16.2 Importance of Proteins and Amino Acids

There are many stages of growth and development which take place throughout a plant’s life. If the particular plant is a crop, then farmers want these stages to occur efficiently to obtain high yields. If the plant is a weed, reducing the crop’s yield potential, then farmers look for ways to inhibit the weed’s cellular growth and development processes.  Proteins are the critical factors in both of these scenarios.  They are involved in catalyzing chemical reactions (enzymes), facilitating membrane transport, intracellular structure and energy generating reactions involving electron transport, just to name a few. Compared to animals, plants contain low levels of proteins due to the large amount of carbohydrate (cellulose) that composes most of a plant’s structure.

Proteins are constructed from even smaller molecules called amino acids. This figure illustrates the general molecular structure of amino acids. Notice that all amino acids contain both an amino group (NH3) and a carboxyl group (COOH); however, individual amino acids differ at the ‘R’ group (See following figures).  Notice that there are two types of ‘R” group, aromatic (i.e. tryptophan) and branched chain (i.e. leucine).

Unlike animals, plants do not derive amino acids by consuming other organisms. Therefore, all 20 of the amino acids must be synthesized by the plant. Proteins have a finite life span and must be constantly translated from mRNA in order for plant growth and development to continue. This means that there must be a ready supply of all 20 amino acids for protein synthesis and ultimately plant growth and development to occur.

A protein is created when a series of amino acids are bound together. The arrangement of the amino acids can be described at three different levels: primary structure, secondary structure and tertiary structure. The primary structure is simply the order in which the amino acids are bound together. This specific order is initially encoded in the DNA sequence of the gene that provides the blueprint for the protein.  The secondary structure of a protein describes the way the string of amino acids fold. The specific bonding interactions among amino acids, resulting from the primary structure, will determine if it is a coil or folding arrangement.  Finally, the tertiary structure of a protein describes the overall shape of the protein.

Since protein primary structure (amino acid sequence) is determined by DNA sequences, changing a gene’s DNA sequence can change the primary structure of a protein. This in turn may change a protein’s secondary structure and tertiary structure. With an altered shape, the protein may function differently or may not function at all.

For example, plants resistant to glyphosate have EPSP synthase enzymes with a slightly different amino acid sequence than susceptible plants. This is due to a simple DNA mutation. The details of which will be explained later.

Both the aromatic and branched chain amino acids are produced by tightly controlled enzymatic pathways.  Because plants must produce all 20 amino acids, chemical compounds that inhibit amino acid production can have herbicidal activity. There are different herbicide classes that interfere with the production of these 2 different types of amino acids.

There are two herbicide Modes of Action and three Sites of Action that negatively affects plants in this manner:

• Amino Acid Synthesis Inhibitors
  • Group 2. ALS Inhibitors (ALS) (HRAC Group B)
  • Group 9. EPSP Inhibitors (EPSP)  (HRAC Group G)
• Nitrogen Metabolism Inhibitor
  • Group 10. Glutamine Synthetase Inhibitors (GSI) (HRAC Group H)