The molecular basis for the gene-for-gene relationship of Vm-resistance in apple to Venturia inaequalis was investigated. Incompatible reactions involved a hypersensitive response (HR), which was accompanied by the accumulation of dark brown pigments and autofluorescent materials in epidermal and mesophyll cells at the site of invasion. Cell-free culture filtrates of the avirulent isolate elicited an HR in the Vm host (h5) leaves, but not in the susceptible host (h1). The elicitor activity was resistant to boiling but was abolished by proteinase K digestion. Elicitation of HR was used to monitor purification of the avirulence factor, AVRVm, from liquid cultures of the avirulent isolate following ultrafiltration, acetone precipitation and ion-exchange chromatography. The purest fraction contained three major proteins all with low isoelectric points (pI 3.0-4.5). The fraction also elicited HR on the differential host h4, but not on other resistant hosts (h2, h3 and h6) tested. Three candidate AVRVm proteins were identified and amino acid sequences were obtained using Edman degradation and mass spectrometry. Nucleotide sequences corresponding to these proteins were found in databases of V. inaequalis expressed sequence tags. There were no polymorphisms evident between avirulent and virulent isolates (representing races 1 and 5 respectively) either at genomic DNA or cDNA level of the full open reading frames. RT-PCR revealed that all genes were expressed in both avirulent and virulent isolates during in vitro and in planta growth. All three genes showed similar levels of expression between avirulent and virulent isolates during their in vitro growth. However, preliminary RT-PCR experiments showed that two of these genes were likely to be expressed at lower levels in the virulent compared with the avirulent isolate during compatible infection. Implications of this difference in expression and the future experiments to identify the genuine AvrVm gene were discussed.
| Identifer | oai:union.ndltd.org:ADTP/276449 |
| Date | January 2004 |
| Creators | Win, Joe |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
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