Molecular aspects of avirulence and pathogenicity of the tomato pathogen Cladosporium fulvum

G.F.J.M. van den Ackerveken

Research output: Thesisinternal PhD, WU

Abstract

<p>The molecular understanding of host-pathogen interactions and particularly of specificity forms the basis for studying plant resistance. Understanding why a certain plant species or cultivar is susceptible and why other species or cultivars are resistant is of great importance in order to design new strategies for future crop protection by molecular plant breeding.<p>In this thesis molecular aspects of avirulence and pathogenicity of the tomato pathogen <em>Cladosporium fulvum</em> are described. The interaction <em>C. fulvum</em> - tomato is an excellent model system to study fungus - plant specificity as the communication between pathogen and plant is confined to the apoplast (intercellular space). The ability to obtain intercellular fluid from <em>C. fulvum</em> -infected tomato leaves enabled the isolation and characterization of plant and fungal compounds that might play an important role in pathogenesis and/or the induction of resistance. The purification and characterization of a race-specific peptide elicitor provides the basis for most of the experiments described in this thesis. This peptide was thought to be produced only by races of <em>C. fulvum</em> avirulent on tomato genotypes carrying the resistance gene <em>Cf9</em> , on which the elicitor induced necrosis.<p>Molecular aspects of avirulence of <em>C. fulvum</em> were first studied by the isolation and characterization of the cDNA encoding the AVR9 race-specific peptide elicitor (Chapter 2). The peptide was shown to be indeed produced by <em>C. fulvum</em> . Races virulent on tomato genotype <em>Cf9</em> , lack the <em>avr9</em> gene and do not produce the peptide elicitor thereby evading recognition by tomato genotypes carrying the corresponding resistance gene <em>Cf9.</em> To prove that the <em>avr9</em> gene is a genuine avirulence gene, races virulent on tomato genotype <em>Cf9</em> , were transformed with the cloned <em>avr9</em> gene (Chapter 3). The cultivar-specificity of the transformants was changed from virulent to avirulent on tomato genotype <em>Cf9.</em> The <em>avr9</em> gene can therefore be considered to be a genuine avirulence gene, the first fungal avirulence gene cloned. Additional proof for the role of the avirulence gene <em>avr9</em> in specificity was provided by the disruption of <em>avr9</em> in two races avirulent on tomato genotype <em>Cf9</em> , by gene replacement, resulting in transformants virulent on tomato genotype <em>Cf9</em> , (Chapter 4).<p>The <em>avr9</em> gene encodes a 63 amino acids precursor protein. Removal of a signal peptide results in an extracellular peptide of 40 amino acids. Proteases of <em>C. fulvum</em> are involved in further processing this extracellular peptide by removal of N-terminal amino acids resulting in peptides of 32, 33 and 34 amino acids. Plant factors are responsible for further processing, resulting in the stable peptide elicitor of 28 amino acids (Chapter 5).<p>The avirulence gene avr9 is highly expressed in <em>C. fulvum</em> while growing inside the tomato leaf. The expression of <em>avr9</em> is induced in <em>C. fulvum</em> grown <em>in vitro</em> under conditions of nitrogen limitation. The high expression of <em>avr9</em> in <em>C. fulvum</em> growing inside the tomato leaf might be caused by nitrogen limiting conditions in the apoplast (Chapter 6).<p>Pathogenicity of <em>C. fulvum</em> was studied at the molecular level by the isolation of two genes encoding extracellular proteins (ECPs). <em></em> The <em>ecp1 </em> and <em>ecp2</em> genes were isolated via the amino acid sequence of ECP1, and polyclonal antibodies raised against ECP2, <em></em> respectively (Chapter 7). The expression of the <em>ecp</em> genes is highly induced <em>in planta</em> as compared to the <em>in vitro</em> situation. The availability of the cloned <em>ecp</em> genes now enables us to study the role and importance of these genes during pathogenesis by reporter gene analysis and gene disruption.<p>Two models describing the <em>C. fulvum</em> -tomato interaction are presented, dealing with basic compatibility and race-specific incompatibility, respectively (Chapter 8). The improved understanding of pathogen recognition can be exploited in future research to elucidate the role of putative receptors in the resistant plant involved in perception of elicitors and induction of active plant defence.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • de Wit, P.J.G.M., Promotor
Award date12 Nov 1993
Place of PublicationS.l.
Publisher
Print ISBNs9789054851486
Publication statusPublished - 1993

Keywords

  • tomatoes
  • passalora fulva
  • plant pathogenic fungi
  • solanum lycopersicum
  • molecular biology
  • gene expression
  • pleiotropy

Fingerprint

Dive into the research topics of 'Molecular aspects of avirulence and pathogenicity of the tomato pathogen Cladosporium fulvum'. Together they form a unique fingerprint.

Cite this