Abstract
The extensively cultivated coffee species Coffea arabica is an allotetraploid resulting from a recent hybridization between two wild diploid Coffea species. We describe in this paper the first identification and functional assessment of homoeologous gene copies in C. arabica. When cloning the CaWRKY1 gene encoding a transcription factor of the WRKY superfamily associated with plant defense responses to pathogens (Ganesh et al. in Plant Sci 170:1045-1051, 2006), two distinct gene copies (CaWRKY1a and CaWRKY1b) were obtained from C. arabica. Southern blots experiments and phylogenetic analysis of the WRKY1 gene in related diploid Coffea species showed that CaWRKY1a and CaWRKY1b are homoeologous sequences in the allopolyploid coffee genome and are probably close descents of the extant Coffea canephora and C. eugenioides WRKY1 genes. To verify if CaWRKY1a and CaWRKY1b were both functional, gene expressions were monitored in C. arabica plants challenged by the rust fungus Hemileia vastatrix, the root-knot nematode Meloidogyne exigua, and after several abiotic treatments. Real-time quantitative polymerase chain reaction (PCR) assays showed that CaWRKY1 homoeologs were concomitantly expressed and displayed the same altered patterns of expression in leaves and roots during biotic and abiotic treatments. These results suggest that CaWRKY1a and CaWRKY1b were functionally retained in the coffee genome after allopolyploidization and that no functional divergence occurred between the duplicated genes in the C. arabica genome. This study provides the first molecular data on sub-genome-specific expression in allopolyploid coffee. The origin of the C. arabica sub-genomes is discussed with regards on the probable progenitors of this important crop species.