Recent Advances in Carbon and Nitrogen Metabolism in C3 Plants

Abstract

C and N are the most important essential elements constituting organic compounds in plants. The shoots and roots depend on each other by exchanging C and N through the xylem and phloem transport systems. Complex mechanisms regulate C and N metabolism to optimize plant growth, agricultural crop production, and maintenance of the agroecosystem. In this paper, we cover the recent advances in understanding C and N metabolism, regulation, and transport in plants, as well as their underlying molecular mechanisms. Special emphasis is given to the mechanisms of starch metabolism in plastids and the changes in responses to environmental stress that were previously overlooked, since these changes provide an essential store of C that fuels plant metabolism and growth. We present general insights into the system biology approaches that have expanded our understanding of core biological questions related to C and N metabolism. Finally, this review synthesizes recent advances in our understanding of the trade-off concept that links C and N status to the plant's response to microorganisms.

Keywords: C and N interactions; assimilation; fixation; omics; plant-microbiome interactions; starch; stress; transport.

Figure 1

Overview of the C and N flow among plant organs. Plants use the xylem–phloem system to exchange nutrients and the information on the status of water and mineral nutrient acquisition by the root system and the growth requirements of the shoot organs. The C flow from shoots supports roots, symbionts (i.e., fungi and bacteria) that provide plants with nutrients and other benefits, and young (sink) organs, such as buds, flowers, and fruits, via the phloem (noted in red arrow). Xylem (noted in blue arrow) conveys water and nutrient from the roots to the rest of the plant and also provides physical support. Several of the displayed systems may occur independently in nature (e.g., nodules, mycorrhizal fungi, and plant growth-promoting rhizobacteria (PGPR)) and/or may not be present at the same time on the root. Bud, flower, and fruit differentiation and development may appear sequentially on the shoots.

Figure 2

Photosynthesis in C3 plant leaves. 3-PGA: 3-phosphoglycerate and Rubisco: Ribulose-1,5-bisphosphate carboxylase/oxygenase.

Figure 3

Respiration and C utilization in a plant cell. AA: Amino acid; FAS: Fatty acid synthesis; GPT (Glucose phosphate transporter): OPPP: Oxidative pentose phosphate pathway; TCA: Tricarboxylic acid: XPT: Xylose phosphate transporter; XPT: Xylulose 5-phosphate/phosphate translocator; and Xyl-5-P: Xylulose 5-phosphate.

Figure 4

Ammonium and nitrate absorption and assimilation related to carbon metabolism in a plant root cell. NR: Nitrate reductase; NiR: Nitrite reductase; GS: Glutamine synthetase; GOGAT: Glutamate synthase AMT: Ammonium transporter; NRT: Nitrate transporter: PEP: Phosphoenolpyruvate; PEPC: PEP carboxylase; and PK: Pyruvate kinase.

Figure 5

Transport of C and N and regulatory signals through the phloem and xylem. OA: Organic acid; CKs: Cytokinins; ABA: Abscisic acid; CEP: C-terminally encoded peptide; CEPD: CEP downstream; HY5: Elongated hypocotyl 5; and miR393: MicroRNA393.

Figure 6

Starch biosynthesis pathway in leaf plastids according to the “Classic model” (black arrow) and alternative model (blue arrow; see [150,151]). CBC: Calvin–Benson cycle; FBA: Fructose-1,6-bisphosphate aldolase; FBPase: Fructose 1,6-bisphosphatase; PGI: Phosphoglucoisomerase; PGM: Phosphoglucomutase; SuSy: Sucrose synthase; GBSS: Granule bound starch synthase; SS: Starch synthase; BE: Starch branching enzyme; and DBE: Starch-debranching enzyme.

Figure 7

A proposed model of grain chalking under high-temperature stress. MSD1: Mn-type superoxide dismutase 1.