plant biology Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek wo ...

, proteinase inhibitors are a family of small

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

s that serve an integral role in the plant’s defense mechanisms against

herbivory A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...

from

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...

s or

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s that may compromise the integrity of the plant.

Functionality

The proteinase inhibitors work to disrupt the enzymatic ability of the digestive or microbial

enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

that are present in the

stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...

of the attacker resulting in the inability to properly digest the plant material. This causes an interference of proper growth and discourages further wounding of the plant by the attacker. Studies have also recently revealed that some proteinase inhibitors also provide defense for the plant through the possession of

antimicrobial An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals ar ...

properties providing for the inhibition of

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

growth.

Wounding

While proteinase inhibitors are present in plants naturally, production of these proteins for defense is often induced by either wounding of the plant or by chemical signaling through molecules such as

methyl jasmonate Methyl jasmonate (abbreviated MeJA) is a volatile organic compound used in plant defense and many diverse developmental pathways such as seed germination, root growth, flowering, fruit ripening, and senescence. Methyl jasmonate is derived from j ...

. Both wounding of the plant as well as signaling molecules result in the formation of

jasmonic acid Jasmonic acid (JA) is an organic compound found in several plants including jasmine. The molecule is a member of the jasmonate class of plant hormones. It is biosynthesized from linolenic acid by the octadecanoid pathway. It was first isolat ...

, which then induces the

gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

of proteinase inhibitors. Many other signal cascades as well as the translocation of signal molecules through the

phloem Phloem (, ) is the living biological tissue, tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This tran ...

and

xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

of the plant are also necessary for the production of these inhibitors. Once the proteinase inhibitor has been ingested by the insect, it presents itself as a normal substrate for the digestive enzymes binding to the active site on the enzyme. This binding of the inhibitor to the proteinase creates a new complex that is very unlikely to dissociate. Once bound, the active site can no longer be accessed by any other substrate and the enzyme can no longer digest the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

s of the plant. Without proper digestion, the insect is unable to grow, and may starve if it chooses to remain at the wounded plant. Similar inhibition of growth can be seen in pathogens that interact with these inhibitors.

Tomato experiment

In order to discover how the production of the inhibitors was induced, scientists exposed

tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word ...

plants to different forms of

and then

assay An assay is an investigative (analytic) procedure in laboratory medicine, mining, pharmacology, environmental biology and molecular biology for qualitatively assessing or quantitatively measuring the presence, amount, or functional activity of a ...

ed using

radial immunodiffusion Radial immunodiffusion (RID) or Mancini method, Mancini immunodiffusion or single radial immunodiffusion assay, is an immunodiffusion technique used in immunology to determine the quantity or concentration of an antigen in a sample. Description ...

for proteinase inhibitors in leaf juices. A group of tomato plants was sprayed with a solution containing methyl jasmonate, while another group of tomato plants was exposed to methyl jasmonate vapor in air-tight glass chambers.

Control groups In the design of experiments, hypotheses are applied to experimental units in a treatment group. In comparative experiments, members of a control group receive a standard treatment, a placebo, or no treatment at all. There may be more than one tr ...

were sprayed with a solution or exposed to a vapor that did not contain methyl jasmonate. Both experimental groups revealed increased proteinase inhibitor production as a result of exposure to volatile methyl jasmonate in comparison to control groups.

Defense behavior

The production of proteinase inhibitors reveals that plants have the ability to alter their defense behavior in response to a threat or direct attack on plant integrity. This complex defense mechanism serves to not only protect the plant from being eaten, but also from pathogen infection requiring both coordination and communication.

References

{{Reflist, 32em Botany Plant physiology Protease inhibitors