The Vertical and Horizontal Subsystems

Many plant pathosystems have two subsystems called the vertical and horizontal subsystems, named after Vanderplank's (1963) concept of vertical and horizontal resistances. These terms 'vertical' and 'horizontal' are abstract terms with no literal connotations. Vanderplank could equally have labelled the two kinds of resistance 'hard' and 'soft' resistance, or 'alpha' and 'beta' resistance. These terms have proved unpopular, but it is not clear whether this is because of their abstract nature, or because of the concept they describe. However, as the originator of the concept, Vanderplank had the privilege of choosing the words to describe it, and we should respect his precedence.

The term 'vertical' means that a gene-for-gene relationship (see 4) is present. And the term 'horizontal' means that a gene-forgene relationship is not present. Originally, the terms described two kinds of resistance. Being the result of a gene-for-gene relationship, vertical resistance is genetically controlled by single genes. However, not all single-gene resistances involve a gene-forgene relationship. Horizontal resistance results from mechanisms other than a gene-for-gene relationship, and it is usually (but not invariably) polygenically controlled.

Vertical resistance is normally qualitative resistance. That is, it functions either completely or not at all. Because of the nature of the gene-for-gene relationship, vertical resistance operates against some strains of the parasite but not others. In agriculture, vertical resistance provides a complete protection against a parasite, but it is likely to fail on the appearance of a new strain of that parasite. It thus provides a complete but temporary protection. It is 'unstable' resistance (see 10.6).

Horizontal resistance is normally quantitative resistance. That is, it can function at any level between its maximum and its minimum. It operates equally against all strains of the parasite, and it does not fail on the appearance of a new strain. It thus provides an incomplete but permanent protection. It is 'stable' resistance (see 10.6).

The terms 'vertical' and 'horizontal' can be applied to parasitic abilities in the parasite, as well as to resistances in the host. They can also be used to describe subsystems of a pathosystem, populations of both host and parasite, and so on. In every case, they are stating that a gene-for-gene relationship is either present or absent, respectively.

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