Handbook of Plant and Crop Physiology

Chlamydomonas reinhardtii[78], and pea [79], which contain only one LPOR gene. In dark-grown

leaves, LPORA is usually more abundant than LPORB [79]. In contrast to LPORA, whose expression ap-

pears to be correlated with Pchlide synthesis [80], LPORB is constitutively expressed [76]. In plants cul-

tivated under a light/dark regime, LPORA concentration shows diurnal variations [81], which might be

correlated with the variations in the amount of Chl in leaves from plants cultivated in the field [82]. The

accumulation of LPORA and Pchlide at the end of the dark phase of each light/dark cycle [28,81] corre-

lates with the observation of small PLB during this period [83].

All POR proteins have a high degree of homology [77]. LPORA and LPORB amino acid sequences,

as deduced from complementary DNA (cDNA) clones, are very related proteins presenting more than

75% homologies. The homology increases to 82% when the deduced amino acid sequences of the mature

proteins are compared. The two sequences are much more divergent within the signal sequences [76].

Electron microscopy and spectroscopic measurements have demonstrated that LPORB and LPORA are

able to induce the formation of a regular PLB and also to drive the Pchlide-Chlide cycle [84].

D. Chlorophyll Formation in Plants That Contain Both the

Light-Dependent and the Light-Independent Protochlorophyllide

Oxidoreductase

The fact that most of the eukaryotic nonangiosperms and green algae are able to green in the dark has been

recognized for a long time (e.g., Ref. 85). The ability to synthesize Chl in the dark correlates with the ad-

ditional presence of a light-independent Pchlide oxidoreductase (DPOR). Biochemical and molecular ge-

netic data indicate that LPOR and DPOR are not related. In fact, DPOR is probably formed by three dif-

ferent subunits (reviewed in Refs. 8, 9, and 14). Most of the data on Chl formation in plants containing

LPOR and DPOR have been obtained from gymnosperms.

CHLOROPHYLL BIOSYNTHESIS DURING PLANT GREENING 271

Figure 5 (A) The effect of light on (•) the relative amount of LPOR mRNA, () the relative LPOR concen-
tration, () the specific LPOR activity, and (
) the relative Chlide content. (From Ref. 73.) (B) Sodium dode-
cyl sulfate–polyacrylamide gel electrophoresis of the proteins contained in (NI) nonilluminated 2-day-old bean
leaves and leaves after (1) 1 hr, (4) 4 hr, and (16) 16 hr of greening. The band corresponding to LPOR is indi-
cated by the arrow. Ref indicates the protein used as a standard.