The fungus Cochliobolus victoriae causes Victoria blight of oats and Arabidopsis and is pathogenic due to its production of a compound called victorin, which induces programmed cell death in sensitive plants. Victorin sensitivity in Arabidopsis is conferred by the dominant gene LOCUS ORCHESTRATING VICTORIN EFFECTS1 (LOV1), which encodes a coiled-coil-nucleotide binding site-leucine-rich repeat (CC-NB-LRR) protein, a type of protein typically associated with disease resistance. In order to better characterize the pathway leading to victorin sensitivity in Arabidopsis, we undertook an EMS-mutagenesis screen to identify mutants that had lost sensitivity to victorin. We isolated 63 victorin insensitive mutants, including 59 lov1 mutants and four locus of insensitivity to victorin1 (liv1) mutants. The LIV1 gene encodes thioredoxin h5 (ATTRX5), a member of a large family of disulfide oxidoreductases. We found that the victorin response was highly specific to ATTRX5, as the closely-related ATTRX3 could only partially compensate for loss of ATTRX5, even when overexpressed. We also created chimeric ATTRX5/ATTRX3 proteins, which identified the central portion of the protein as important for conferring specificity to ATTRX5 in the victorin response. Furthermore, we found that ATTRX5, but not ATTRX3, is highly induced in sensitive Arabidopsis following victorin treatment. Finally, we determined that only the first of the two active site cysteine residues in ATTRX5 is required for the response to victorin, suggesting that ATTRX5 function in the victorin pathway does not involve its redox activity.
We sequenced the LOV1 gene from the 59 lov1 mutants and found that the spectrum of mutations causing loss of function of LOV1 was similar to that found to cause loss of function of RPM1, a CC-NB-LRR protein with a known function in resistance. This indicates that LOV1 functions in a manner similar to resistance proteins. A survey of victorin sensitivity in 30 Arabidopsis ecotypes revealed that victorin sensitivity is common in worldwide populations and that there is very little genetic variation among LOV1 alleles. The prevalence of functional LOV1 alleles suggests that LOV1 functions as a resistance gene to a naturally-occurring pathogen of Arabidopsis.
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