Abstract
Oxygenation of pheophorbide a is a key step in chlorophyll breakdown. Several biochemical studies have implicated that this step was catalyzed by an iron-containing and ferredoxin-dependent monooxygenase, pheophorbide a oxygenase (PaO). It has been proposed that inhibition of its activity leads to the "stay-green" phenotype. We searched the Arabidopsis genome for a possible PaO encoding gene and identified three such genes,Tic55,ACD1 and ACD1-like. We produced transgenic Arabidopsis plants which expressed antisense RNA as a method to inhibit the expression of these genes. After they were kept under darkness for 5 days and again illuminated, the leaves of the antisense ACD1 plants (AsACD1) were bleached. The rate of decrease in chlorophyll a was not influenced and pheophorbide a was accumulated in senesced AsACD1 leaves. These results demonstrated that ACD1 encodes PaO, and its inhibition led to photooxidative destruction of the cell instead of the "stay-green" phenotype.
