Photoconversion and regeneration of active protochlorophyll(ide) in mutants defective in the regulation of chlorophyll synthesis

Seedlings carrying mutations in regulatory genes for protochlorophyll(ide) synthesis accumulate protochlorophyll(ide) in darkness in amounts exceeding the wildtype level. Thus, +/tig-d¹² and $\text{tig-b}{}^{24}\text{tig-b}{}^{24}$accumulate 2-fold, $\text{tig-o}{}^{34}\text{tig-o}{}^{34}$ 5- to 6-fold, and $\text{tig-d}{}^{12}\text{tig-d}{}^{12}$ 15-fold more protochlorophyll(ide) than the wild type.The amount of photoconvertible protochlorophyll(ide) accumulated in darkness is the same in all genotypes, despite the large differences in total protochlorophyll(ide) content, indicating a constant number of photoconversion sites.When briefly illuminated leaves are returned to darkness, regeneration of active protochlorophyll(ide) from the pool of inactive protochlorophyll(ide) takes place in wild-type and mutant leaves. Compared to the wild type, the rate of protochlorophyll(ide) activation during 4- and 10-min dark periods is higher in +/tig-d¹², $\text{tig-b}{}^{24}\text{tig-b}{}^{24}$, and $\text{tig-o}{}^{34}\text{tig-o}{}^{34}$, but lower in $\text{tig-d}{}^{12}\text{tig-d}{}^{12}$.There was no indication that the accumulation of protochlorophyll(ide) influences the conversion sites of the protochlorophyll(ide) holochrome, as the kinetics of photoconversion of initially active protochlorophyll(ide) in leaves with the genotypes +/+, +/tig-o³⁴, and $\text{tig-o}{}^{34}\text{tig-o}{}^{34}$ are similar or identical.