We are using the rice leaf sheath assay (See methods) and live cell microscopy to understand how the rice blast fungus invades living rice cells to establish this devastating rice disease. Using the same infected tissues, we are using the whole genome M. oryzae microarray (Agilent technologies) to identify fungal and rice genes associated with biotrophic invasion. Key learnings on the cellular biology of blast invasion are reported in Kankanala, Czymmek and Valent (2007) The Plant Cell 19:706-724.
This material is based upon work supported by the National Science Foundation under Grant No. 0446315.
|Blast biotrophic invasive hyphae (IH) fill the first invaded rice cells and then colonize the rice tissue by growing from cell to cell. Using the endocytosis tracer dye FM4-64 and transmission electron microscopy, we have shown that IH are encased in plant membrane, the extra-invasive hyphal membrane (EIHM) as they grow inside the rice cells.|
|We noticed that the fungus searches for specific locations to cross the plant cell wall into the live rice cell next door. Although it has previously been reported that IH constrict as they cross the plant cell wall, our live cell imaging showed the extreme degree of this constriction. The fungus seems to preferentially contact the plant cell wall at pit fields. All of our data suggested that the fungus seeks out and uses plasmodesmata to cross into the next live plant cell.|
|Hemibiotrophic fungi like M. oryzae are believed to invade one or a few host cells using a biotrophic strategy and then to switch to a necrotrophic strategy of killing host cells ahead of their invasion. Our studies show that the rice blast fungus uses a different strategy of hemibiotrophy in which successive cell invasions throughout lesion development are biotrophic, but invaded cells die quietly by the time the fungus moves into the next cell. This is consistent with results showing that the fungus has always colonized rice tissue well ahead of macroscopic symptoms (See Berruyer et al. 2006 Phytopathology 96:346-355).|