Introduction to Horizontal Resistance Breeding

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OPBF Presentation 1 Vertical and Horizontal Resistance Explained Presentation by Raoul Robinson

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This presentation is the first in a planned series. Its purpose is to explain plant breeding to organic farmers, seed savers, and to both amateur and participatory plant breeders. This presentation concerns an unusual method of plant breeding which is designed to reduce, or even to eliminate, the need for crop pesticides. It does this by utilising a special kind of host resistance called horizontal resistance.

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Protection may be Stable or Unstable A stable protection does not break down to new strains of the parasite. It is an enduring protection. An unstable protection does break down to new stains of the parasite. It is an ephemeral protection.

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Examples of Unstable Protection

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Examples of Stable Protection

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The Terms ‘Vertical’ and ‘Horizontal’ These terms were coined by the late J.E. Vanderplank, who was possibly the most important plant pathologist who ever lived. The terms are derived from his classic diagrams that define the two kinds of host resistance.

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This diagram represents a potato cultivar that has the vertical resistance gene R1 to the potato blight fungus Phytophthora infestans. The level of resistance is measured on the vertical axis, with maximum at the top, and minimum at the bottom. The eight races of the blight fungus that are possible with three genes (i.e., Genes 1, 2, & 3) are shown on the horizontal axis. These include one race with no genes, three races with one gene, three races with two genes, and one race with three genes. This cultivar is susceptible to any race that carries gene 1, and it is resistant to any race that lacks gene 1.

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This second diagram represents a potato cultivar that has the vertical resistance gene R2 to potato blight. This cultivar is susceptible to any race of the blight fungus that carries gene 2, and it is resistant to any race that lacks gene 2.

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Resistance Max Min Resistance Max Min R 1 When the two diagrams are compared, it will be seen that differences in this kind of resistance are parallel to the vertical axis of the diagram, and this prompted Vanderplank to call it vertical resistance.

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This diagram represents three potato cultivars that have no vertical resistance genes at all. Consequently, they are susceptible to all races of the blight fungus. However, cultivar A is less susceptible (or more resistant) than cultivar B, which is more resistant than cultivar C. Differences in this kind of resistance are parallel to the horizontal axis of the diagram and this prompted Vanderplank to call it horizontal resistance.

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Why Two Kinds of Resistance? Because there are two kinds of infection

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Infection is the contact made by one parasite individual with one host individual for the purpose of parasitism

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Allo-infection means that the parasite originates away from its host, and that it has to travel to that host. This is the equivalent of an immigrant arriving in an island. Clearly, the first infection of any host must be an allo-infection

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Auto-infection means that the parasite originates on the host that it is infecting. It has no need to travel to that host. This is the equivalent of an immigrant's progeny populating an island. Clearly, auto-infection can occur only after there has been a successful allo-infection.

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Think of allo-infection being the equivalent of cross-pollination Think of auto-infection being the equivalent of self-pollination

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Vertical resistance can control allo-infection only Auto-infection can be controlled only by horizontal resistance But horizontal resistance can also control allo-infection

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This is because all plants have horizontal resistance. But many plants have no vertical resistance.

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We will deal with vertical resistance first but, because it is a somewhat complicated subject. Some viewers may prefer to skip to horizontal resistance, beginning in Slide No. 35

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Vertical resistance is due to the gene-for-gene relationship which is an approximate botanical equivalent of mammalian antibodies and antigens The Gene-for-Gene relationship

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the Gene-for-Gene Relationship The Host Each resistance gene is equivalent to a tumbler in a lock The Parasite Each parasitism gene is equivalent to a notch in a key Vertical Resistance and

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In this gene-for-gene relationship, there are six pairs of genes numbered 1 to 6. This host has three of these genes, namely genes 1,3, & 5, and these genes are the equivalent of the tumblers or ‘wards’ of a lock, which prevent an illicit key from turning the lock.

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Host Parasite This parasite has matching genes 1, 3, & 5, and these genes are the equivalent of slots in a key which allow the key to turn a lock in spite of the wards. When the ‘key’ of the parasite fits the ‘lock’ of the host, the ‘door’ of resistance is opened, and the vertical resistance is matched. It then fails to operate, and is said to have ‘broken down’. 1 3 5 1 3 5

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How many locks and keys can we get from a given number of pairs of genes? The maximum number of different locks and keys is obtained when each lock or key has half of the available genes.

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The Gene-for-Gene Relationship A System of Locking The n/2 Model In the n/2 model, every vertical resistance and every vertical parasitic ability has half of the total genes (n/2) in the gene-for-gene relationship. This provides the maximum number of different locks and keys for a given number of pairs of genes. It is assumed that every lock and key occurs with an equal frequency, and with a random distribution, in both the host and the parasite populations. The probability of an allo-infection being a matching infection is then 1 divided by n/2.

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0 0 1 The gene-for-gene relationship functions as a system of locking with many different locks, and many different keys. When there are no vertical genes, there are no locks and no keys, and the probability of an allo-infection being a matching infection is 1.

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6 20 0.05 0 0 1 Number of pairs of genes (n) Number of n/2 locks and keys Probability of matching allo-infection When there are six pairs of genes in the gene-for-gene relationship, every lock and key has three genes. This produces twenty different locks and keys, and the probability of an allo-infection being a matching infection is one in twenty.

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Parasite: 20 keys Host: 20 locks Pathosystem: 1/20 matching

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6 20 0.05 12 924 0.001 When there are twelve pairs of genes in the gene-for-gene relationship, every lock and key has six genes. This produces 924 different locks and keys. The probability of an allo-infection being a matching infection is then approximately one in a thousand.

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0 0 1 6 20 0.05 12 924 0.001 20 184,756 0.000005 When there are twenty pairs of genes in the gene-for-gene relationship, every lock and key has ten genes. This produces 184,756 different locks and keys. The probability of an allo-infection being a matching infection is then approximately one in two hundred thousand.

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Note that, as the number of pairs of genes increases arithmetically, the number of n/2 locks and keys increases geometrically. This is a highly effective system of protection produced with only a few Mendelian genes.

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What happens when every door in the town has the same lock, and every householder has the same key that fits every lock? A system of locking is ruined by uniformity. This is exactly what we have done with vertical resistance in agriculture – we have ruined its system of locking with genetically uniform clones, pure lines and hybrid varieties

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This is why vertical resistance breaks down to new strains of the parasite Vertical resistance is unstable resistance

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Pathosystem: all matching Parasite: one key Host: one lock

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Vertical resistance is single-gene resistance, and it has a gene-for-relationship Horizontal resistance is many-gene (i.e., polygenic) resistance, and it has no gene-for-relationship

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Let us now consider auto-infection and horizontal resistance

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Matching allo-infection Auto-infection Asexual reproduction in the parasite produces a clone, in which every auto-infecting individual has the same key Every part of one host has the same lock. All auto-infection is matching infection, and it can be controlled only by horizontal resistance

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Every host suffers auto-infection It follows that every host has horizontal resistance to all of its parasites But the level of horizontal resistance is low in most modern cultivars due to the vertifolia effect

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Vertical Resistance Matching Allo-infection Horizontal Resistance Key in Lock Open Door Behind that door is a knight in shining armour repelling all intruders Summary

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The first infection of a host must be an allo-infection It must be a matching infection if it is to be a successful infection Auto-infection can occur only after a matching allo-infection Vertical resistance can control allo-infection only Auto-infection can be controlled by horizontal resistance only

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The Two Kinds of Resistance Compared Vertical Resistance Qualitative Many agro-ecosystems High Profile Expensive Very technical Few Cultivars Unstable Horizontal Resistance Quantitative One agro-ecosystem Low Profile Cheap Very easy Many Cultivars Stable

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With the minimum level of horizontal resistance, and without pesticides, the loss of crop is total. With the maximum level of horizontal resistance, and without pesticides, the loss of crop is negligible. Horizontal resistance is quantitative. How Useful is Horizontal Resistance?

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Horizontal resistance does not conflict with either the yield or quality of the crop product. Breeding crops for horizontal resistance is easy, and it can be undertaken by amateurs, particularly when they cooperate in a plant breeding club

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Comprehensive Horizontal Resistance The epidemiological competence of parasites can differ widely from one agro-ecosystem to another. Horizontal resistance to one species of parasite does not normally confer resistance to any other species of parasite. A cultivar which has all its horizontal resistances balanced within one agro-ecosystem, will be unbalanced in another agro-ecosystem A cultivar can have comprehensive horizontal resistance in only one agro-ecosystem

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This means that each agro-ecosystem must have a separate breeding program for each species of crop However, most agro-ecosystems are quite large, and breeding for horizontal resistance is easy The ideal solution is very many plant breeding clubs throughout the world, made up of amateur plant breeders

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The techniques of breeding crops for horizontal resistance will be given in later presentations

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Finis