Plant Storage Products (Carbohydrates, Oils and Proteins)

Abstract

The majority of foods consumed by humans and their domesticated animals as food sources are ultimately obtained from plants, especially seeds. The storage products in seeds are predominately carbohydrates, oils and proteins, which are synthesised and stored in specialised tissues during seed development. Ultimately the storage products ensure successful establishment of the new plant, and the vigour of the young seedling. For example, the reserves are utilised following germination to support early growth of the seedling, allowing it to survive before it commences photosynthesis and autotrophic growth. Some of the storage compounds of seeds play a direct protective role, allowing the seed to withstand water loss during the final stages of its development, and to survive in the dry state for long periods under adverse environmental conditions. Molecular, proteomic and other approaches are elucidating the regulatory networks of genes and encoded proteins that underlie the biochemical and physiological basis of seed maturation, and the accumulation of stored compounds.

Key Concepts:

  • Seed proteins directly provide more than half of the global intake of dietary protein in humans.

  • In seeds, storage products (carbohydrates, oils and proteins) are accumulated during maturation and are utilised following germination to support early growth of the seedling. Likewise, storage products accumulated over winter in tree bark, tubers and perennial weed roots provide nutrients for rapid resumption of growth in the spring.

  • There are regulatory networks of genes and encoded proteins that control the accumulation of stored compounds during seed maturation.

  • Some of the storage products accumulated in seeds and plants are related to their longevity and protect them against environmental stresses.

  • The genetic engineering of the storage products of seeds and plants is directed towards improving their nutritional, stability and food processing properties.

Keywords: carbohydrate; protein; oil; phytin; seeds; nutrition; seed quality; seedling vigour

Figure 1.

Steps in starch biosynthesis in seeds (a) and schematic representation of the levels of organisation within the starch granule (b–e). (a) In step 1 of starch biosynthesis, the enzyme ADPGlcPPase catalyses the formation of ADPglc and inorganic pyrophosphate from glc‐1‐phosphate and ATP. In step 2, starch synthase (SS) adds glucose units from ADPglc to the nonreducing end of a growing α (1→4)‐linked glucan chain by an α (1→4)‐linkage and releases ADP and Pi. In step 3, the starch‐branching enzyme (SBE) cleaves an α (1→4)‐linked glucan chain and forms an α (1→6)‐linkage between the reducing end of the cleaved chain and the C6 of another glucose residue in an α (1→4)‐linked chain, thus creating a branch. Reproduced with permission Srivastava ; Copyright © Elsevier Press. (b–e) Schematic representation of the levels of organisation within the starch granule. Boxes in (c)–(e) represent the area occupied by the structure of the preceding panel. (b) Two branches of an amylopectin molecule, each consisting of α (1→4)‐linked glucan chains (amylose) attached by α (1→6)‐linkage. (c) A cluster within an amylopectin molecule showing the association of adjacent branches. (d) Clusters are arranged to form alternating crystalline and amorphous lamellae. Crystalline lamellae are more closely packed and represent a more ordered arrangement of amylose and amylopectin chains than amorphous lamellae; the two together form the semicrystalline zone. (e) Slice through a starch granule showing alternating zones of semicrystalline and amorphous zones. (b)–(e) Reproduced with permission from Smith AM (1999) Making starch. Current Opinions in Plant Biology2: 223–229. Copyright © Elsevier Press.

Figure 2.

Storage protein deposition in storage parenchyma cells of pea seed. Electron micrograph of a cell from a developing pea cotyledon. Numerous proteins storage vacuoles are visible. Storage protein is being accumulated as electron‐dense deposits at the periphery of the proteins storage vacuoles. At the end of seed development, the proteins storage vacuoles will be completely filled with protein. The nucleus is at the centre, surrounded by large, electron dense, starch grains. Reproduced with permission from Vitale A and Chrispeels MJ (1992) Sorting of proteins to the vacuoles of plant cells. Bioessays14: 151–160. Micrograph by Craig S, CSIRO, Plant Industry.

Figure 3.

Synthesis and processing of seed storage proteins of dicotyledonous seeds (2S albumins, 7S vicilins, and 12S legumins). Signal peptides are shown in solid black; propeptides are shown by hatched boxes and glycosylation sites are shown by inverted symbols. Processing events that take place enroute to the vacuole are shown on the left, those occurring within the vacuole are shown on the right. Lower figure shows the proteolytic cleavage site used by vacuolar endoproteases, in which there is cleavage of the peptide bond on the C‐terminal side of an asparagine (Asn) residue of the protein. The other amino acid is often, but not always, glycine. Reproduced with permission from Srivastava . Copyright © Elsevier.

Figure 4.

Protein bodies in the endosperm of developing rice grains are derived from the endoplasmic reticulum or from the vacuole. The rough endoplasmic reticulum (RER) of developing rice endosperms is composed of two distinct domains, the cisternal‐ER (CER), where the polysomes translate mRNA for a globulin‐like protein (glutelin), and the protein body‐ER (PBER), which contains polysomes translating mRNA for prolamin (oryzenin). The prolamin is sequestered in the ER lumen and directly distends to form a protein body, whereas the globulin passes via the Golgi apparatus to the vacuoles. Storage protein in a mature rice grain is 5% oryzenin and 85% glutelin.

Figure 5.

Simplified scheme for the synthesis of triacylglycerols in plants. Synthesis involves three cell compartments: the cytosol, the plastid and the endoplasmic reticulum, the latter becoming modified to form the oil bodies. Numbered steps require the following enzymes: (1) desaturase; (2) hydroxylase; and (3) elongase. Polyunsaturation and hydroxylation take place on acyl groups incorporated temporarily into phosphatidylcholine (Pc). Mammalian tissues are not able to desaturate oleate to linoleate: hence, this fatty acid, an essential part of the diet, must be obtained from plant sources.

Figure 6.

Seeds of the severe abi3 mutant of Arabidopsis (abi3‐6) as compared to those of the wild‐type plants. The mutant seeds are green, reduced in their storage reserve content and desiccation‐intolerant. Reproduced with permission from Nambara E, Keith K, McCourt P and Naito S (1994) Isolation of an internal deletion mutant of the Arabidopsis thaliana ABI3 gene. Plant and Cell Physiology35: 509–513. Copyright © Oxford University Press.

Figure 7.

Model of some of the regulatory steps that control seed development (including reserve biosynthesis) based on research on the model plant Arabidopsis. The orange line indicates that the maturation programme can be bypassed in various mutants. There are negative mechanisms that repress seed maturation and developmental programmes during seedling development and vegetative growth. ABA, abscisic acid; GA, gibberellic acid; PcG, polycomb group; PCGP, polycomb group protein; PRC2, polycomb repressive complex 2; PKL, PICKLE; HDAC, histone deacetylase; HIS, HIGH‐LEVEL EXPRESSION OF SUGAR‐INDUCIBLE; VAL, VP1/ABI3‐LIKE; LEC, LEAFY COTYLEDON; FUS3, FUSCA3; TAN, TANMEI; AGL 15, AGAMOUS‐like 15; ABI3, ABSCISIC ACID INSENSITIVE3; SP, seed storage protein; WRI, WRINKLED; TAG, Triacylglycerol. Reproduced with permission from North et al. .

Figure 8.

Post‐genomics strategies applied to seeds for dissecting their composition and/or identifying regulatory and metabolic networks during developmental processes such as reserve biosynthesis and deposition. These strategies can be carried out at the level of the transcriptome, proteome, metabolome, ionome and hormonome (hormone and hormone metabolite profiling). Further, the approaches may be ‘untargeted’ or ‘targeted’ (e.g. confined to a specific tissue, developmental stage or organelle). Seed drawings courtesy of Dr Ying Zeng.

Figure 9.

NMR showing distribution of lipid in a western white pine seed shortly after imbibition. Photo courtesy of Feurtado A and Grewal M, Simon Fraser University and Plant Biotechnology Institute, Canada. Reproduced with permission from Kermode . Copyright © Elsevier.

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Kermode, Allison R(Nov 2011) Plant Storage Products (Carbohydrates, Oils and Proteins). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001325.pub2]