chlorophyll-a and succinylacetone

Fumarylacetoacetate hydrolase (FAH) catalyses the final step of the tyrosine degradation pathway, which is essential to animals but was of unknown importance in plants until we found that mutation of Short-day Sensitive Cell Death1 (SSCD1), encoding Arabidopsis FAH, results in cell death under short-day conditions. The sscd1 mutant accumulates succinylacetone (SUAC), an abnormal metabolite caused by loss of FAH. Succinylacetone is an inhibitor of δ-aminolevulinic acid (ALA) dehydratase (ALAD), which is involved in chlorophyll (Chl) biosynthesis. In this study, we investigated whether sscd1 cell death is mediated by Chl biosynthesis and found that ALAD activity is repressed in sscd1 and that protochlorophyllide (Pchlide), an intermediate of Chl biosynthesis, accumulates at lower levels in etiolated sscd1 seedlings. However, it was interesting that Pchlide in sscd1 might increase after transfer from light to dark and that HEMA1 and CHLH are upregulated in the light-dark transition before Pchlide levels increased. Upon re-illumination after Pchlide levels had increased, reactive oxygen species marker genes, including singlet oxygen-induced genes, are upregulated, and the sscd1 cell death phenotype appears. In addition, Arabidopsis WT seedlings treated with SUAC mimic sscd1 in decline of ALAD activity and accumulation of Pchlide as well as cell death. These results demonstrate that increase in Pchlide causes cell death in sscd1 upon re-illumination and suggest that a decline in the Pchlide pool due to inhibition of ALAD activity by SUAC impairs the repression of ALA synthesis from the light-dark transition by feedback control, resulting in activation of the Chl biosynthesis pathway and accumulation of Pchlide in the dark.

Topics: Aldehyde Oxidoreductases; Aminolevulinic Acid; Arabidopsis; Arabidopsis Proteins; Cell Death; Chlorophyll; Gene Expression Regulation, Plant; Heptanoates; Hydro-Lyases; Hydrolases; Light; Lyases; Oxygen; Protochlorophyllide; Reactive Oxygen Species; Seedlings; Transcriptome

Steady-state mRNA levels of the three nuclear genes cab1, rbcS1 and rbcS2 (coding for the light-harvesting chlorophyll-binding protein (LHCP) and the small subunit of ribulose 1,5-bisphosphate carboxylase, respectively) and of the two plastid-encoded genes rbcL and psaA2 (coding for the large subunit of the carboxylase and a member of the P700 chlorophyll a protein, respectively) have been investigated in synchronized Chlamydomonas cells in response to light and inhibitors interfering with chlorophyll synthesis. The accumulation of cab1, rbcS1 and psaA2 transcripts is light-dependent, whereas transcripts from rbcS2 and rbcL genes are present in high amounts in the light and in the dark. Dioxoheptanoic acid, an inhibitor blocking chlorophyll synthesis prior to porphyrin formation, does not affect the accumulation of all five mRNAs. However, inhibition of chlorophyll synthesis by incubating cells with dipyridyl, cycloheximide or nitrogen promotes the accumulation of porphyrin compounds, but specifically prevents the accumulation of light-dependent transcripts. Although functionally unrelated, these inhibitors are known to block an Fe-dependent oxygenase, which is involved in the formation of the isocyclic ring in the chlorophyll molecule. The data are explained as a control by chlorophyll precursors over the accumulation of light-dependent transcripts.

Topics: 2,2'-Dipyridyl; Chlamydomonas; Chloramphenicol; Chlorophyll; Cycloheximide; DNA Probes; Hemin; Heptanoates; Iron; Levulinic Acids; Light; Light-Harvesting Protein Complexes; Nucleic Acid Hybridization; Photochemistry; Photosynthetic Reaction Center Complex Proteins; Plant Proteins; Protein Precursors; Pyridazines; RNA, Messenger; Transcription, Genetic