Macronutrients
Proteins
Plants naturally have an incomplete amino acid composition as it relates to human amino acid requirements. This can be overcome by including foods containing the amino acids that are low in the other food, thus creating a complete source of amino acids. This technique is often used in the traditional meals from vegetarian cultures (2).
In most developed countries it is not difficult for individuals to attain complete sources of amino acids required for human health. However, in developing countries there are more challenges to overcome such as production costs, climatic restrictions on what crops can be grown, some traditional eating habits, and an overall lack of food (2).
Modifying Amino Acid Content
Research in this area has been focused on creating a complete amino acid source in plants. This is a difficult task, because plants are complex organisms. Protein expression is tissue specific, meaning that the functional requirements of each tissue dictate what proteins are manufactured there. Even within the tissue, proteins in individual cells are separated based on their function. This separation makes it essential to develop transformation systems that not only introduce the genetic information of interest, but that also work within the control mechanisms, such as tissue-specific expression (2). The introduced gene will only create the desired result when it is expressed in the correct tissue.
Figure 5A. Amino acids. Amino acids are made from simple atoms such as hydrogen, carbon, nitrogen ,oxygen, and sulfur. Amino acids always have an amino end (consisting of nitrogen and two hydrogens) and an acid end (consisting of two oxygens and a hydrogen). "R" in the bottom diagram represents the location where 20 different combinations of simple elements are attached to form a unique amino acid.
Figure 5B. Proteins or polypeptides are made by connecting amino acids together. For instance, methionine and glycine attaching together forms a simple polypeptide.
Figure 5C. The three parts of a gene. The parts of the gene - the promoter and termination sequences - control the start and end of transcription, respectively, while the coding region encodes the amino acid sequence that forms the protein.
In order to plan the correct addition of genetic information into the plant, several factors have to be considered. The most important consideration is the tissue or tissues in which the new or modified protein will be expressed and the proteins that will be modified. Plant protein in the diet comes primarily from plant seeds and to a lesser extent from the leaves. The function of protein in seeds is to provide nitrogen to the developing plant. Since that is their only function, modification of these proteins has less potential for disrupting other processes in the plant than modification of proteins in other plant tissues (2). For that reason, the seed is often the target for amino acid profile improvements in plants.
There are several approaches to enhancing amino acid content in plants. The first approach is to simply increase the amount of amino acids made in the plant to provide more protein per serving.
A second approach focuses on a change in the amount of high quality proteins in the plant. This involves making the amino acid components of the protein more available. Techniques for accomplishing this include increasing the transcription rate of the associated amino acid genes, increasing the stability or the rate of translation of the mRNA that encodes for the protein, or by making the protein product less susceptible to degradation after it is made.
The third approach is to increase the nutritional quality of the proteins made in the plant seeds. Proteins that are already made in a particular plant can have new amino acids introduced into them or new proteins that have the desired amino acid composition can be expressed (2).
In summary, most of the transgenic plants that are being developed are of three major types (2):
- Genes of normally occurring protein within the plant is modified to add additional essential amino acids to its composition
- A gene for a naturally occurring protein from another species is transformed into the plant to provide the complementary essential amino acids
- A new gene for a designed protein (de novo) with a more desirable amino acid composition is introduced into the plant
Example
The synthesis of lysine, which is derived from asparate, is mainly controlled by two enzymes: aspartate kinase (AK) and dihydrodipicolinate synthetase (DHDPS). These enzymes are regulated by end-product feedback inhibition by lysine. In other words, when lysine accumulates the pathway by which it is created slows down. If the feedback inhibition can be overcome, production of lysine above normal amounts will occur. (2).
When bacterial genes were used to decrease the inhibition feedback system in transgenic plants, phenotypic abnormalities occurred in the plant. It was determined that promoters were needed to control when the amino acid was produced during the development of the plant (2). This illustrates the need for the desired amino acid to be produced in not only the correct tissue, but also at the correct stage of the plant’s development to minimize negative effects on the plant.