|Resistance to Plant Diseases|
Plants, the primary producers in all food chains, are besieged by a host of biological agents throughout their life cycles. Each species of plant is attacked by at least one hundred different kinds of mycoplasmas, viruses, bacteria, fungi, nematodes, and insects.
In addition, regional weather patterns change, and plants are often faced with unfavorable environmental conditions such as heat, drought, or cold. Plants must contend with the chemicals used by people to deal with unwanted plants. These herbicides are designed to kill the plant or to restrict its growth.
When plants are exposed to biotic or abiotic stress, varying degrees of damage may occur, but many plants manage to protect themselves. Collectively, their defense mechanisms make up what is referred to as plant resistance.
Although all plants possess some means of defense against stress, the term resistant plant is usually reserved for varieties that have the ability to produce a larger crop of good quality fruit than would other varieties placed under the same stress conditions.
Types of Resistance
Resistance is often categorized according to its intensity. An immune cultivar is one that will never be consumed or injured by a particular stress under any condition.
Few, if any, cultivars are considered immune to biotic or abiotic factors known to induce stress in other cultivars of the same plant species. A high-resistance cultivar is one that exhibits only slight damage from a specific biotic or abiotic stressing agent under a given set of conditions.
A low resistance cultivar is one that demonstrates less damage from a stressing agent than the average for the species. Susceptible and highly susceptible cultivars show increasing damage from a biotic or abiotic agent greater than the average for the species.
Resistance also varies according to environmental conditions, genetic control, number of pests, and plant age. Multiple resistance refers to cultivars that are resistant tomultiple factors. Resistance under field conditions (called field resistance) may be considerably different from the resistance observed in the laboratory or greenhouse.
Resistance may be controlled by a single gene (monogenic), by a few genes (oligogenic), or by many genes (polygenic). The terms “horizontal resistance” or “general resistance” are used when describing resistance that is expressed equally against all biotypes of a pest species, and “vertical resistance,” or “specific resistance,” refers to resistance expressed against only some of the biotypes of a pest species.
Resistance may be expressed at any stage of the life cycle from seedling through maturity. On occasion, host plants may pass through a particularly susceptible stage of growth quickly, thereby avoiding infestation by a large number of pests.
Mechanisms of Resistance
Plants commonly develop tolerances to stresses such as heat, cold, drought, or salt. Non preference is a phenomenon in which the plant is merely ignored by a particular pest. The plant has no food or shelter to offer the pest and is not suitable for egg-laying; therefore, the plant is not a potential host.
Antibiosis refers to a mechanism in which the plant exerts some deleterious action on the pest. For example, the plant may produce a substance that inhibits some essential function of the pest’s biology, such as reproduction or development, usually leading to death of the pest.
Both structural and biochemical defenses can be preexisting or induced by stress. Preexisting defense structures include the waxy surfaces of many leaves, thickness of the cuticle that covers the epidermal cells, characteristics of openings into the plant, and thickness and toughness of the cell walls of the plant cells.
After a pest invades a plant, inducible changes in structure can provide some degree of defense. After invasion by pests such as fungi, bacteria, viruses, and nematodes, some plants will form layers of cork tissue that seal off the invading organisms and prevent them from reaching the remainder of the plant.
Other structural defense strategies include the formation of structures called tyloses to seal off the infected vascular tissue or the deposition of gums around lesions. Both tyloses and gum deposits prevent the spread of the agent. In some instances, plants will form abscission layers that seal off a section of leaf and cause it to die along with the pest.
Although structural barriers provide some degree of defense against invading organisms, chemicals produced by the plant during or after the induction of stress appear to be much more important in conferring resistance.
There are several preexisting biochemical defense systems. Although plants do not produce antibodies to specific invading pests, some type of immunological response appears to be operating.
Plants that are resistant to specific pathogens do not contain the antigens, chemicals that induce the resistance response, that are found in the susceptible plants. Some cultivars maintain resistance by limiting the production of certain chemicals that are essential nutrients for invading pathogens.
Other preexisting defense mechanisms include the presence in the plants’ cells of chemicals that inhibit the growth of an invading pest or the release into the environment of chemicals that either inhibit or kill potential pathogens.
When injured by a biotic agent, chemicals, or environmental factors, plants respond with a series of biochemical reactions aimed at limiting the injury and healing the wound. This response is much more pronounced in resistant plants than in susceptible plants.
The biochemical response to stress shows tremendous variation. Many resistant plants respond to a pest invasion by releasing phenolics or other toxic compounds. Fungi produce a group of toxic substances called phytoalexins in response to an invasion.
Many plants respond to stress by the induced synthesis of proteins and other enzymes that form an immune layer around the infected site. When resistant plants are confronted with the oxidative stress that usually accompanies environmentally induced stress, they increase the production of antioxidant enzymes.
Enzymes produced by invading organisms are often responsible for the damage suffered by the host plant, but some resistant plants produce substances that either resist or inactivate these enzymes.
Some invading organisms produce toxins that damage the host plant, and plants resistant to these organisms generally produce chemicals that detoxify the toxins. Other plants develop resistance by altering certain biochemical pathways or initiating a hypersensitive response.
Genetically Engineered Resistance
With the advent of recombinant DNA (deoxyribonucleic acid) technology in the 1970’s, the development of new traits such as resistance was no longer limited to mutation or natural selection from a limited pool of genes.
Scientists first developed transgenic animals and plants in the early 1980’s, and industry has made widespread use of genetically modified organisms. Despite the beneficial applications, potential risks and ethical issues associated with the technology have led to controversy.
Genetic engineering has been used extensively in agriculture. Products of modified organisms are used to protect plants from frost and insects. In 1986 the U.S. Environmental Protection Agency approved the release of the first genetically modified crop plant; by the end of the 1990’s, more than one thousand others had been field-tested.
Plants have been designed to resist disease, drought, frost, insects, and herbicides as well as to improve the nutritional value or flavor of foods.