The capacity of green oilseeds to utilize photosynthesis to drive biosynthetic processes

Abstract

Seeds of many plant species are green during embryogenesis. To directly assess the influence of light on the physiological status of green oilseeds in planta, Brassica napus and soybean (Glycine max) seeds were rapidly dissected from plants growing in the light or dark. The activation state of malate dehydrogenase, which reflects reduced thioredoxin and NADP/NADPH ratios, was found to be as high in seeds exposed to light as in leaves and to decrease in the dark. Rubisco was highly activated (carbamylated) in both light and dark, most likely reflecting high seed CO(2) concentrations. Activities of Rubisco and phosphoribulokinase were sufficient to account for significant refixation of CO(2) produced during B. napus oil biosynthesis. To determine the influence of light on oil synthesis in planta, siliques on intact plants in full sunlight or detached siliques fed (3)H(2)O were partly covered with aluminum foil. Seeds from light and dark sections were analyzed, and fatty acid accumulation was found to be higher in seeds exposed to light than seeds from dark sections. The spectrum of light filtering through silique walls and the pigment composition of developing B. napus embryos were determined. In addition to a low chlorophyll a/b ratio, the carotenoid pigments of seeds can provide additional capture of the green light that filters through siliques. Together, these results demonstrate that even the low level of light reaching seeds plays a substantial role in activating light-regulated enzymes, increasing fatty acid synthesis, and potentially powering refixation of CO(2).

Figure 1.

Transmission spectra of a B. napus leaf and about a 3-week-old green silique. Each spectrum is an average of two measurements over different areas.

Figure 2.

The activation state of chloroplast NADP-dependent MDH in B. napus leaves and developing seeds and in soybean seeds measured from dark-adapted (2 h) material and samples harvested from sunlight. In all samples, the activation state is significantly higher in light, demonstrating the presence of photosynthetic electron transport in light. The values are an average ± sd of three to four independent determinations.

Figure 3.

Carbamylation state of Rubisco in B. napus leaves and seeds harvested from dark or light. For comparison, carbamylation ratios were also measured from soybeans. The values are an average ± sd of three to four independent determinations.

Figure 4.

Immunoblot of protein extracts from leaves and developing seeds of B. napus and soybean. Analysis was performed using antibody against soybean PRK. Lanes 1 and 2 contained 5 μg of protein extracted from B. napus and soybean leaf, respectively, and lanes 3 and 4 contained 20 μg of protein extracted from developing B. napus and soybean seeds. PRK size is about 43 kD.

Figure 5.

Influence of light on lipid synthesis in seeds inside siliques. A, Comparison of FA content of B. napus seeds exposed to light or dark. Either the basal or the top half of 13 siliques on plants growing outside was covered with aluminum foil so that one-half of the seeds was shaded. After a 10-h period of sunlight, the seeds of each half of the 13 siliques were analyzed. The difference between the light treatments was statistically significant (paired Student's t test; P < 0.001). For siliques 2, 3, and 8, the tip was dark, for others the base. B, Sugar contents of same seeds analyzed in A. C, Incorporation of ³H from ³H₂O into lipids of individual seeds from sections of a silique either exposed to light or shaded with foil tubes. Results are expressed as 100 × (dpm in lipid extract/dpm in aqueous extract of seeds). The mean and sd for five seeds is shown. For odd-number siliques, the tip was dark; for even-number siliques, the base was covered.

Figure 6.

Simplified comparison of leaf photosynthesis with the metabolism of green oilseeds in light. Only the major reactions involved in photosynthesis or conversion of Suc to oil are shown. For cofactors, only the fate of NADPH, produced by photosynthesis, is indicated. A, In an autotrophic leaf, photosynthetic CO₂ is incorporated by Rubisco into PGA, which is reduced to TP. Altogether, five-sixths of the produced TP is used to recycle RuBP, the acceptor molecule for CO₂. B, In B. napus embryos, the major pathway of carbon flow during oil synthesis, based on biochemical analyses, stable isotope-labeling patterns, expressed sequence tag, and microarray analysis, is indicated by thick arrows. Embryo chloroplasts contain the full enzymatic set to catalyze glycolysis. The Rubisco reaction is added to the scheme by considering synthesis of RuBP from imported hexose and considering PGA metabolism to pyruvate rather than recycling. OPPP production of NADPH contributes approximately 38% of NADPH (Schwender et al., 2003) but is omitted from the figure for simplicity. Suc, Sucrose; Glc, glucose; Glc-6-P, glucose-6-phosphate; TP, triose phosphate; Pyr, pyruvate; AcCoA, acetyl-CoA; RPPP, reductive pentose phosphate pathway.