ALOYSIUS IBEAGHA, SANJAY JAIN, RALPH HÜCKELHOVEN, KARL-HEINZ KOGEL
Justus-Liebig-Universität Giessen, Institute of Phytopathology and Applied Zoology, Germany
Spot blotch of wheat, caused by the hemibiotrophic fungal pathogen Bipolaris sorokiniana (teleomorph: Cochliobolus sativus) is a cereal disease of increasing global concern in wheat and barley cropping systems. The pathogen is transmitted through contaminated or infected seeds and can adversely affect germination, development of the root system and above ground plant parts which may kill the seedling within a couple of days. Due to the increasing food demand and limited agricultural land, an effective control of spot blotch needs to be achieved. Hence, the identification of parental stocks possessing an adequate level of resistance to Bipolaris sorokiniana is urgently required.
We investigated the interaction of several differentially resistant wheat genotypes to the attack of Bipolaris sorokiniana to uncover the mechanism of host resistance. A microscopic dissection of early fungal interaction revealed that fungal penetration into the leaf epidermal layer was stopped either by papilla-like cell wall apposition (CWA) and/or by a hypersensitive reaction (HR) of single cells. Restriction of post-penetration growth of the fungus within a single epidermal cell (biotrophic phase) by a cell death reaction was also observed and termed ``post-penetration HR'' or ``encapsulation''.
In case the fungus successfully overcame epidermal defence, its spreading within the mesophyll tissue (necrotrophic phase) could be restricted by some of the wheat genotypes. The restriction of fungal spreading after successful infection of the host mesophyll tissue was found to be one of the most important mechanisms limiting the emergence of disease outbreak. We discuss the role of fungal toxins as chemical determinants of resistance or susceptibility. These data might be used in strategic breeding programs to build up disease resistance in cereals to hemibiotrophic pathogens.
Keywords: Spot blotch, cell wall apposition, hypersensitive reaction, encapsulation, spreading factor, hemibiotrophy