Exchanges at the Plant-Oomycete Interface That Influence Disease

Abstract

The microbial eukaryotes known as oomycetes comprise more than 1,500 species, including many important phytopathogens. Most exhibit filamentous growth and feed osmotrophically. Oomycetes appear fungus-like but are classified as stramenopiles along with brown algae and diatoms (Beakes et al., 2012). Unlike most fungi, oomycetes are diploid, have cell walls made primarily of cellulose and b-glucans instead of chitin, make aseptate hyphae, undergo oogamous reproduction, and produce few secondary metabolites (Fawke et al., 2015). Oomycetes exhibit diverse lifestyles across terrestrial and aquatic niches. While best known as pathogens of leaves, stems, roots, and fruit, some oomycetes are endophytes, infect animals, or are saprophytes (Lamour and Kamoun, 2009; Ploch and Thines, 2011; Aram and Rizzo, 2018). Many are highly host adapted, unculturable on artificial media, and grow only on living plants as biotrophs. Examples include downy mildew pathogens such as Plasmopara viticola, which infects grapevine (Vitis vinifera), and Albugo candida, which causes white rust on crucifers (Kamoun et al., 2015). The obligate pathogens typically cause minimal damage to the plant but reduce yield and raise susceptibility to secondary infection or abiotic stress. Many oomycetes are hemibiotrophs, which start infections like biotrophs but cause necrosis late in the disease cycle. Most belong to the genus Phytophthora, including Phytophthora cinnamomi, which infects hundreds of agricultural, forest, and ornamental hosts; Phytophthora infestans, which blights potato (Solanum tuberosum) and tomato (Solanum lycopersicum); and Phytophthora sojae, which colonizes soybean (Glycine max) and lupines. Some species, such as Ph. cinnamomi, shift to necrotrophy early in infection, while others, such as Ph. infestans, make the transition much later, reflecting differences in how the species balance the two

Tracking host infection and reproduction of Peronospora salviae‐officinalis using an improved method for confocal laser scanning microscopy

“…Moreover, it was also possible to detect callose deposition and accumulation by the plant in downy mildew‐infected leaf tissue of sage, basil, and red dead‐nettle. Callose depositions are a well‐known, nonspecific immune response of plants towards mechanical and pathogenic injury (Kortekamp, 2005; Judelson and Ah‐Fong, 2019). Encapsulations of haustoria by callose are known for several downy mildew infections, such as Plasmopara viticola on Vitis vinifera , Bremia lactucae on Lactuca sativa (Sedlarova and Lebeda, 2001; Kortekamp, 2005; Diez‐Navajas et al , 2008), and P eronospora parasitica and Hyaloperonospora arabidopsidis on A. thaliana (Donofrio and Delaney, 2001; Fabro et al ., 2011).…”

“…It is assumed that encasements restrict the nutrient uptake by the pathogen, impair effector translocation, or concentrate plant‐derived antimicrobials. Oomycetes, on the other hand, have developed countermeasures against this plant defence system to secure their nutrient source (Judelson and Ah‐Fong, 2019). We found that in the case of P .…”

Uncovering oomycete metabolism using systems biology

“…They colonize the apoplastic space in the leaf mesophyll and form feeding structures, so called haustoria, inside host cells. At the site of the haustoria and the apoplastic hyphae, there is a close interface between pathogen and host, through which effectors, enzymes and small molecules can be exchanged (Judelson and Ah-Fong, 2018). Oomycetes typically reproduce by forming sporangia on hyphae that emerge from stomata a few days after infection (Judelson and Blanco, 2005).…”

“…These components predominantly contained pathways that could be associated with plant compounds such as 'drug metabolism', 'naphthalene degradation', and 'terpenoid biosynthesis'. It is therefore conceivable that these components originate from secreted enzymes involved in interactions with the host or the environment (Judelson and Ah-Fong, 2018). In comparison to the plantpathogen network, the obligate biotroph network lacks 209 compounds, of which 65 are seed compounds and 65 are part of the primary component.…”

“…From there, P. infestans colonizes the mesophyll; hyphae grow in the apoplast while forming intracellular feeding structures, called haustoria. Both the apoplastic hyphae and haustoria provide close contact with the plant, facilitating the exchange of effectors and nutrients (Judelson and Ah-Fong, 2018).…”