Abstract
Phytophthora palmivora (Butler) is an hemibiotrophic oomycete capable of infecting over 200 plant species including one of the most economically important crops, Theobroma cacao L. commonly known as cocoa. It infects many parts of the cocoa plant including the pods, causing black pod rot disease. This review will focus on P. palmivora’s ability to infect a plant host to cause disease. We highlight some current findings in other Phytophthora sp. plant model systems demonstrating how the germ tube, the appressorium and the haustorium enable the plant pathogen to penetrate a plant cell and how they contribute to the disease development in planta. This review explores the molecular exchange between the oomycete and the plant host, and the role of plant immunity during the development of such structures, to understand the infection of cocoa pods by P. palmivora isolates from Papua New Guinea.
Highlights
Within the order Peronosporales, the largest genus with over 120 described species, Phytophthora is a hemibiotrophic phytogen capable of infecting a wide range of hosts, including many agricultural crops, worldwide [1,2]
NPP8 and a Suppressor of Necrosis 1 protein (SNE1) in P. palmivora (Ppal)–cocoa infection studies. The latter, SNE1, previously characterized in P. infestans, was shown to translocate to the plant nucleus and suppressed the action of secreted NLPs from Phytophthora that are expressed during the necrotrophic growth phase, as well as programmed cell death mediated by the Avr3a/R3a protein interaction [107]
The same authors [146] proposed that extra-xylary tissue hardness associated with fiber content or deposition of suberin, callose and lignin could hinder the progress of fungal pathogens and that bark hardness, acting as a mechanical barrier, had contributed to the slow rate of tissue colonization of Ppal in the canker-resistant cocoa line used in greenhouse studies, leading to the use and selection of resistant cultivars with acceptable horticultural traits [147]
Summary
Within the order Peronosporales, the largest genus with over 120 described species, Phytophthora is a hemibiotrophic phytogen capable of infecting a wide range of hosts, including many agricultural crops, worldwide [1,2]. Some black pod-causing Phytophthora species have distinct geographical distributions (Table 1) while Phytophthora palmivora (Ppal, Butler) [5], which was originally isolated from Palmyra palm (Borassus flabellifer) in 1907, has a pantropical geographical distribution and is found in virtually all cocoa production areas [1,4,6] It has a wide host range of over 200 plant species in the tropics [1,4,6]. The reduction of high humidity and avoidance of excess water are some of the practices in greenhouses/glasshouses for Phytophthora disease control [19] Ppal reproduces both asexually, via the motile zoospores, and sexually, via the formation of oospores caused by the contact of two structures found at the mycelium tips: the female oogonium (the sac which contains the developing oospore) and the male structure, the antheridium. The A2 compatibility type is predominant on cocoa throughout the world [20,21]
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