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

Abstract

Summary 1. Non‐photosynthetic fixation of CO2/HCO3‐ occurs both under light and dark conditions and involve the addition of carbon to substrates which in higher plants are derived originally from carbon reduced to carbohydrates during photosynthesis. Despite the endergonic nature of these carboxylations, the advantages offered seem to be sufficient to outweigh the disadvantages of energy loss. 2. Non‐photosynthetic carbon incorporation into metabolism is dealt mainly in relation to PEP carboxylase, acetyl‐CoA carboxylase, carbamoyl phosphate synthetase and phosphoribosylaminoimidazole carboxylase while other carboxylases await further characterization or discovery. The extent to which a carboxylase participates depends upon the need for products of its activity in metabolism. 3. Non‐photosynthetic carbon fixation is intricately involved in several pathways of metabolism throughout the ontogeny of plants. The roles in relation to leaf carbon metabolism, respiratory metabolism, nitrogen metabolism, lipid and isoprenoid biosynthesis, purine and pyrimidine metabolism and metabolism associated with the action of growth regulators have been described. The fixation reactions appear to be largely concerned with the production of intermediary metabolites, circumvention of energy barriers in metabolism and regulation of plant metabolism. In addition, the activity of PEP carboxylase is involved in ionic balance and pH‐stat. 4. Malate derived by way of PEP carboxylase and NAD‐malate dehydrogenase acts as an effective osmoticum and a counter‐ion for K+ accumulation in actively growing plant cells. In addition, malate may enter the TCA cycle or can be decarboxylated by cytoplasmic NADP‐malic enzyme converting NADH to NADPH. Wherever it has been sought in different plant tissues, some evidence for PEP carboxylase and metabolism of malate has always been found. 5. Almost every plant process spanning from seed development and germination to flowering and fruit‐set requires the essential participation of non‐photosynthetic carbon fixation in regulating certain metabolic and cellular functions but it does not contribute in a major way to the carbon nutrition of plants. It is largely the tissue type that appears to determine which of the roles is predominant at any one time.

Photosynthetic Carbon Fixation Characteristics of Fruiting Structures of Brassica campestris L.

“…Gluconeogenic tissues of fatty seeds have been reported to catalyze appreciable incorporation of '4CO2 into sugars in dark (4,5). Nonphotosynthetic fixation of CO2 by PEP-carboxylase has been reported to occur both under light and dark conditions (3). This enzyme in seeds may thus, help in reducing respiratory CO2 losses, thereby contributing to seed dry matter in addition to the dry matter deposited as a result of net CO2 fixation by podwall.…”

Kinetic characterization of phosphoenolpyruvate carboxylase extracted from whole-leaf and from guard-cell protoplasts of Vicia faba L. (C3 plant) with respect to tissue pre-illumination

“…The carboxylation reaction is catalysed by the ubiquitous plant enzyme PEP carboxylase (PEPC); formed OAA is reduced to the more stable malate. In addition to non-photosynthetic roles (review: Basra & Malik, 1985), PEPC is the primary catalyst in the initial uptake of CO2 in both the CAM and C4 auxiliary photosynthetic pathways (review: Black, 1973). Thus, several isoenzymes have evolved to fulfill the different physiological roles (reviews: O'Leary, 1982;Andreo et al, 1987); in addition to having different properties intrinsically, at least some PEPCs are reversibly modified posttranslationally (Jiao & Chollet, 1991).…”

DCMU inhibits in vivo nitrate reduction in illuminated barley (C 3 ) leaves but not in maize (C 4 ): A new mechanism for the role of light?

“…Therefore, NADH could indeed be a limiting factor. C 4 leaves seem to possess a superior ability to supply NADH in the cytosol of MCs as discussed above, and additionally, the phosphoenolpyruvate carboxylase (PEPC) activity of C 4 leaves could help generate carbon skeletons for continued NO 3 -assimilation (Basra and Malik 1985). Only the C 4 PEPC gene among the PEPC gene family shows light inducibility and MC-specific expression in leaf (Kausch et al 2001).…”