ISOLATION AND CLASSIFICATION OF CHLOROPHYLL-DEFICIENT XANTHA MUTANTS OF ARABIDOPSIS-THALIANA

Mutant lines of Arabidopsis thaliana that are either blocked at variou s steps of the biosynthetic pathway of chlorophyll (Chi) or that are d isturbed in one of the subsequent steps leading to the assembly of an active photosynthetic membrane were isolated by screening for Chl-defi cient xantha (ran) mutants. Only mutants that segregated in a 3:1 rati o, that contained the same carotenoid spectrum as etiolated wild-type seedlings and less than 2% of the Chi of wild-type control seedlings, and whose Chi content was not affected by the addition of sucrose to t he growth medium were selected for a more detailed analysis. As a fina l test for the classification of the selected mutants, light-grown ran mutants were vacuum infiltrated and incubated with the common precurs or of tetrapyrroles, delta-aminolevulinic acid (ALA), in the dark. Two major groups of mutants could be distinguished. Some of the mutants w ere blocked at various steps of the Chi pathway between ALA and protoc hlorophyllide (Pchlide) and did not accumulate the latter in the dark. The other mutants accumulated Pchlide in the dark regardless of wheth er exogenous ALA was added. This latter group could be subdivided into mutants with a biochemical lesion in a recently discovered second lig ht-dependent Pchlide reduction step that occurs in green plants and mu tants that have blocks in the assembly of Chi protein complexes. In th e present work a total of seven different loci could be defined geneti cally in Arabidopsis that affect the synthesis of Chi and its integrat ion into the growing photosynthetic membrane.