Types and genetic mechanism of resistance to biotic stress-horizontal and vertical resistance to crop plants

1. Types and genetic mechanism of
resistance to biotic stress-horizontal and
vertical resistance to crop plants
Submitted to: Dr. J. k. kshirsagaar
Asst. prof. of Genetics And Plant Breeding
Dr. sharadchandra pawar college of Agriculture, Baramati
Submitted by: Ashish R. Bachhav

2. Biotic stress
1.Biotic stress is stress that
occurs as a result of damage
done to an organism by other
living organisms. Biotic
stress
Bacteria
Fungi
Viruses
Weeds
Insects
Parasites

3. Disease Resistance
• Disease resistance refers to the ability of an
organism, typically a plant or an animal, to withstand
and defend against the harmful effects of pathogens
(such as bacteria, viruses, fungi, and other
microorganisms) that can cause diseases.
• This resistance can manifest in various ways,
including reduced susceptibility to infection, faster
recovery from infections, and less severe symptoms
when exposed to pathogens.

4. Genetic resistance
• Genetic resistance in plants refers to the innate ability of certain plant
varieties to withstand or tolerate infections by specific pathogens due to
genetic traits they possess.
• These traits are inherited from one generation to the next and are often the
result of a long process of natural selection or deliberate breeding efforts.
• Genetic resistance is a valuable approach in agricultural practices as it can
help reduce the need for chemical pesticides and promote more sustainable
and resilient crop production.

5. Vertical resistance
• vertical resistance, also known as monogenic or major gene resistance
• it is a type of genetic resistance in plants that is controlled by a single
major gene or a small number of genes.
• This gene imparts a high level of resistance to a specific pathogen, often
providing complete protection against the pathogen's attack.
• This type of resistance is often the result of a well-defined gene-for-gene
interaction between the plant and the pathogen.

6. 1.Gene-for-Gene Interaction:
i. Vertical resistance is based on a specific interaction between a resistance gene (R
gene) in the plant and an avirulence gene (Avr gene) in the pathogen.
ii. When a plant carries the R gene that corresponds to a specific Avr gene in the
pathogen, it triggers a defense response that prevents infection or limits the
pathogen's growth.
2.Hypersensitive Response (HR):
i. The interaction between the R gene and the Avr gene often leads to a
hypersensitive response (HR) in the plant.
ii. This involves rapid and localized cell death at the site of infection, which helps
contain the spread of the pathogen.

7. 3.Specificity:
i. Vertical resistance is highly specific to particular pathogen strains that carry the
corresponding Avr gene.
ii. If the pathogen population evolves to lack the Avr gene, the plant's resistance may be
overcome.
4.Durability:
i. While vertical resistance can be highly effective, it can also be short-lived if the
pathogen population evolves to overcome it.
ii. Pathogens can mutate or recombine to produce strains that lack the Avr gene
recognized by the plant's R gene.
Examples:
1) interaction between rice plants carrying the Xa21 gene and the bacterium Xanthomonas
oryzae pv. Oryzae
2) interaction between wheat plants carrying the Pm3 gene and various strains of the wheat
powdery mildew fungus.

8. Horizontal resistance
• Horizontal resistance, also known as polygenic resistance or non-race-specific
resistance.
• is a type of genetic resistance in plants that involves the combined effects of
multiple genes, each contributing a small amount to overall resistance against a
pathogen.
• Unlike vertical resistance, which is controlled by a single major gene, horizontal
resistance is controlled by a network of genes with additive effects.
• This type of resistance provides a more complex and durable defense against a
wide range of pathogen strains.

9. 1.Polygenic Nature:
Horizontal resistance involves the interaction of multiple genes scattered throughout
the plant's genome. Each gene contributes a small, incremental effect to the overall
resistance level.
2.Complex Mechanisms:
The mechanisms behind horizontal resistance are often complex and involve
multiple pathways of defense response. This complexity makes it more challenging
for pathogens to overcome the resistance by mutating a single gene.
3.Non-Specific Defense:
Horizontal resistance is not limited to specific pathogen strains or races. Instead, it
provides a general level of protection against a broad spectrum of pathogens,
reducing the potential for rapid breakdown due to pathogen evolution.

10. 4.Durability:
Horizontal resistance tends to be more durable over time because it involves
multiple genes that pathogens must collectively overcome. This makes it less likely
that a single mutation or recombination event in the pathogen population will render
the resistance ineffective.
5.Quantitative Trait Loci (QTLs):
The genes contributing to horizontal resistance are often referred to as quantitative
trait loci (QTLs). These loci are associated with specific genomic regions that
influence the level of resistance.

11. 6.Breeding for Horizontal Resistance:
Breeding for horizontal resistance is more complex than breeding for vertical
resistance because it involves multiple genes with smaller effects. It often requires
the use of molecular markers and advanced statistical methods to identify and
combine beneficial QTLs.
Examples:
Quantitative resistance is commonly observed in plant-pathogen interactions, such
as resistance against fungal diseases like wheat rust or bacterial diseases like
bacterial spot in tomatoes. In these cases, no single gene provides complete
protection, but the cumulative effects of multiple genes enhance overall resistance.

12. Difference between horizontal and vertical resistance
Aspect Horizontal Resistance Vertical Resistance
Number of Genes Multiple genes with small effects Single major gene
Specificity
General protection against many
strains
Highly specific to one pathogen
Durability More durable over time Can be short-lived if pathogen evolves
Pathogen Evolution Less prone to pathogen evolution Vulnerable to pathogen adaptation
Interaction with Pathogen
Recognizes multiple pathogen
components
Recognizes specific effector or Avr protein
Hypersensitive Response Not always accompanied by HR Often leads to HR (ETI)
Activation Often induced by beneficial microbes Can be constitutive or induced
Disease Range Provides broader resistance Targeted against specific pathogen
Breeding Complexity Complex due to polygenic nature Simplified due to single gene
Example Quantitative resistance against rust
R gene-mediated resistance to
Xanthomonas
Mode of Action Combined additive effects of genes
Recognition of specific pathogen
component

13. Adult Plant Resistance
• Definition: Adult plant resistance (APR) is a type of resistance that manifests
later in the plant's life cycle, often after the seedling stage. It's typically weaker in
the early stages of growth but becomes more effective as the plant matures.
• Mechanism: APR can be mediated by both major and minor genes. It involves a
combination of physiological, biochemical, and molecular changes that limit
pathogen growth and disease progression.
• Timing: Unlike seedling resistance, which is often rapidly overcome by evolving
pathogen populations, APR provides protection over a longer period, including
the adult stage of the plant.
• Example: Wheat varieties showing adult plant resistance to leaf rust may not
show strong resistance in the seedling stage but exhibit reduced symptoms as the
plant matures.

14. Slow rusting resistance
1.Definition: Slow rusting resistance is a form of quantitative resistance
where the plant exhibits a reduced rate of disease development over time
when exposed to a rust pathogen. This resistance mechanism is characterized
by a gradual increase in disease symptoms rather than a rapid and severe
disease outbreak.
2.Mechanism: Slow rusting resistance is typically controlled by multiple
quantitative trait loci (QTLs) that contribute to the overall level of resistance.
These QTLs may have small individual effects but collectively provide a
higher level of protection against rust pathogens.

15. 1.Features:
1.Durable: Slow rusting resistance is known for its durability over time. It is less likely to break
down due to pathogen evolution because it involves a combination of genes that are more
difficult for the pathogen to overcome.
2.Partial Resistance: Slow rusting resistance often results in partial protection against rust
diseases. It doesn't provide complete immunity but significantly reduces the severity of disease
symptoms and slows down disease progression.
2.Practical Significance:
1.Slow rusting resistance is highly valuable in agricultural and breeding contexts. Crops with
slow rusting resistance can provide extended periods of protection against rust pathogens,
allowing more time for farmers to implement disease management strategies.
2.Breeding efforts often focus on identifying and incorporating slow rusting resistance genes into
crop varieties to develop more durable and sustainable rust-resistant cultivars.

16. Examples
1.One of the most well-known examples of slow rusting resistance is in
wheat against stem rust. Slow rusting resistance in wheat involves the
combined effects of multiple genes that contribute to reduced disease
severity and delayed disease progression.
2.Similar mechanisms of slow rusting resistance have been observed in
other rust diseases affecting various crops.