Flor showed that the inheritance of both resistance in the host and parasite ability in the parasite is controlled by pairs of matching genes. One is a plant gene called the resistance (R) gene. The other is a parasite gene called the avirulence (Avr) gene. Plants producing a specific R gene product are resistant towards a pathogen that produces the corresponding Avr gene product.
Person was the first scientist to study plant pathosystem ratios rather than genetics ratios in host-parasite systems. In doing so, he discovered the differential interaction that is common to all gene-for-gene relationships and that is now known as the Person differential interaction.
There are several different classes of R Genes. The major class are the NBS-LRR genes. The protein products of these R genes contain a nucleotide binding site (NBS) and a leucine rich repeat (LRR). Within this class of R genes are two subclasses: -
However there is an extracelluar LRR class of R genes, including rice Xa21 for resistance against Xanthomonas and the cf genes of tomato that confer resistance against Cladosporium fulvum. The proteins have classic receptor-kinase formats - an extracelluar LRR, a membrane spanning region and an intracellular protein kinase domain. The reason that these particular R proteins have extracellular domains is that the pathogens they protect against have an extracellular lifestyle.
The Pseudomonas tomato resistance gene (Pto) belongs to a class of its own. It encodes a Ser/Thr kinase but has no LRR. It requires the presence of a linked NBS-LRR gene, prf, for activity.
R gene specificity (recognising certain Avr gene products) is believed to be conferred by the leucine rich repeats. LRRs are multiple, serial repeats of a motif of roughly 24 amino acids in length, with leucines or other hydrophobic residues at regular intervals. Some may also contain regularly spaced prolines and arginines.
LRRs are involved in protein-protein interactions, and the greatest variation amongst resistance genes occurs in the LRR domain. LRR swapping experiments between resistance genes in flax rust resulted in the specificity of the resistance gene for the avirulence gene changing.
Most resistance genes are autosomal dominant but there are some, most notably the mlo gene in barley, in which monogenic resistance is conferred by recessive alleles. mlo protects barley against nearly all pathovars of powdery mildew.
There is no common structure between avirulence gene products, except that most are secreted proteins. Because there would be no evolutionary advantage to a pathogen keeping a protein that only serves to have it recognised by the plant, it is believed that the products of Avr genes, sometimes known as effector proteins, play an important role in virulence in genetically susceptible hosts.
They are probably targets of proteins involved in plant innate immunity, as homologues of Avr genes in animal pathogens have been shown to do this. The AvrBs3 family of proteins possess DNA binding domains, nuclear localisation signals and acidic activation domains and are believed to function by altering host cell transcription.
It was originally believed that gene-for-gene resistance was conferred by a direct interaction between the R gene product and the Avr gene product, but experiments failed to show this. This lack of evidence for a direct interaction led to the formation of the guard hypothesis.
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, i.e. the Rpm1 gene in Arabidopsis 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.
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.