Handbook of Plant and Crop Physiology

ever, one exception: the light-dependent NADPH:Pchlide oxidoreductase (LPOR, EC 1.3.1.33). This en-

zyme has two amazing properties: (1) it requires light for activity and (2) in the absence of light, the en-

zyme forms stable ternary complexes with its cofactor (NADPH) and its substrate (Pchlide) without re-

acting with them [11]. It is therefore obvious that the transformation of Pchlide to Chlide constitutes a

strong regulating point of Chl synthesis, especially in plants, which are unable to synthesize Chl in the

dark (see Sec. III.D). The reactions catalyzed by PORs are the main topic of this chapter.

III. TRANSFORMATION OF PCHLIDE TO CHLIDE UNDER A FIRST

ILLUMINATION IN ANGIOSPERMS: INFLUENCE OF THE GROWTH

CONDITION

In angiosperms, chloroplast biogenesis invariably begins with the photoreduction of photoactive Pchlide

to Chlide because the formation of the first Chlide molecules initiates the synthesis of the chloroplast-en-

coded proteins, which will be used for the assembly of the photosynthetic apparatus [12].

Most of the data about the development of the photosynthetic apparatus including pigment bio-

genesis have been obtained using etiolated plants (reviewed in Refs. 13 and 14). Although the etiolated

plants—Dubrunfaut [15] considered them ill—cannot be taken as a model for plants that develop in na-

ture, they can probably be used to study chloroplast development in plants cultivated in the field. In

fact, modern agricultural methods bury the seeds deep in the soil and, therefore, the leaves start to grow

almost in the absence of light. In situ measurements demonstrate that in these conditions, the leaves

perceive light when they reach a level approximately 2 mm below the soil surface [16] (reviewed in

Ref. 17). It is likely that at this moment the proplastids are already developed into etioplasts. Even if

the seeds fall on the ground, in the natural environment, the embryonic leaves can hardly see the light

before germination. When the appropriate conditions exist, the seed germinates, i.e., the radicle

emerges from the seed [18] (Figure 2). This event modifies the light environment of the embryonic

leaves because the radicle can conduct light to them as an optic guide would do [19]. In the literature,

the terms designating the material used for greening experiments are often confusing. Therefore,

throughout this chapter, the terms old and young leaves were used to designate etiolated leaves with

etioplasts and embryonic leaves with proplastids, respectively.

It is important to note that plant species can be classified in different groups on the basis of the Pch-

lide chemical form (either monovinyl or divinyl) accumulated during the night and Chlide chemical form

produced at daybreak and later [20]. Spectroscopic measurements using isolated LPOR have indicated

that the mechanism of Pchlide reduction is not significantly affected by the group to which a plant be-

longs [21]. In addition, the presence of an 8-ethyl or an 8-vinyl at the Pchlide ring B does not significantly

influence the spectral properties of the different Pchlide forms in the red region (600–800 nm). In con-

trast, in the blue region (400–500 nm), significant differences can be observed [22–24].

CHLOROPHYLL BIOSYNTHESIS DURING PLANT GREENING 267

Figure 2 Germination of seeds of Acersp. in nature. Only the radicle is out of the seeds. Pictures taken on
March 14, 1999 in the park of the Hluboka castle (Hluboka nad Valtavou, Czech Republic).