what is the difference between plant resistance and plant tolerance?

Plant tolerance and plant resistance are ways that plants deal with stressors in their environment. Resistance and tolerance are plants’ best defense mechanisms.

At the most basic level, the difference between tolerance and resistance is related to how the plant defends itself. In the case of tolerance, the plant has strategies that helps it to survive despite dangers within their local environment. Contrastingly, plant resistance starts at the environmental or genetic level.

In other words:

Tolerance is a plant’s ability to grow and produce an acceptable yield despite a pest attack. Tolerance is typically attributed to plant vigor, regrowth of damaged tissue and a plant’s ability to produce additional stems/branches – factors that enables a plant to avoid, tolerate or recover from damage from inclement weather, pests or herbivores, under conditions that would typically cause a greater amount of injury to other plants of the same species.

Resistance means that a plant is immunized from a particular stressor – typically a biotrophic pathogen infection. The host (i.e. the plant) has a resistance gene that prevents the proliferation of the pathogen, while a pathogen typically contains an avirulence gene which triggers plant immunity.

Two main types of resistance exist: ecological resistance/pseudo-resistance and genetic resistance. 

Ecological resistance is resistance related to favorable environmental conditions at a given location at a particular time. There are three forms of ecological resistance:

1. Host evasion – occurs when a host passes through the susceptible stages very quickly or during a period when pests are fewer.  This type of resistance applies to an entire species population.
2. Induced resistance – resistance stemming from some type of changed condition for the plant, such as an increase in available nutrients or water
3. Escape – this is more or less luck as there is an absence of infestation or injury to a host plant as a result of incomplete infestation

Genetic resistance is resistance related to plant genetics. There are several types of genetic resistance.

1. Resistance based on the number of genes
   • Monogenic – controlled by a single gene which makes it easy to both incorporate and exclude in plant breeding programs
   • Oligogenic – controlled by a few genes
   • Polygenic – controlled by several genes
   • Major Gene Resistance – controlled by one or a few major genes (vertical resistance) 
   • Minor Gene Resistance – controlled by many minor genes; the cumulative effect of minor genes is called adult/mature/field/horizontal resistance
2. Resistance based on biotype reaction
   • Vertical Resistance – specific resistance against specific biotypes
   • Horizontal Resistance – effective against all known biotypes; nonspecific resistance
3. Resistance based on miscellaneous factors
   • Cross-Resistance – when resistance against a primary pest results in resistance to a secondary pest
   • Multiple-Resistance – different environmental stresses (e.g. insects, diseases, nematodes, heat, drought, etc.) results in a new resistance
4. Resistance based on evolution
   • Sympatric Resistance – resistance that is acquired through the coevolution of a plant and insect (which is why it is important to protect our native pollinators!); governed by major genes
   • Allopatric Resistance – not governed by a co-evolution; governed by many genes

There are also three mechanisms of plant resistance

1. Antixenosis (non-preference) resistance mechanisms are those by which host plant has characteristics that result in non-preference for insects in terms of shelter, oviposition, feeding, etc.
2. Morphological or chemical factors that influence insect behavior and results in the poor establishment of insects. Plant shape and color can also be an important influencing factor.
3. Antibiosis, i.e. the negative effect of a host plant on the biology of an insect. This can include decreased rates of survival, development and/or reproduction and is a result of biochemical and biophysical factors. Antibiosis may be a result of the presence of toxic substances, the absence of essential nutrients or a nutrient imbalance. Physical factors, such as thick cuticles, glandular hairs and silica deposits, also contribute to antibiosis.

Both plant tolerance and plant resistance are extremely important to IPM efforts as it helps to reduce costs and amount of other control tactics that need to be implemented. This includes improving the efficacy of insecticides and promoting non-chemical benefits like shifts in predator-prey relationships and reduced pest populations.

The benefits of the use of resistant cultivars are numerous as this method has a high level of specificity, is eco-friendly, lower cost and adaptable to a given situation. The benefits are limited only by the amount of time required to develop new cultivars in breeding programs and by a lack of known resistance genes that can be used. To identify and capture relevant genes, crop wild relatives are often used.

sources

• Gatehouse, J. A. (2002). Plant resistance towards insect herbivores: a dynamic interaction. New phytologist, 156(2), 145-169.
• Rausher, M. D. (2001). Co-evolution and plant resistance to natural enemies. Nature, 411(6839), 857.
• Strauss, S. Y., & Agrawal, A. A. (1999). The ecology and evolution of plant tolerance to herbivory. Trends in Ecology & Evolution, 14(5), 179-185.
• Vinocur, B., & Altman, A. (2005). Recent advances in engineering plant tolerance to abiotic stress: achievements and limitations. Current opinion in biotechnology, 16(2), 123-132.

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