Difference between revisions of "Disease resistance to viral diseases"

Revision as of 17:54, 26 March 2013

Tim C. Keating, Davidson '13

Plant viruses

Plant viruses represent a huge part of agricultural planning, crop losses and quality reduction around the world. Although simple biologically, the mechanisms they use to survive are complex and not yet well understood. Resistance to plant viruses is an important attribute that is widely prevalent in nature, and often the subject of any plant genetic engineering ventures.

Signs that this blueberry plant has been virally compromised.

Viral life cycle

Diagram of a typical viral life cycle.

• Entry into plant host cell ⇒ un-coating of nucleic acid ⇒ translation of viral proteins⇒ replication of viral nucleic acid⇒ assembly of progeny⇒ cell-cell movement and infection⇒ systemic and plant-plant movement

• Most viruses make it to un-coating viral nucleic acid, even in nonhost species.

What defenses do plants have against viruses?

• Genetic defenses- “R genes” are genes that allow a plant to be a less susceptible host.
• Resistance to most plant viruses is the norm, while the exception is plant susceptibility
• Non-host resistance is said to be the case when discussing a virus that cannot infect any genotype of a certain plant species, which is harder to study
• Host resistance refers to the appearance of a resistant genotype within a population previously susceptible to a particular virus. This phenomenon is easier to study in a genetic context.
• Pathogen resistance is a complex term- it is sometimes hard to tell whether the pathogen is still able to enter the plant and reproduce without the plant showing physical signs, or if the pathogen was stopped all together from entering the plant, as resistance may develop at any point in the pathway towards disease.
• 80% of genetic viral resistance has been shown to be monogenic, with the remainder polygenic or oligogenic

Mechanisms of resistance

• Dominant resistance is most commonly associated with a “recognition” event, leading to a signal cascade that most comonly interferes with viral replication and movement
• Recessive resistance is derived from incompatible viral and host elements, preventing viruses from utilizing crucial cellular machinery of the host for their own use

Schematic of viral resistance from "Genetics of Plant Viral Resistance."

1. HR- hyper-sensitive response
   • Mediated cell death surrounding an infected plant area
2. Single-cell resistance
   • ER- Extreme resistance. Limits viral activity to a single cell by eliminating viral reproduction before it starts. Protease inhibitors?
   • Active resistant response that occurs before cell-to-cell movement can occur
3. Inhibition of within cell or cell-to-cell movement
   • Viruses use movement proteins (MP) to move around a cell, which may be inhibited
   • HR mediated cell death inhibits viral infection to a particular place within a plant
4. Systemic and plant-plant resistance
   • Viruses infiltrate bundle sheath cells and other types to enter the phloem sieve elements, which are the nutrient transport highways of plants much like our circulatory system
   • Once the phloem sieve element cells have been infiltrated, the infection becomes systemic.
   • Systemic movement is more difficult to study, less R genes. Many plants block entry into companion cells or exit from the phloem sieve elements.

Cross and vertical section of a plant showing phloem sieve elements; the circulatory system of plants

Specific known genes

Previously described viral resistance genes

• Table (File:Table.pdf) of known viral resistance genes that show both dominant and recessive inheritance. (Supplementary materials from Genetics of Plant Viral Resistance Media:Annurev.phyto.43.011205.141140 (1).pdf.)