Gene-for-genes interactions between cotton R genes and Xanthomonas campestris pv. malvacearum avr genes

Area of Science:

• Plant Pathology
• Molecular Genetics
• Bacterial Genetics

Background:

• Six plasmid-borne avirulence (avr) genes were previously identified in Xanthomonas campestris pv. malvacearum strain XcmH.
• Understanding these avr genes is crucial for deciphering host-pathogen interactions in cotton diseases.

Purpose of the Study:

• To localize six cloned avr genes from XcmH.
• To investigate the interaction patterns of these avr genes with cotton resistance (R) genes.
• To elucidate the evolutionary and genetic mechanisms underlying race changes in X. campestris pv. malvacearum.

Main Methods:

• Subcloning and Tn5-gusA insertional mutagenesis for gene localization.
• Southern blot hybridization and DNA sequence analysis for gene comparison.
• Analysis of spontaneous race change mutants to identify genetic mechanisms.

Main Results:

• All six avr genes were localized on cloned fragments.
• AvrB4 conferred avirulence on cotton lines with B1 or B4 resistance loci.
• The B1 locus conferred resistance against multiple avr genes.
• XcmH avr genes show high similarity to each other and related Xanthomonas species genes, differing mainly in repeat motif multiplicity.
• Avrb6 shares 97% DNA sequence identity with related avr genes, with conserved inverted terminal repeats.
• Multiple copies of avr genes and high homology likely explain gene-for-gene interactions and frequent race changes.
• Race change mutants exhibited avr locus deletions, indicating recombination as a key mechanism.

Conclusions:

• The studied avr genes in Xanthomonas campestris pv. malvacearum are highly homologous and exist in multiple copies.
• These genetic characteristics contribute to gene-for-gene interactions and facilitate rapid evolution of new pathogen races.
• Intergenic recombination is identified as the primary mechanism for generating race diversity in this cotton pathogen.