The Guard Hypothesis
In only some cases is there direct interaction between the R gene product and the Avr gene product. For example both FLS2 and XA21 interact with the microbial peptides. In contrast, for the NBS-LRR class of R genes, direct interaction has not been shown for most of the R/avr pairs. This lack of evidence for a direct interaction led to the formation of the guard hypothesis for the NBS-LRR class of R genes.
This model proposes that the R proteins interact, or guard, a protein known as the guardee which is the target of the Avr protein. When it detects interference with the guardee protein, it activates resistance.
Several experiments support this hypothesis, e.g. the Rpm1 gene in Arabidopsis thaliana is able to respond to two completely unrelated avirulence factors from P. syringae. The guardee protein is RIN4, which is hyperphosphorylated by the Avr proteins. Another high profile study that supports the guard hypothesis shows that the RPS5 pair uses PBS1, a protein kinase as a guardee against AvrPphB.
Yeast two-hybrid studies of the tomato Pto/Prf/AvrPto interaction showed that the Avirulence protein, AvrPto, interacted directly with Pto despite Pto not having an LRR. This makes Pto the guardee protein, which is protected by the NBS-LRR protein Prf. However, Pto is a resistance gene alone, which is an argument against the guard hypothesis.
Read more about this topic: Gene-for-gene Relationship
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