Fungi and Oomycetes

By Marissa Brasher

This week in OUTPACE we discusses two distinct types of phytopathogenic attackers, Fungi and Oomycetes. Fungi are the most common plant pathogens with a total of 100,000 fungal species, 300 of these species have been linked to disease in animals while approximately 8,000 species attack plants. Molds and yeasts are widely distributed in the air, dust, fomites, and normal flora aiding the distribution of these infectious agents.  Upon infection, plants appear off-color or yellow, and may be weaker or show signs of wilting. There are three different fungal life styles on plant hosts: biotrophic, hemi-biotrophic/necrotrophic, and endophyte. While biotrophic and hemi-biotrophic/ necrotrophic life styles will eventually kill their host, endophytes live symbiotically with their hosts.

Plants present both primary and secondary defense responses to pathogen infections. Their primary defense is structural, this includes their cuticles, cell walls, suberin layers, and seed coat. Their secondary defense responders are hormonal. Biotrophic plants secrete Salicylic Acid as their defense while Necrotrophs secrete Jasmonates. Fungal pathogens have evolved around these physical barriers through natural openings such as the stomata, wounds, and insect vectors. Some species of fungal pathogens contain a structure called appressoria which is understood to be important the penetration of the plant’s surface. Some appressorias are melanized. Melanized appressoria power penetration by building enormous turgor pressure, the pressure forces the penetration peg through the leaf. The penetration peg grows into a haustorium, which are specialized for nutrient and signal exchange.

Our second lecture this week focuses on Oomycetes. Oomycetes has been previously classified under Fungi, they liken in morphology and physiology but are phylogenetically unrelated. Oomycetes have now been classified in Chromista, there are approximately 500-800 known species. Unlike fungi, their cell walls lack chitin, instead they consist of glucans and cellulose. Their hyphae have no cross walls (septae) and can reproduce asexually or sexually. Asexual reproduction is carried out through zoospores to preserve their genomes. Zoospores have two types of flagella, anteriorly directed is the tinsel type, posteriorly directed is the whiplash type. Sexual reproduction is carried out through a resting spore, an oospore.  Oomycetes are found in water and soil, they can be saprobic on both dead plants and animals. Oomycetes infections can cause downy mildews.

Some are considered deadly plant pathogens. For example, one of the most infamous oomycetes species is Phytophthora infestans, which translates to plant destroyer. This pathogen is responsible for none other than Ireland’s Great Famine of the 1840’s. Phytophthora infestans caused an outbreak in potato late blight, unaware of the pathogen, farmers lost almost all of their potato crops for years causing mass starvation and emigration. Another infamous oomycetes species is Phytophthora ramorum which can cause Foliar Blight or Sudden Oak Death (SOD).  In non-oak hosts the pathogen causes Foliar Blight; however, in oak hosts the pathogen causes Sudden Oak Death. S.O.D. affects mostly true oaks and tanoaks in the United States.

We also conducted a fungal infection on Arabidopsis using the pathogen Botrytis cinerea. Fungal spores were mixed with potato dextrose to later infect the Arabidopsis. We used two strains of Arabidopsis for infection, Col-0, the wild type, and Pad2. Col-0 is more resistant to infection than Pad2. 6 leaves were separated from each strain and taken downstairs for infection. A pipette was then used to deliver 20 microliters of the spore solution on top of the leaves. The leaves were stored downstairs after infection.

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After a few days, we received our infected leaves to quantify the disease severity on the Arabidopsis. Lesions may develop upon infection, leaves may also appear discolored and shriveled. Below is a comparison of the Arabidopsis leaves prior and post infection.

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Before infection                                                      After infection

We expressed the disease severity in the two strains into charts. Below are some photos of the classroom’s data.

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For example, my chart in the middle of the bottom row proves that my Col-0 strain was more resistant than Pad2. The Pad2 strain infection overall produced mostly yellow leaves which shows more disease severity than a light green or gray color. These results conclude our discussions on Fungi and Oomycetes.

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