Systems of Locking

There is little doubt that the gene-for-gene relationship functions as a system of locking. Each individual plant host has a biochemical 'lock', consisting of several vertical resistance genes. And each individual parasite has a biochemical 'key', consisting of several vertical parasitism genes. Each resistance gene in the host is the biochemical equivalent of a tumbler in a mechanical lock. And each parasitism gene in the parasite is the biochemical equivalent of a notch in a mechanical key. At the time of allo-infection, the parasite's key either fits the host's lock, or it does not fit. If the key fits, the infection is described as a matching infection, the 'door' of host resistance is opened, and the parasite is able to enter. If the key does not fit, the infection is described as a non-matching infection, the 'door' of host resistance remains closed, and the parasite is denied entry.

The penetration of a stigma by a pollen germ tube is very similar to the penetration of a host by a fungal pathogen germ tube. And the system of pollen self-incompatibility, which prevents self-pollination, is genetically and operationally very similar to the gene-for-gene relationship. However, this analogy must not be pressed too far.

For ease of discussion, it should be remembered that the resistance and parasitic ability based on the gene-for-gene relationship are described as vertical. The 'lock' described here is vertical resistance, and the 'key' is vertical parasitic ability.

Robinson (1976) suggested that the epidemic could be subdivided into the esodemic and the exodemic. He defined the esodemic as that part of the epidemic that involves auto-infection only. The exodemic is that part of the epidemic that involves allo-infection only. Clearly, vertical resistance controls the exodemic by reducing the proportion of allo-infections that are matching infections. But not all exodemics are controlled by vertical resistance. That is, a vertical subsystem may occur in a discontinuous pathosystem, but it need not necessarily do so. The esodemic can be controlled by horizontal resistance only and, because every epidemic has an esodemic, horizontal resistance is universal. Equally, because some pathosystems do not have a gene-for-gene relationship, it follows that horizontal resistance can also control the exodemic.

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