Transcriptional Responses of Eucalyptus grandis Against the Stem Canker Pathogen Chrysoporthe austroafricana Suggests a Role for Ethylene and Terpene Mediated Defences

The E. grandis genome provides researchers with a platform to investigate host responses to pests and pathogens that pose a threat to this important wood fibre crop. One such pathogen, Chrysoporthe austroafricana, causes stem cankers on eucalypts. We exploited this system by investigating the transcriptomic responses of a tolerant (Egr-T) and susceptible (Egr-S) E. grandis clone against C. austroafricana. RNA-sequencing of these interactions generated a minimum of 39 million PE reads per dataset and was mapped to the E. grandis genome using Tophat v2.0. The Cuffdiff analysis revealed ~1500 differentially expressed (DE) genes in both Egr-T and Egr-S. Comparison of the tolerant and susceptible associated transcripts revealed two noteworthy responses which may contribute to the disparate phenotypes. Gene ontology terms associated with the positive regulation of ethylene (ET) were found in Egr-T, whereas terms for the negative regulation of ET were found in Egr-S. ET, which is known to contribute to defense against necrotrophs, appears to be suppressed in Egr-S. This could be the result of pathogen manipulation. The up-regulation of all but one gene associated with the mevalonate pathway for sesquiterpene biosynthesis was observed in both Egr-T and Egr-S in stems; however expression profiling of a monoterpene precursor was up-regulated in Egr-T and not Egr-S leaves. This suggests a difference in the systemic induction of the monoterpenes in the two genotypes which may provide added protection against secondary pests and pathogens. This study points to important mechanisms which may provide Eucalyptus with enhanced tolerance to pathogens.