title: Phytophthora megakaryaandP. palmivora, Causal Agents of Black Pod Rot, Induce Similar Plant Defense Responses Late during Infection of Susceptible Cacao Pods.
abstract: Phytophthora megakarya(Pmeg) andPhytophthora palmivora(Ppal) cause black pod rot ofTheobroma cacaoL. (cacao). Of these two clade 4 species, Pmeg is more virulent and is displacing Ppal in many cacao production areas in Africa. Symptoms and species specific sporangia production were compared when the two species were co-inoculated onto pod pieces in staggered 24 h time intervals. Pmeg sporangia were predominantly recovered from pod pieces with unwounded surfaces even when inoculated 24 h after Ppal. On wounded surfaces, sporangia of Ppal were predominantly recovered if the two species were simultaneously 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 3 days post inoculation to assess their abilities to alter/suppress cacao defense. Expression of 4,482 and 5,264 cacao genes was altered after Pmeg and Ppal infection, respectively, with most genes responding to both species. Neural network self-organizing map analyses separated the cacao RNA-Seq gene expression profiles into 24 classes, 6 of which were largely induced in response to infection. Using KEGG analysis, subsets of genes composing interrelated pathways leading to phenylpropanoid biosynthesis, ethylene and jasmonic acid biosynthesis and action, plant defense signal transduction, and endocytosis showed induction in response to infection. A large subset of genes encoding putative Pr-proteins also showed differential expression in response to infection. A subset of 36 cacao genes was used to validate the RNA-Seq expression data and compare infection induced gene expression patterns in leaves and wounded and unwounded pod husks. Expression patterns between RNA-Seq and RT-qPCR were generally reproducible. The level and timing of altered gene expression was influenced by the tissues studied and by wounding. Although, in these susceptible interactions gene expression patterns were similar, some genes did show differential expression in aPhytophthoraspecies dependent manner. The biggest difference was the more intense changes in expression in Ppal inoculated wounded pod pieces further demonstrating its rapid progression when penetrating through wounds.
authors.affiliation: Sustainable Perennial Crops Laboratory, United States Department of Agriculture/Agricultural Research Service, Beltsville Agricultural Research Center-West, Plant Sciences Institute Beltsville, MD, USA.
authors.forename: Shahin S
authors.initials: SS
authors.lastname: Ali
authors.affiliation: Sustainable Perennial Crops Laboratory, United States Department of Agriculture/Agricultural Research Service, Beltsville Agricultural Research Center-West, Plant Sciences Institute Beltsville, MD, USA.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Shao
authors.affiliation: Physics Department, University of Texas at Dallas Richardson, TX, USA.
authors.forename: David J
authors.initials: DJ
authors.lastname: Lary
authors.affiliation: Sustainable Perennial Crops Laboratory, United States Department of Agriculture/Agricultural Research Service, Beltsville Agricultural Research Center-West, Plant Sciences Institute Beltsville, MD, USA.
authors.forename: Mary D
authors.initials: MD
authors.lastname: Strem
authors.affiliation: Sustainable Perennial Crops Laboratory, United States Department of Agriculture/Agricultural Research Service, Beltsville Agricultural Research Center-West, Plant Sciences Institute Beltsville, MD, USA.
authors.forename: Lyndel W
authors.initials: LW
authors.lastname: Meinhardt
authors.affiliation: Sustainable Perennial Crops Laboratory, United States Department of Agriculture/Agricultural Research Service, Beltsville Agricultural Research Center-West, Plant Sciences Institute Beltsville, MD, USA.
authors.forename: Bryan A
authors.initials: BA
authors.lastname: Bailey
keywords: Phytophthora; RNA-Seq; black pod rot; cacao; plant-pathogen interaction; self-organizing map