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Title: Phytophthora megakarya and P. palmivora, closely related causal agents of cacao black pod induce similar reactions when infecting pods of a susceptible cacao genotype

item ALI, SHAHIN - Non ARS Employee
item Shao, Jonathan
item Strem, Mary
item LARY, DAVID - University Of Texas
item Meinhardt, Lyndel
item Bailey, Bryan

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2017
Publication Date: 2/14/2017
Citation: Ali, S., Shao, J.Y., Strem, M.D., Lary, D.J., Meinhardt, L.W., Bailey, B.A. 2017. Phytophthora megakarya and P. palmivora, closely related causal agents of cacao black pod induce similar reactions when infecting pods of a susceptible cacao genotype. Frontiers in Plant Science. 8:169.

Interpretive Summary: Phytophthora palmivora and Phytophthora megakarya cause black pod rot on cacao, a disease which destroys cacao yields and threatens chocolate industry supplies. P. palmivora is spread around the world causing 15 to 30% reductions in cacao yields. P. megakarya is found only in Africa, one of the major cacao suppliers. P. megakarya is very aggressive on cacao causing complete yield losses if not managed. As with most disease situations, whether involving animals or plants, understanding the genetics of both the pathogen and disease host is critical to optimizing methods for managing disease. In this study we characterize the genetic responses of cacao to infection by both P. megakarya and P. palmivora. We used a susceptible cacao genotype to build a foundation for future studies which will include resistant cacao genotypes. Despite their differences in aggressiveness in the field, the two Phytophthora species induced very similar genetic responses in susceptible cacao. In cacao, resistance is controlled by many genes and these genes are thought to contribute resistance to both Phytophthora species. Even though a susceptible cacao genotype was used in this study, many genes potentially involved in providing resistance to cacao were identified. As stated, this study forms a basis for detailing the resistance responses of cacao against Phytophthora in the future. The results will be used by scientists developing cacao genotypes resistant to Phytophthora infection benefiting producers and consumers by helping stabilize cacao yields and commodity supplies.

Technical Abstract: Phytophthora megakarya (Pmeg) and Phytophthora palmivora (Ppal) cause black pod rot of Theobroma cacao. Of these two clade 4 species; Pmeg is more virulent and is displacing Ppal on cacao in many cacao production areas in Africa. To understand the advantages Pmeg has over Ppal, we compared symptoms and species specific sporangia production when the two species were co-inoculated onto pod pieces in staggered 24 h time intervals. Sporangia of Pmeg were predominantly recovered from pod pieces with un-wounded surfaces even when applied 24 h after Ppal. If the surface was wounded prior to inoculation, sporangia of Ppal were predominantly recovered if the two species were simultaneous applied or Ppal was applied first but not if Pmeg was applied first. Pmeg demonstrated an advantage over Ppal when infecting un-wounded surfaces while Ppal had the advantage when infecting wounded surfaces. RNA-Seq was carried out on RNA isolated from control and Pmeg and Ppal infected pod pieces 65 h after inoculation. Expression of 4482 and 5264 cacao genes was altered after Pmeg and Ppal infection, respectively, with most genes being induced by both species. Machine learning SOM class analyses separated the cacao RNA-Seq gene expression profiles into 24 classes, 7 of which tended to be induced and 7 of which tended to be repressed in response to infection. Using KEGG analysis, subsets of genes composing interrelated pathways leading to phenylpropanoid biosynthesis, ethylene and jasmonic acid biosynthesis and action, and plant defense signal transduction, and endocytosis showed up-regulation in response to infection. In addition, a large subset of genes encoding putative PR-proteins showed differential expression in response to infection. A subset of cacao genes (36) were used to validate the RNA-Seq expression data and compare genes expression patterns in wounded and unwounded pods and in leaves inoculated with the two Phytophthora species. Expression patterns between RNA-Seq and RT-qPCR were generally reproducible in unwounded pod pieces at 65 h post inoculation, but the level of gene expression was influenced by the tissues studied and wounding. Although in these susceptible interactions changes in gene expression were generally similar, some genes did show differential expression in a Phytophthora species dependent manner. The biggest difference was the more intense changes in expression for 11 of the 36 genes studied in Ppal inoculated wounded pod pieces further demonstrating its rapid progression when penetrating through wounds.