Biochemical composition of lipids and nucleic acids in forage crops

Managing the lipid complex: how to increase the nutritional value of forage crops and potatoes

The quality of green forage and silage depends directly on the composition and amount of lipids in plants. These compounds serve as a crucial source of energy and essential fatty acids for livestock animals, which cannot be synthesized in their bodies. The bulk of plant fats is concentrated in the most valuable parts of the grasses — leaves and inflorescences, whereas stems and roots are poor in them.

• The share of polyunsaturated fatty acids in grass lipids is 75–80%
• Lipid content in the raw mass of potatoes — 0.1–0.15%

Polyunsaturated fatty acids — linoleic and linolenic — predominate in the composition of lipids in forage crops. Potatoes also contain these valuable acids along with palmitic and oleic acids. However, fats are distributed unevenly in the tubers: their maximum concentration is in the periderm (peel), and their content in the core is minimal. To increase the yield of lipids, starch, and protein per hectare, it is necessary to provide the crop with balanced mineral nutrition taking into account its biological characteristics.

Nucleic acids and nucleotides: energy balance and cell growth

Nucleic acids control all key processes in a plant: from cell division and organ growth to the inheritance of traits and protein synthesis. They are high-molecular polymers consisting of individual nucleotides. Each nucleotide is assembled from a nitrogenous base, a sugar (ribose or deoxyribose), and a residue of phosphoric acid. Free nucleotides are also part of the most important enzymes and serve as energy accumulators.

For an agronomist, it is important to understand the pathways of synthesis of these compounds, as they are directly related to the nitrogen and carbon nutrition of plants. The synthesis of different groups of nitrogenous bases has its own peculiarities. For example, purines are formed on the basis of carbohydrate components, while pyrimidine bases are built from the simplest nitrogenous compounds. The building blocks of nucleic acids are divided into several main groups.

The energy status of a cell depends on the ability of adenylic acid to attach phosphate groups. This is how adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are formed — the main energy carriers in the plant organism. Energy released during respiration, fermentation, and photosynthesis is accumulated in these compounds. Adjacent nucleotides are linked into chains by a phosphodiester bond between the sugar of one unit and the phosphate of another.

Localization and functions of nucleic acids in a plant

The synthesis of plant protein and the transmission of cultivar traits depend directly on the work of nucleic acids. In a plant cell, DNA is mainly concentrated in the chromosomes of the cell nucleus. However, about 1% of DNA is located outside the nucleus — in mitochondria and chloroplasts, which is extremely important for the processes of respiration and photosynthesis. RNA is predominantly localized in the cytoplasm and is responsible for the direct assembly of protein molecules.

To build proteins, the cell uses three main types of RNA, each of which performs its own task:

• Messenger RNA (mRNA) — copies information from DNA and serves as a template for creating a specific protein. Its diversity corresponds to the number of types of proteins in the cell.
• Ribosomal RNA (rRNA) — together with proteins, it forms ribosomes, on which protein chains are physically assembled. It possesses a complex three-dimensional structure.
• Transfer RNA (tRNA) — carries amino acids to the site of protein synthesis. Each molecule consists on average of 80 nucleotides.

• Share of rRNA in the cell — 80%
• Share of tRNA in the cell — 15%
• Share of mRNA in the cell — 3–5%
• DNA outside the cell nucleus — ~1%

Nucleotide composition of DNA and RNA in various crops

The nucleotide composition of DNA is absolutely identical in all organs of the same plant. This is because all cells of an organism carry identical genetic information. In plants, nucleotides of the AT type (adenine + thymine) predominate, and their amount exceeds the sum of guanine and cytosine.

Analysis data show the ratio of nucleotides in the DNA of various agricultural crops and forest species:

The nucleotide composition of RNA also follows certain patterns. Unlike DNA, RNA contains uridylate instead of thymidylate, and guanylate predominates among the bases. Analyses of various vegetative and generative organs show a stable distribution of these components.

Nucleic acids, like proteins, have primary, secondary, tertiary, and quaternary structures. The primary structure of nucleic acids is the sequence of nucleoside phosphates in a linear chain. The secondary structure is the spatial arrangement of DNA or RNA polynucleotide chains. The secondary structure of nucleic acids is created by the interaction of adjacent monomeric units of the polynucleotide chain, and in the case of double-stranded molecules or molecular sections, by the interaction of nucleotide residues located opposite each other in the double helix.

American biochemist Erwin Chargaff formulated the rules (Chargaff's rules) that govern the structure of nucleic acids.

1) in a DNA molecule, the molar sum of A and G (purine bases) is equal to the molar sum of C and T (pyrimidine bases): A+G = C+T;

2) in DNA molecules, the number of A residues is always equal to the number of T residues; G and C are in the same ratio: A=T, G=C.

3) the ratio of molar sums (G+C)/(A+T) in a DNA molecule, as well as (G+C)/(A+U) in an RNA molecule, is specific to different organisms and therefore can serve as an important molecular biological characteristic. This ratio is called the coefficient of specificity of nucleic acids. Geneticists use this indicator to determine the degree of similarity and difference between organisms.

It turns out that in some organisms, (A+T) nucleotides predominate in DNA. Such DNA is characteristic of humans, higher animals, and plants, as well as most fungi. Therefore, the DNA of such organisms is called AT-type DNA.

In the DNA of many microorganisms (unicellular animals, yeast fungi, most bacteria, and viruses), (G+C) nucleotides predominate, that is, they are characterized by GC-type DNA.

The DNA of many objects is not just a simple extended helix but possesses a specific tertiary structure. A double-stranded DNA helix can undergo further folding into a supercoil in certain sections, and the molecule can also take on a circular form (for example, in leaf chloroplasts) or fold into a globule.

The tertiary structure of tRNA is a globule extended in the shape of the letter "L". Its base is a "cloverleaf" secondary structure, folded such that bonds form between nitrogenous bases, as well as between nitrogen atoms, amino- and hydroxy-groups of the bases on one side, and phosphate groups and hydroxyl groups of pentoses on the other. Finally, hydrophobic interactions occur between stacks of bases, forming the two parts of the "L" shape.

Thus, the tertiary structure of nucleic acids is organized through the interaction of nucleic residues belonging to different elements of their secondary structure.

The quaternary structure is known only for RNA. It represents a block of two identical subunits, the bond between which is maintained by the complementary interactions of their nitrogenous bases.

Nucleic acids of higher plants differ somewhat from those of animals and microorganisms in their nucleotide composition. In particular, the DNA of higher plants is characterized by a relatively high percentage of 5-methylcytosine. Plant ribosomal RNA belongs to the GC-type. The value of the (G+C):(A+U) ratio varies across plant species within the range of 1.03–1.36. In animal RNA, this ratio varies from 0.7 to 1.6. Plant RNA differs slightly from animal RNA in the composition of additional nucleotides in water-soluble RNA. The content of nucleic acids in plants does not exceed 10% of the amount of protein. However, nucleic acids play a decisive role in intracellular metabolism. Convincing evidence of this is the ubiquity of nucleic acids in the organic world, their concentration in the most critical structures of the cell, and in areas of intensive growth and new tissue formation.

In assessing the place and role of nucleic acids in vital phenomena, the existence of a constant interaction between nucleic acids and the most active part of proteins – the constitutional proteins of the cytoplasm and nucleus of animal and plant cells – is of particular importance. Protein is the basis of life, and the attitude towards it and the nature of the interaction with it is the best proof of the significance of a particular component of the cell structure in vital processes. The following genetic and functional links between nucleic acids and proteins in an organism are known: a) products of protein metabolism – amino acids and their derivatives – are the starting material in the synthesis of nitrogenous bases of nucleic acids; b) nucleic acids participate in the synthesis of the protein molecule; c) nucleic acids have a multifaceted influence on the structure and physicochemical properties of the protein molecule; d) nucleic acids and proteins, by interacting with each other, form nucleoproteins, which constitute the basis of the organization of living substance.

The relationships between protein and nucleic acids in an organism are extremely complex and diverse. In the process of interaction between protein and nucleic acid, a qualitatively new state of matter is created, one of the remarkable properties of which is the ability for self-reproduction, which, as is known, lies at the basis of such vital phenomena as growth, reproduction, and the transmission of an organism's traits.

Nitrogen, phosphorus, and sulfur are direct building materials that are part of nucleotides. But to initiate the synthesis and degradation of nucleic acids, the plant requires helper elements: magnesium, potassium, calcium, iron, and trace elements. They work as enzyme cofactors, controlling all biochemical reactions in cells.

The level of iron supply directly affects the rate of protein creation in a plant cell. With good iron nutrition, translation is accelerated, causing reserves of free RNA to be consumed faster and its total content in the cell to decrease. This is a natural process of converting accumulated substances into protein structures.

Effect of trace elements on enzymes and RNA stability

Manganese activates enzymes of nucleic acid metabolism — phosphodiesterases, nucleotidases, and nucleoside kinases. Other metal ions also selectively regulate enzymatic activity. The direction of their action depends on the specific element:

• Barium, calcium, and zinc ions increase ribonuclease activity;
• Copper and cobalt activate deoxyribonucleases;
• Molybdenum activates xanthine oxidases.

The role of trace elements is not limited only to working with enzymes. They are capable of forming stable complex compounds with nucleic acids and their precursors. This helps to hold purine and pyrimidine bases via covalent bonds and to maintain the correct spatial shape of the RNA molecule. Boron stands apart in this process as it is not an enzyme cofactor. Due to the structure of its electron shells, it binds directly to molecules of proteins, lipids, polysaccharides, and nucleic acids, stabilizing their supramolecular structure.

Optimization of mineral plant nutrition through fertilizer application is a direct tool for managing nucleic acid metabolism in the field. Precise selection of doses and types of fertilizers based on the chemical composition of the soil and the biological characteristics of the crop allows for directing metabolism into the desired path. As a result, the plant fully realizes its genetic potential for productivity.