Introductory Plant Pathology

Genetics and Plant Disease
Host Variability
Variation in Nature

General Mechanisms of Variability

Hybridization
Mutation
Cytoplasmic Inheritance

Philosophical Point to Ponder

Native plant species have survived because of their ability to acquire and maintain horizontal resistance. Man's intervention into the culture of plant species, away from their natural habitats, has lead to need to introduce and maintain vertical resistance in cultivated species.

This, perhaps, is not all bad as it keeps plant breeders, horticulturists and ethnobotanists employed.

TYPES of PLANT RESISTANCE TO PATHOGENS

True Resistance

For annual crops not vegetatively propagated
If gene is overcome - crop fails
Pathogen specific

Broadly based non-specific
Perennial species, or for annual crops
Probably most common in the wild

For cultivated species the most useful resistance results from a combination of horizontal and vertical resistance.

Apparent resistance

Disease Escape
- For any number of reasons a plant may simply escape infection and, thereby, no disease results.
- Plants may be genetically predisposed to "escape"of they grow such that they are not vulnerable to infection when a pathogen is present.
- The placement and topography of a leaf surface around a stoma may enable a plant to "escape" infection.
Tolerance
- Even though a plant may be infected and diseased, it may still produce a "good" crop.
- All plants are infected and parasitized by any number of organisms. If it were not for tolerance (that is the ability to endure though infected); the vast majority of plants would succumb.

The Gene-for-Gene Concept

Postulated by H.H. Flor, it introduced the concept that both host and parasite genetics played a role in the determination of whether or not a resistance reaction would be observed. Do not misunderstand the concept. Flor put forward the concept that the expression of resistance by the host was dominant while conversely the expression of Avirulence by the parasite was dominant. This concept was carried forward by others, namely Clayton Person, to advance a theory that there was a single gene in the host the interacted with a single gene in the parasite. This theory was put forward at about the same time that the "one gene- one protein" idea was being proven in biochemistry. The Gene-for-Gene idea of Person has not been proven with the rigor required by the biochemists; however, the gene-for-gene concept of Flor is still valid.

If one takes the position of a Darwinian that the ability to be genetically plastic and adapt to changing needs is the key to survival then one must postulate that all organisms have some mechanism to introduce genetic variability. The classic Mendelian scheme applies generally to eukaryotes but alternate schemes must occur in some prokaryotes that do not undergo sexual reproduction.

Natural Variability

If plants have existed with their parasites for eons, is it not obvious that resistance in nature is the rule and; therefore, the place to find resistance genes may be in natural populations.?

This graphic seeks to depict the accommodation phenomenon often refereed to as "Selection Pressure" that occurs in nature.

In this Image:

N = the steady state condition where parasitism is occurring without the demise of either of the interactants.
A = a point in time when the host develops a gene that provides it with an advantage over the parasite. In common usage, we might term this a "resistance" gene.
A to B = a time when the host "thrives" and the expense of the parasite. (The green line representing the fortunes of the host while the brown line those of the parasite)
B to C = a time of unestimable length during which the host is greatly advantaged. At point C, a change occurs in the parasite which allows it to "overcome" the resistance gene. This change (positive mutation) could be any number of attributes from alteration in a receptor site, to enzyme, to development of a detoxification mechanism.
C to E = a time when the parasite is able to out-compete the host.
E to F = a time similar to B to C but with the advantage to the parasite.
F to G = the host develops a new resistance mechanism.

During this process, some assumptions have been made. For illustrative purposes, it is assumed that the action of neither host nor parasite leads to the extinction of the other. It is also shown that the magnitude of each subsequent acquisition of resistance (virulence) genes results in a lower magnitude than the following. This is predicated, in this example, on considering only a single change which results in a single reciprocal change. In nature, this is probably no the case. None-the-less, natural systems do seek equilibrium.

This harmonic "ringing" pattern represents a large number of cycles over a long period of time. As one can appreciate, this pattern is applicable for single interactions, however, it is plausible that, in the long run, one could probably envisage this pattern as representing the totality of a host-parasite interaction over time.

Enter man and a "host-improvement" plan.

The scenario depicted on this graphic demonstrates the intervention of plant breeding. A time period "A" a host variety containing resistance gene R1 against virulence gene r1 (remember resistance is dominant in the host and virulence is recessive in the pathogen) is planted into a "field" containing a potential pathogen.

As may be observed, the parasite is initially disadvantaged, recovers and overcomes the resistant host. When the host is declines, an additional resistance gene is introduced. This additive process of resistance genes and continued development of new virulences of the pathogen continues. In as much as there is no evidence that any pathogen has ever been eradicated by this process, it is incumbent on the breeder to maintain resistance against all known pathogens during the process of developing new resistance. Thus, man's intervention into the process necessitates the pathogen to continually develop increasingly new virulence.

With the advent of transgenic methodologies, one might have hoped that intervention by biotechnology into the process would significantly shift the balance in favor of the host. However, to this date, there is not convincing evidence the genes inserted by biotechnological methods significantly reduce selection pressure on the affected pathogens. Therefore, is it reasonable to expect them to be of any longer effective duration in the field than "naturally selected" genes. Evidence from the Bacillus thuringensis literature would certainly not support optimism in that regard.

Thus, what are some Plant Health Management Strategies that reduce selection pressure:

Crop Rotation
Mixed Variety plantings (include some susceptible genotypes)
?
?

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:

Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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