Carbon Assimilation in Photoheterotrophic Cells of Peanut (Arachis hypogaea L.) Grown in Still Nutrient Medium

Abstract

The relative transport of photosynthetic and dark carboxylation products in photoheterotrophic cells of Arachis hypogaea L. var. TMV-3 at varied phases of growth were determined. Despite the presence of an equally competent photosynthetic apparatus as determined from "CO2 incorporation rates in the dark and light, pulse-chase experiments revealed little or no change in the radioactivity of the C3 intermediates but rapid disappearance of label from the dark carbon assimilates (malate and other tricarboxylic acid cycle intermediates) with a simultaneous increase in the aminoacid pool in early log-phase (10 days old) cells. However, significant flow of carbon through the photosynthetic intermediates resulting in the accumulation of sugars occurred in the late log-phase (34 days old) cells. Limitation of exogenous sugar in the nutrient milieu and depletion of reserve carbohydrates stored in starch of the chloroplasts of the cells were considered as the decisive factors in promoting transport of C3 cycle intermediates through the reductive pentose phosphate pathway in photoheterotrophic cells. The observed drain of radioactivity even from the small amounts of tricarboxylic acid cycle intermediates synthesized during photosynthesis into glutamate indicated that the transport of carbon through the nonautotrophic pathway is not controlled by these factors.The rates and early products of carbon assimilation have been determined for photoheterotrophic tissue and cell cultures of carrot (1 1), tobacco (3, 18, 21), Ruta graveolens (7), Chenopodium rubrum (14), Portulaca oleracea (15), Kalanchoe crenata (17), and Chamaecereus sylvestril (22). However, the relative changes in the transport of the early formed photosynthetic carbon through the reductive pentose phosphate pathway using pulse-chase experiments have not been precisely monitored in these cultures. In the only system analyzed thus far to this effect, little decrease or increase in photosynthetic products was reported (15). This was attributed to the reduced cellular transport and absence of translocation supposedly present in the cultured cells. Yet another important contributing factor was the supply of carbohydrate stored as starch in the chloroplasts of the cultured cells. With a reduced demand for photosynthates due to the availability of sugar in the nutrient medium and plenty of reserve carbohydrate stored in starch, a reduced flow of carbon in the cultured cells was anticipated (15).In a previous paper, we reported the establishment of highly chlorophyllous cell suspension culture from the callus proliferation of leaf explants ofArachis hypogaea L. var. TMV-3 (23). The cells of this culture possessed well-differentiated photochemical appa- ratus and were particularly devoid of stored starch in the chloroplasts during the late log-phase growth, a situation favoring the transition to photoautotrophy. We now report on the nature of the early products of photosynthetic carbon assimilation and their interconversion in such cells. MATERIALS AND METHODS Pla...

Photosynthetic Characteristics of a Photoautotrophic Cell Suspension Culture of Soybean

“…2B), was comparable to that of photomixotrophic tobacco cells (17) (1), and was lower than that reported for photomixotrophic peanut cells (20). As has been generally observed in plants, a higher Chl content is correlated with a lower rate of 02 evolution on a Chl basis (13,20 (Fig.…”

“…2B). The addition of HCO3 (1.5 mM) stimulated 02 evolution and was necessary to obtain maximum rates, as has been observed in other photosynthetic cell cultures (18,20). Oxygen evolution became saturated at 400 ME m 2 s-', higher than the intensity used for cell growth (250-300 ME m-2 s-'), a characteristic which has been observed in other plant cells (13).…”

Carbohydrate Metabolism and Activity of Pyrophosphate: Fructose-6-Phosphate Phosphotransferase in Photosynthetic Soybean (Glycine max, Merr.) Suspension Cells

“…Cultures of photoautotrophic cells derived from a number of species have been increasingly employed to study primary carbon metabolism (13,19,27). The type of metabolism displayed by a photosynthetic cell culture depends upon several parameters that include the light regime, supplemental phytohormones, inorganic and organic composition ofthe medium, and the metabolic capacities of the cells under study (19).…”

“…The type of metabolism displayed by a photosynthetic cell culture depends upon several parameters that include the light regime, supplemental phytohormones, inorganic and organic composition ofthe medium, and the metabolic capacities of the cells under study (19). The presence of an exogenous sugar source, such as sucrose, is a predominant factor influencing metabolism in cultured photosynthetic cells (18,19,27). For certain photosynthetic cells, the addition of sugar to the growth medium causes the cells to change from autotrophic metabolism to a "mixotrophic" type metabolism, where carbon is assimilated from the medium as well as CO2 (27 and references therein).…”

Non‐photosynthetic Fixation of Carbon Dioxide and Possible Biological Roles in Higher Plants

“…The increased carboxylation of the in vitro grown cells in the early log-phase is mainly centred around malate formation via PEP carboxylase and its turnover through TCA cycle (Nesius & Fletcher, 1975;Dohler & Kohlenbach, 1976;Nato & Mathieu, 1978;Nishida, Sato & Yamada, 1980;Seeni & Gnanam, 1982). The flow of carbon from TCA cycle into the amino acid pool in the late log-phase in some studies is consistent with the protein and amino acid requirements of cells (Hunt & Fletcher, 1976;Seeni & Gnanam, 1982). Nato & Vidal (1983) showed a twofold higher in vivo I4CO, fixation rate of tobacco cells in exponential growth phase and the analysis of labelled metabolites revealed the participation of PEP carboxylase activity for 5 0 % of the total CO, fixation.…”