Regulation of etioplast pigment-protein complexes, inner membrane architecture, and protochlorophyllide alpha chemical heterogeneity by light-dependent NADPH : protochlorophyllide oxidoreductases A and B

The etioplast of dark-grown angiosperms is characterized by the prolamellar body (PLB) inner membrane, the absence of chlorophyll, and the accumulatio n of divinyl and monovinyl derivatives of protochlorophyll(ide) a [Pchl(ide ) a]. Either of two structurally related, but differentially expressed Ligh t-dependent NADPH:Pchlide oxidoreductases (PORs), PORA and PORE, can assemb le the PLB and form dark-stable ternary complexes containing enzymatically photoactive Pchlide-F655. Here we have examined in detail whether these pol ypeptides play redundant roles in etioplast differentiation by manipulating the total FOR content and the PORA-to-PORB ratio of etiolated Arabidopsis seedlings using antisense and overexpression approaches. FOR content correl ates closely with PLB formation, the amounts, spectroscopic properties, and photoreduction kinetics of photoactive Pchlide, the ratio of photoactive P chlide-F655 to non-photoactive Pchl(ide)-F632, and the ratio of divinyl- to monovinyl-Pchl(ide). This last result defines FOR as the first endogenous protein factor demonstrated to influence the chemical heterogeneity of Pchl (ide) in angiosperms. It is intriguing that excitation energy transfer betw een different spectroscopic forms of Pchl(ide) in etiolated cotyledons rema ins largely independent of FOR content. We therefore propose that the PLB c ontains a minimal structural unit with defined pigment stoichiometries, wit hin which a small amount of non-photoactive Pchl(ide) transfers excitation energy to a large excess of photoactive Pchlide-F655. In addition, our data suggests that FOR may bind not only stoichiometric amounts of photoactive Pchlide, but also substoichiometric amounts of non-photoactive Pchl(ide). W e conclude that the typical characteristics of etioplasts are closely relat ed to total FOR content, but not obviously to the specific presence of PORA . or PORE.