The gene-for-gene relationship was discovered by Harold Henry FlorFlor H.H. (1942) Inheritance of pathogenicity in "Melampsora lini". "Phytopath." 32:653-669] Flor H.H. (1947) Inheritance of reaction to rust in flax. "J. Agric. Res.", 74:241-262] [Flor, H.H. (1955) Host-parasite interaction in flax rust - its genetics and other implications. "Phytopathology" 45:680-685.] [Flor H.H. (1971) Current status of the gene-for-gene concept. "Annu Rev Phytopathol." 9:275-296. DOI|10.1146/annurev.py.09.090171.001423] who was working with rust ("
Melampsora lini") of flax(" Linum usitatissimum"). Flor was the first scientist to study the genetics of both the host and parasite and to integrate them into one genetic systems.Robinson, Raoul A. (1987) Host Management in Crop Pathosystems, Macmillan Publishing Company]
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.
PersonPerson, C.O. (1959) Gene-for-gene relationships in parasitic systems. "Can. J. Bot.", 37:1101-1130.] was the first scientist to study plant
pathosystemratios 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.Robinson, Raoul A. (1987) Host Management in Crop Pathosystems, Macmillan Publishing Company]
Classes of Resistance Gene
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
nucleotidebinding site (NBS) and a leucine rich repeat(LRR). Within this class of R genes are two subclasses: -
*One subclass has an amino-terminal Toll/Interleukin 1 receptor homology region (TIR). This includes the "N" resistance gene of
*The other subclass does not contain a
TIRand instead has a leucine zipperregion at its amino terminal.
proteinproducts encoded by this class of resistance gene are located within the plant cell cytoplasm.
However there is an extracelluar LRR class of R genes, including
riceXa21 for resistance against " Xanthomonas" and the "cf" genes of tomatothat confer resistance against "Cladosporium fulvum". The proteins have classic receptor- kinaseformats - 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.
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.
pecificity of Resistance Genes
R gene specificity (recognising certain Avr gene products) is believed to be conferred by the
leucinerich repeats. LRRs are multiple, serial repeats of a motif of roughly 24 amino acids in length, with leucines or other hydrophobicresidues at regular intervals. Some may also contain regularly spaced prolines and arginines. [Dickinson, M. Molecular Plant Pathology, 2003.]
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 rustresulted in the specificity of the resistance gene for the avirulence gene changing. [ Brody J DeYoung & Roger W Innes. Plant NBS-LRR proteins in pathogen sensing and host defense. Nature Immunology 2006. Volume 7, pages 1243-1249.]
Recessive Resistance Genes
Most resistance genes are
autosomal dominantbut 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
virulencein 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 DNAbinding domains, nuclear localisation signals and acidic activation domains and are believed to function by altering host cell transcription. [Lahaye, Thomas and Bonas, Ulla. Molecular Secrets of bacterial Type III effector proteins. Trends in Plant Science, 2001. Volume 6, issue 10, pages 479-485]
The Guard Hypothesis
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
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.
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