Abstract
Understanding the genetic basis of pathogen susceptibility in various crop plants is crucial to increasing the stability of food, feed, and fuel production. Varietal differences in defence responses provide insights into the mechanisms of resistance and are a key resource for plant breeders. To explore the role of salicylic acid in the regulation of defence in cacao, we demonstrated that SA treatment decreased susceptibility to a pod rot pathogen, Phytophthora tropicalis in two genotypes, Scavina 6 and Imperial College Selection 1, which differ in their resistance to several agriculturally important pathogens. Transient overexpression of TcNPR1, a major transcriptional regulator of the SA-dependent plant immune system, also increased pathogen tolerance in cacao leaves. To explore further the genetic basis of resistance in cacao, we used microarrays to measure gene expression profiles after salicylic acid (SA) treatment in these two cacao genotypes. The two genotypes displayed distinct transcriptional responses to SA. Unexpectedly, the expression profile of the susceptible genotype ICS1 included a larger number of pathogenesis-related genes that were induced by SA at 24h after treatment, whereas genes encoding many chloroplast and mitochondrial proteins implicated in reactive oxygen species production were up-regulated in the resistant genotype, Sca6. Sca6 accumulated significantly more superoxide at 24h after treatment of leaves with SA. These experiments revealed critical insights regarding the molecular differences between cacao varieties, which will allow a better understanding of defence mechanisms to help guide breeding programmes.
Highlights
Theobroma cacao, the seeds of which are used to world (Wood and Lass, 2008)
To explore the role of salicylic acid in the regulation of defence in cacao, we demonstrated that SA treatment decreased susceptibility to a pod rot pathogen, Phytophthora tropicalis in two genotypes, Scavina 6 and Imperial College Selection 1, which differ in their resistance to several agriculturally important pathogens
Both genotypes demonstrated an SA response that resulted in decreased lesion size and pathogen growth, the SA effect was greater in Scavina 6 (Sca6) (4-fold reduction in pathogen biomass accumulation) than in Imperial College Selection 1 (ICS1) (1.7-fold) (Fig. 1E, F)
Summary
Theobroma cacao (cacao), the seeds of which are used to world (Wood and Lass, 2008). ∼30−40% of annual make chocolate, is an economically important crop providing cacao production is lost to pathogens due to its very high disincome to small-scale farmers in tropical regions all over the ease susceptibility (Hebbar, 2007; Argout et al, 2008). Scavina 6 (Sca6) and Imperial College Selection 1 (ICS1), are of special importance to the study of cacao disease resistance because they differ in their tolerance to the above-mentioned pathogens; Sca is a more resistant variety and ICS1 is highly susceptible (Yamada and Lopes, 1999; Brown et al, 2005; Faleiro et al, 2006). Several quantitative trait loci (QTLs) have been mapped for resistance to WBD and black pod rot in Sca; the mechanistic differences underlying the variation in susceptibility between these two varieties are still unclear (Risterucci et al, 2003; Brown et al, 2005; Faleiro et al, 2006). A fuller understanding of the genes associated with susceptible and resistance responses would be extremely useful for cacao breeding programmes and the selection of new varieties with higher resistance
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