Barbara Valent, Kansas State University

Follow the Effectors: Understanding Ancient and Emerging Blast Diseases on Rice and Wheat

The ascomyceteous fungus, Magnaporthe oryzae (synonym of Pyricularia oryzae), causes disease on diverse grass host plants including the world’s major cereal crops. M. oryzae occurs as host-adapted populations including the Oryza pathotype (MoO), causing the ancient rice blast disease, and the Triticum pathotype (MoT), causing the recently emerged wheat blast disease. Wheat blast has spread and restricted wheat production within South America since it was first reported in Brazil in 1985. Movement and establishment of this explosive disease in Bangladesh in 2016 underscores the importance of developing sustainable control strategies for blast diseases. The hemibiotrophic rice blast fungus repeatedly invades living rice cells as it spreads within host tissue for 4 to 5 days before macroscopic lesions appear. During biotrophic invasion, the MoO fungus specifically expresses genes encoding dozens of effectors, termed cytoplasmic effectors, which are delivered inside living rice cells to take control. In contrast to effectors that remain in the host extracellular space, the fungus secretes cytoplasmic effectors using a nonconventional secretion system and concentrates them into a specialized interfacial structure, the biotrophic interfacial complex (BIC). So far, at least 30 of the BIC-localized effectors that are translocated into the cytoplasm of invaded rice cells move into neighboring rice cells through plasmodesmata, the natural communication channels between plant cells, presumably to prepare these host cells before further fungal invasion. New higher-resolution live cell imaging shows that cytoplasmic effectors are packaged in vesicles in BICs and the surrounding rice cytoplasm, leading to our current working hypothesis that the fungus manipulates plant endocytosis mechanisms for internalizing effectors inside living rice cells. Initial results suggest that the wheat blast fungus shares the same cytoplasmic effectors and infection strategies identified for the rice blast fungus. Current status and interesting contrasts between the ancient and emerging blast diseases will be discussed.