IDENTIFICATION AND CHARACTERIZATION OF RACE 1 BACTERIAL SPECK RESISTANCE IN A WILD RELATIVE OF TOMATO
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Pseudomonas syringae pv. tomato (Pst) is a persistent pathogen of tomato that causes bacterial speck disease. On tomato, resistance conferred by the gene Pto is effective against race 0 Pst strains which express the effector proteins AvrPto and/or AvrPtoB; however, race 1 strains of Pst, which do not express AvrPto/AvrPtoB but rather a different repertoire of effectors, evade Pto-mediated resistance. Race 1 strains of Pst are becoming increasingly common, and no simply-inherited genetic resistance to such strains is known. It was discovered that a locus in Solanum lycopersicoides, termed Pseudomonas tomato race 1 (Ptr1), confers resistance to race 1 Pst strains by recognizing the type III effector AvrRpt2. In Arabidopsis and apple, strains of Pst and Erwinia amylovora expressing AvrRpt2 degrade the RIN4 protein, thereby activating RPS2 or Mr5-mediated immunity, respectively. Ptr1 also recognized homologs of AvrRpt2 from diverse bacteria including one in Ralstonia pseudosolanacearum and this correlated with the ability of AvrRpt2 to degrade RIN4. Using site-directed mutagenesis of AvrRpt2, we found that, like RPS2, activation of Ptr1 requires AvrRpt2 proteolytic activity. Ptr1 detection of AvrRpt2 activity suggests it likely encodes an NLR protein or possibly a guardee such as RIN4. Ptr1 was identified by cloning of candidate NLR-encoding genes located in the Ptr1 region and testing using Agrobacterium-mediated transient expression in Nicotiana glutinosa identified one gene for the ability to activate the plant immune system in response to AvrRpt2 in the presence of tomato Rin4. Interestingly, while overexpression of Ptr1 in N. glutinosa leaves caused localized cell death, co-expression of Ptr1 with tomato Rin4 prevented this cell death. The protein encoded by Ptr1 has little similarity to RPS2 or Mr5, which suggests that Ptr1 is a third example of convergent evolution in different plant species for recognition of AvrRpt2. In summary, the Ptr1 gene has the potential to become an important component (along with Pto) in controlling bacterial speck disease. Further research focused on studying the mechanism of action between Ptr1 and Rin4 may contribute to a better understanding of the recognition of the type III effector AvrRpt2 in tomato.
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131 pages
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2019-12
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AvrRpt2; Bacterial Speck; Pseudomonas syringae pv. tomato; Ptr1; Rin4; Solanum lycopersicoides
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Martin, Gregory B.
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Bogdanove, Adam Joseph
Smart, Chris
Smart, Chris
Degree Discipline
Plant Pathology and Plant-Microbe Biology
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Ph. D., Plant Pathology and Plant-Microbe Biology
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Doctor of Philosophy
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dissertation or thesis