Phytophthora palmivora

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1. Classification

Higher order taxa

Chronmalveolata; Stramenophile; Oomycete; Peronosporales; Phytophthora; Palmivora

Species

Phytophthora palmivora

2. Introduction

Fig. 1. Black rot due to P. palmivora. [4].

Phytophthora palmivora is a water mold responsible for fruit, pod, and palm rot in many agricultural products such as the cacao plant, coconuts, citrus fruits, and papaya. It relies on a wet, windy environment to spread sporangia and infection typically occurs in tropical climates[1]. The recent discovery of P. palmivora in the Western Hemisphere contributes to researchers’ understanding of the preferred climate for optimal pathogenesis. Additionally, the spread of root rot caused by this aggressive plant pathogen poses a problem for citrus fruits[2].. The biological implications of P. palmivora have been shown to progress at a much higher rate than its sister taxa Phytophthora parasitica and Phytophthora citrophthora[2]. There is increasing interest in research focused on inhibition of pathogenic activity in agriculture. The lack of understanding on how to combat the damaging effects of P. palmivora as it continues spread across the globe is regarded as a serious cause for concern[2]. Furthermore, there is lack of research on the defining characteristics of P. palmivora’s genome. Much of what is known about this species is derived from information obtained from other species in the genus[3].

3. Ecology

P. palmivora is part of the Phytophthora genus, which contains several species that are plant pathogens. P. palmivora is responsible for the death of cacao crops throughout tropical regions in the world, as well as other agricultural products in India and Southeast Asia. [9]. Ongoing research concerning pathogenesis in crops has taken a variety of approaches; these include accelerating plant cell death, and adding fungal agents. Historically, P. palmivora has been most prevalent in the Southeast Asian tropics and the Pacific Islands however more recently the presence of P. palmivora has been documented in Southern India and other tropical areas such as Ecuador and Hawaii[9]. Within the past five years P. palmivora has been discovered in Turkey, where it has devastated the English Lavender crop[10].

4. Morphology

Fig. 2. Chlamydospores of P. palmivora. Bar = 10 µm. [11].

Fig. 3. Papillate sporangium and short pedicel of P. palmivora. Bar = 10 µm. [11].

Cultured specimens of P. palmivora produce a stellate pattern, with aerial mycelia on certain media, while also producing branching hyphae. After 3-5 days, cells will begin producing ellipsoid, ovoid, pyriform, or obpyriform near spherical sporangia[2]. Upon flooding with distilled water, zoospores are released from sporangia, while incubation in a dark environment results in the production of globular chlamydospores from mycelium[2]. Phytophthora palmivora can reproduce sexually or asexually via sporangia and zoospores respectively. Raindrops and wind transport the spores to new territory. Warmer (tropical) conditions are necessary to activate the spores[1].

5. Physiology

P. palmivora is an obligate aerobe and grows best at 30 degrees Celsius. P. palmivora shows a preference for galactose over other common monosaccharides and grows well with raffinose present[12]. Unlike other species of Phytophthora, P. palmivora grows well in environments containing glutamate and aspartate. These two amino acids are known for inhibiting reproduction in this genus[12]. Furthermore, it grows poorly in arginine, in contrast to other species in this genus. P. palmivora does not grow well in ammonium nitrate or ammonium sulfate[12]. Ammonium is a common compound that inhibits reproduction in Phytophthora. P. palmivora does not display a preference for phosphates[12]. Growth of P. palmivora is greater in the presence of salts containing Mg+ and Ca+ over K+[12]. P. palmivora mimic fungal growth and development in many ways, however, they do not require ergosterol for vegetative growth. Ergosterol is present in many fungi and is often the dominant sterol required for growth and reproduction[12]. Additionally, it is targeted by many fungicides14. Not requiring ergosterol makes P. palmivora resistant to fungicides[13]. While the reproductive phases are dependent on other sterols like cholesterol, this alternate metabolite makes elimination of the pathogen difficult[14].

6. Genome Structure

While much still remains unknown about P. palmivora as a species, inferences about its genome structure can be made based on the genome of sister species, Phytophthora capsici.  The genome of P. capsici has been widely used as a reference for studying many oomycetes due to the extent its genomic size and structure are conserved among genus members, as well as the high levels of synteny among the species[3]. The assembly of genes captured in sequencing efforts were oriented into 917 scaffolds, where half of the genome was contained within 29 larger scaffolds comprised of at least 706. Overall, 17,123 genes fell into 18 linkage groups and 2,682 genes resembling transposable elements were found[3]. Overall, 86% of the sequenced DNA was determined to be non-repetitive sequences[3]. Though there is little to no specific information about P. palmivora, the Phytophthora genus points to some unique features. Phytophthora has been shown to exhibit high phenotypic variation, perhaps due to sequences mimicking transposable elements, which are abundant in Phythophtora genomes[15]. The exact genetic reason for this variation, which persists even in reproduction, is still unknown. The genus may have unique transcription machinery: promoters from other species are non-functional within Phytophthora, it has few introns, and often no TATA box[15]. The combination of high variance and unique cellular mechanisms could explain the persistence of P. palmivora as a pathogen.

7. Current Research

The major focus of research on P. palmivora is its function as a plant pathogen and how to inhibit its growth without being toxic to the surrounding environment. Further characterization of optimal growth conditions for P. palmivora will aid in research concerning its inhibition[2]. In light of the challenges, P. palmivora imposes on the production of a wide variety of crops. For this reason there has been a major shift towards researching inhibition of the pathogenic effects of P. palmivora. Like many other oomycetous organisms, P. palmivora subsists utilizing alternative biochemical pathways, such as its dependence on differing sterols for growth and reproduction, from true fungi[2],[13]. This allows P. palmivora to be more aggressive pathogens[2],[13]. Current research seeks to find sustainable alternatives to traditional harsh fungicides[16]. Some success has been seen in utilizing fungi of the genus Chaetomium as antagonists against the growth of P. palmivora through its development of sporangia[2]. This is done by competitively establishing growth of fungal colonies over existing P. palmivora colonies and degrading mycelium through the release of antibiotics and lytic enzymes common in organisms of the Chaetomium genus[2]. This results in either partial or complete death of P. palmivora colonies. The introduction of bioactive compounds during sporangium formation is the most effective in regards to time to inhibit pathogenic activity[2]. Treatment of P. palmivora with phosphate, an environmentally innocuous agent, has been shown to trigger a defense mechanism in the infected organism. This treatment is ideal because it enhances pre-existing plant responses against pathogens to minimize the spread of infection and death of the plant by signaling a stress response within the pathogenic bacteria[17]. When treated with potassium phosphate, Arabidopsis plants were able to generate oxidative burst, which released superoxides and phenolic compounds and signaled a stress response to surrounding cells. This response kills off infected cells and makes 70% of infected cells unviable to allow the spread of the pathogen[17]. It was demonstrated that oxidative species are responsible for cell death rather than the phosphonate alone, showing that phosphonate is an indirect cause of pathogen inhibition[17].

References

[1] [Hunter, J. E., and I. W. Buddenhagen. "Field Biology and Control of Phytophthora Parasitica on Papaya (Carica Papaya) in Hawaii." Annals of Applied Biology 63.1 (1969): 53-60. Web.]
[2] [Hung PM, Wattanachai P, Kasem S, Poaim S. Biological control of Phytophthora palmivora causing root rot of pomelo using Chaetomium spp. Mycobiology. 2015; 43:63–70.]
[3] [Lamour,K., Hu, J., Lefebvre, V., Mudge, J., Howden, A., Huitema, E., (2014) Illuminating the Phytophthora capsici Genome. In Dean, A., Lichens-Park, A., Kole, C. (Eds), Genomics of Plant-Associated Fungi and Oomycetes: Dicot Pathogens (121-132). Springer Berlin Heidelberg.]
[4] [“PlantVillage: Cocoa (cacao): Diseases and Pests in South America." N.p., n.d. Web. 24 Oct. 2016.]
[5] ["Taxonomy Browser: Phytophothora Palmivora." NCBI. N.p., n.d. Web. 24 Oct. 2016.]
[6] ["Phylogenomics Reveals a New ‘megagroup’ including including most photosynthetic eukaryotes" N.p., n.d. Web. 24 Oct. 2016.]
[7] [Fry, William. "Phytophthora Infestans: The Plant (and R Gene) Destroyer." FRY - 2008 - Molecular Plant Pathology - Wiley Online Library. N.p., n.d. Web. 27 Nov. 2016.]
[8] [Goss, Erica M., Meg Larsen, Gary A. Chastagner, Donald R. Givens, and Niklaus J. Grünwald. "Population Genetic Analysis Infers Migration Pathways of Phytophthora Ramorum in US Nurseries." PLoS Pathog PLoS Pathogens 5.9 (2009): n. pag. Web.]
[9] [Ordoñez, M. E., D. A. Jácome, C. B. Keil, R. J. Montúfar, and T. A. Evans. "First Report Of Phytophthora Palmivora Causing Bud Rot on Palmito (Bactris Gasipaes) in Ecuador." Plant Disease 100.6 (2016): 1248. Web. 26 Sept. 2016.]
[10] [Dervis, S., M. Arslan, C. U. Serce, S. Soylu, and I. Uremis. "First Report of a Root Rot Caused by Phytophthora Palmivora on Lavandula Angustifolia in Turkey." Plant Disease 95.8 (2011): 1035. Web. 26 Sept. 2016.]
[11] ["Illustration of Key Morphological Characteristics of Phytophthora Species Identified in Virgina Nursery Irrigation Water." N.p., n.d. Web. 24 Oct. 2016.]
[12] [Leonian, Leon H. 1925. Physiological studies on the Genus Phytophthora. American Journal of Botany 12 (7): 444 – 498.]
[13] [Grant, B. R., W. Greenaway, and F. R. Whatley. "Metabolic Changes during Development of Phytophthora Palmivora Examined by Gas Chromatography/Mass Spectrometry." Microbiology 134.7 (1988): 1901-911. Web.]
[14] [Weete, John D., Maritza Abril, and Meredith Blackwell. "Phylogenetic Distribution of Fungal Sterols." PLoS ONE 5.5 (2010): n. pag. Web.]
[15] [Kamoun, S. "Molecular Genetics of Pathogenic Oomycetes." Eukaryotic Cell 2.2 (2003): 191-99. Web.]
[16] [Timmusk, Salme. "Biofilm Forming Paenibacillus Polymyxa Antagonizes Oomycete Plant Pathogens Phytophthora Palmivora and Pythium Aphanidermatum." Journal of Biotechnology 136 (2008): n. pag. Web.]
[17] [Guest, D., Guest, D, Guest, R. "Defence Responses Induced by Potassium Phosphonate in Phytophthora palmivora-challenged Arabidopsis Thaliana." Physiological and Molecular Plant Pathology 67.3-5 (2005): 194-201. Web.]


Edited by [Jennifer Talbot], student of Jennifer Talbot for BI 311 General Microbiology, 2016, Boston University.