A histological assessment of the infection strategy of Exserohilum turcicum in maize

Abstract

Northern leaf blight is a lethal foliar disease of maize caused by the fungus Exserohilum turcicum. The aim of this study was to elucidate the infection strategy of the fungus in maize leaves using modern microscopy techniques and to understand better the hemibiotrophic lifestyle of E. turcicum. Leaf samples were collected from inoculated B73 maize plants at 1, 4, 9, 11, 14 and 18 days post‐inoculation (dpi). Samples were prepared according to standard microscopy procedures and analysed using light microscopy as well as scanning (SEM) and transmission electron microscopy (TEM). Microscopic observations were preceded by macroscopic observations for each time point. The fungus penetrated the leaf epidermal cells at 1 dpi and the disease was characterized by chlorotic leaf flecks. At 4 dpi the chlorotic flecks enlarged to form spots, and at 9 dpi hyphae were seen in the epidermal cells surrounding the infection site. At 11 dpi lesions started to form on the leaves and SEM revealed the presence of hyphae in the vascular bundles. At 14 dpi the xylem was almost completely blocked by hyphal growth. Hyphae spread into the adjacent bundle sheath cells causing cellular damage, characterized by plasmolysis, at 18 dpi and conidiophores formed through the stomata. Morphologically, lesions started to enlarge and coalesce leading to wilting of leaves. This study provides an updated, detailed view of the infection strategy of E. turcicum in maize and supports previous findings that E. turcicum follows a hemibiotrophic lifestyle.

Time-Course RNAseq Reveals Exserohilum turcicum Effectors and Pathogenicity Determinants

“…Interest in the candidate effectors SIX13-like and SIX5-like were further fueled by the presence of E. turcicum in the xylem of maize plants (Hilu and Hooker, 1964;Kotze et al, 2019). These candidates were uniquely expressed during in planta infection.…”

“…SIX genes have been identified in five non-Fusarium species. Two of these species are known to invade host xylem cells during infection (E. turcicum and V. nonalfalfae) similar to F. oxysporum, while the other three species (Ustilaginoidea virens, C. orbiculare, and C. gloeosporioides) do not colonize the xylem (Pandey et al, 2012;Li et al, 2013;Yadeta and Thomma, 2013;Marton et al, 2018;Kotze et al, 2019). Although some of the SIX effectors are required for pathogenicity, lead to host responses upon recognition, and can interact with R-genes, the biological function of the majority of these genes is unknown (Ma et al, 2015;Niu et al, 2016).…”

“…After penetration, E. turcicum grows in or between epidermal cells and slowly advances in or between adjacent mesophyll cells with some hyphae growing toward the xylem vessels. The hyphae of E. turcicum colonized xylem vessels, and at this stage of infection, disease symptoms are limited to light, chlorotic flecks (Hilu and Hooker, 1964;Kotze et al, 2019). In susceptible interactions, fungal proliferation in the xylem is rapid, where after hyphae grow from the xylem into adjacent mesophyll cells, leading to widespread tissue necrosis and characteristic elongated NLB lesions (Jennings and Ullstrup, 1957;Hilu and Hooker, 1964;Kotze et al, 2019).…”

“…The hyphae of E. turcicum colonized xylem vessels, and at this stage of infection, disease symptoms are limited to light, chlorotic flecks (Hilu and Hooker, 1964;Kotze et al, 2019). In susceptible interactions, fungal proliferation in the xylem is rapid, where after hyphae grow from the xylem into adjacent mesophyll cells, leading to widespread tissue necrosis and characteristic elongated NLB lesions (Jennings and Ullstrup, 1957;Hilu and Hooker, 1964;Kotze et al, 2019). In monoor polygenic-resistant maize plants, proliferation of E. turcicum hyphae in xylem is limited, and small necrotic flecks result from slowly advancing hyphae in the mesophyll (Hilu and Hooker, 1965).…”

Benefits of maize resistance breeding and chemical control against northern leaf blight in smallholder farms in South Africa

“…Leonard and Suggs. 11,15 This fungus also undergoes a sexual phase named Setosphaeria turcica, a rare form observed in the laboratory. Some publications refer to the fungus by this name.…”