Abstract
In addition to angiosperms, most plants are able to synthesize chlorophyll (Chl)-generating green tissues in total darkness. In this study, 140 plants of the angiosperm Pachira macrocarpa were divided into five groups. Among them, one group was grown for 2 weeks under natural light conditions, whereas the others were grown in complete darkness (0 μmol m−2 s−1). Dark-grown plants were then treated with 0~6% glucose for another 8 weeks. The budding and greening ratios, ultrastructure of chloroplasts (ChlPs) of newly developed leaves, and green pigment contents of pre-illuminated mature and young leaves, and totally dark-grown newly developed leaves were measured. Results showed that glucose inhibited the budding and promoted the greening of newly developed leaves. Pre-illuminated mature and young leaves were able to synthesize green pigments during the 2 weeks of dark adaption. Dark-grown newly developed leaves contained high levels of green pigments at 2 and 3 weeks after budding. Green pigments of glucose-fed newly developed leaves had increased, whereas they had decreased in control leaves. In addition, ChlPs of dark-grown glucose-fed newly developed leaves contained both giant grana and prolamellar bodies (PLBs), usually found in shade plants and etiolated seedlings, respectively. The higher the glucose concentration was, the greater the numbers of grana, thylakoids, and PLBs. Glucose increased the green pigment contents and grana formation in newly developed leaves in a dark condition, and the mechanisms are discussed.
Highlights
Chloroplasts (ChlPs) harbor the photosynthetic apparatus, containing chlorophylls (Chls), pigment-protein complexes, and the thylakoid system
The gradient of glucose concentrations (Figure 1c–f) resulted in a gradient of green color in darkgrown newly developed leaves, indicating that newly developed leaves of dark-grown plants had synthesized some green pigments during the 2 weeks, but these leaves were unable to continue this biochemical phenomenon in complete darkness
Our study provides a beginning for studying leaf greening and thylakoid development in newly developed leaves of dark-grown plants
Summary
Chloroplasts (ChlPs) harbor the photosynthetic apparatus, containing chlorophylls (Chls), pigment-protein complexes, and the thylakoid system. In ChlPs of higher plants, Chl biosynthesis requires light, and the limiting step is the transfer of the protochlorophyllide (Pchlide) metabolite to chlorophyllide (Chlide) by light-dependent Pchlide oxidoreductase (POR; LPOR), when the protein structure changes and reduces nicotinamide adenine dinucleotide phosphate (NADPH) oxidation to synthesize Chlide [1,2]. Many angiosperm seedlings are etiolated in completely dark growth conditions due to a lack of DPOR genes [6]. Algal cells can be cultured and survive under dark conditions if fed glucose, sucrose, or glycerol as carbon sources, and they synthesize Chl using their DPOR genes [6,7]. Whether other green pigments can be synthesized in angiosperms under dark conditions while being fed glucose still remains unknown
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