Chloroplast Biogenesis 601 CONVERSION OF DIVINYL PROTOCHLOROPHYLLIDE TO MONOVINYL PROTOCHLOROPHYLLIDE IN GREEN ( ING ) BARLEY , A DARK MONOVINYL

In higher plants, most of the chlorophyll a is formed via the divinyl and monovinyl chlorophyll monocarboxylic biosynthetic routes. These two routes are strongly interconnected prior to protochlorophyllide formation in barley (Hordeum vulgare L. cv Morex), a dark monovinyl-light divinyl plant species, but not in cucumber (Cucumis sativus L. cv Beit Alpha MR), a dark divinyl-light divinyl plant species (BC Tripathy, CA Rebeiz, 1986 J Biol Chem 261: 13556-13564). It is shown that in dark monovinyl-light divinyl plant species such as barley, the divinyl and monovinyl monocarboxylic routes become interconnected at the level of protochlorophyllide during transition from the divinyl to the monovinyl protochlorophyllide biosynthetic mode. In cucumber, a dark divinyl-light divinyl plant species, in which the monovinyl monocarboxylic biosynthetic route becomes preponderant only after an abnormally long sojourn in darkness, the conversion of divinyl to monovinyl protochlorophyllide does not take place on the barley time-scale of incubation. Photobiology, ABL, University of Illinois, Urbana, monocarboxylic biosynthetic route is not as preponderant as the DV monocarboxylic route, the DV and MV monocarboxylic routes were found to be weakly interconnected (15, 17). From in vitro investigations, the DV and MV monocarboxylic routes did not appear to be interconnected at the level of Pchlide, either in etiolated barley or in etiolated cucumber (15, 17). Indeed, although it is firmly believed that DV Pchlide is convertible to MV Pchlide in higher plants (5) by conversion of the vinyl group at position 4 of the macrocycle to an ethyl group (Fig. 1), this hypothesis has not yet been corroborated by experimental evidence (15, 17). During ongoing investigations of the regulation of the MV and DV Chl monocarboxylic biosynthetic routes in higher plants, preliminary experimental evidence suggested that in some plant species, DV Pchlide may be convertible to MV Pchlide, under certain conditions. In this work, it is shown that during the transition from the DV Pchlide to the MV Pchlide biosynthetic state, DV Pchlide can be partially converted to MV Pchlide in barley, a DMV/ LDV plant species. Under similar incubation conditions, the conversion of DV Pchlide to MV Pchlide did not appear to take place in cucumber, a DDV/LDV plant species. The discovery of the ubiquitous occurrence of DV3 and MV Pchlides in higher plants (1, 3) has led to a reevaluation of the Chl biosynthetic pathway (14). Protochlorophyllide is the main precursor of Chl in green plants. Considerable experimental evidence now indicates that, in higher plants, MV and DV Pchlides are formed from MV and DV Proto, respectively. via two parallel MV and DV monocarboxylic Chl biosynthetic routes (15, 17). Furthermore, on the basis of the MV or DV monocarboxylic biosynthetic routes that predominate at night or in daylight, higher plants have been observed to fall into one of four greening groups (3, 4, 15), namely: dark divinylllight divinyl (DDV/LDV), dark monovinyl/light monovinyl (DMV/LMV), DMV/LDV, and DDV/ LMV. It has also been demonstrated that in etiolated DMV/ LDV plant species such as barley, which are poised in the MV Pchlide biosynthetic mode (3, 4), the DV and MV monocarboxylic biosynthetic routes are strongly interconnected prior to DV Pchlide formation (15, 17). On the other hand, in etiolated DDV/LDV plant species such as cucumber in which the MV ' Supported by Research Grant National Science Foundation DMB 85-07217, by funds from the Illinois Agricultural Experiment Station, and by the John P. Trebellas Photobiotechnology Research Endowment