to phytochromobilin, indicatin g that pcd1 plants are severely impaired in their ability to convert h eme to BV IX alpha. This provides clear evidence that the conversion o f heme to BV IX alpha is an enzymatic process in higher plants and tha t it is required for synthesis of the phytochrome chromophore and henc e for normal photomorphogenesis.