Discovery of chemical clue may lead to solving cacao's black pod rot mystery

The finding of relatively high levels of the antimicrobial compound clovamide in the leaves of a disease-resistant strain of cacao has significant implications for breeding trees that can tolerate black pod rot, according to Penn State researchers who conducted a novel study.

The discovery is significant because this fungal disease is a serious problem in all areas of the world where cacao is grown, noted researcher Mark Guiltinan, J. Franklin Styer Professor of Horticultural Botany and professor of plant molecular biology, College of Agricultural Sciences. Black pod rot, caused by the fungus Phytophthora, causes pod losses of up to 30% and kills as many as 10% of the trees annually.

"This is the first time that clovamide has been implicated in cacao resistance to pathogens, and the innovative method we used to measure the compound in the leaves could have a major impact in the quest to develop highly productive, disease-resistant varieties of cacao," Guiltinan said. "But these results may have consequences for advancing disease resistance in other plant species, as well."

In the study, leaves of a cacao variety known as "Scavina 6," which is tolerant of the fungus that causes black pod rot, were found to accumulate dramatically higher levels of clovamide and several other metabolites known to be involved in responses to stress—compared to a susceptible cacao strain, "Imperial College Selection 1." Clovamide was the most abundant compound in Scavina 6 leaf extracts, with concentrations up to 58-fold higher than in Imperial College Selection 1.

Researchers discovered this dramatic difference in clovamide accumulation by comparing the abundance of hundreds of specialized metabolites in the two cacao strains using liquid chromatography and tandem mass spectrometry at Penn State's Metabolomics Facility, part of the Huck Institutes of the Life Sciences.

"These results pointed to clovamide as a potential contributor to disease resistance, and laboratory tests supported this hunch," Guiltinan said.

Laboratory assays demonstrated that the compound inhibits the growth of three pathogens of cacao in the Phytophthora genus. Also, in another experiment, clovamide inhibited the activity of the enzymes proteinase and pectinase, which are known to break down plant cell walls. Based on this evidence, it is likely that clovamide prevents pathogens' ability to break down cell walls while killing and digesting plant tissue, the researchers said.

The study's findings, published today (Dec. 23) in Frontiers in Plant Science, promise to enhance plant-breeding efforts to develop highly productive varieties of cacao that can withstand the onset of black pod rot, lead researcher Ben Knollenberg suggested. A postdoctoral scholar in plant science at Penn State, he said the method researchers used to measure clovamide levels in leaves could greatly streamline and condense the breeding process.

"The Scavina 6 strain has been utilized as a parent in cacao-breeding programs for decades, but it hasn't been clear why it's resistant to black pod rot, which makes breeding for resistance difficult," he said. "Breeders cross Scavina 6 with higher-yielding varieties, for example, and they get a bunch of seeds or offspring, which may or may not have inherited the disease resistance. Evaluating the disease resistance of this new generation of trees requires years of growth and measurements in field trials, which requires land, labor and money."