next phase of battle

After overcoming the leaf surface protection against fungal invasion, the battle inside the leaf begins. This corn journal blog in 2018 illustrates the next phase of the battle between host and pathogen. 

​A recent report in Science (Vol.359, 1399-1403) describes how a single fungal gene controls plant cell-to-cell invasion by the rice blast fungus.  It is an interesting description of how a plant pathogen manages to invade a plant cell and manages to travel cell-to-cell, evading the plant immunity system, eventually killing significant sufficient tissue to sporulate and spread to other plant tissue.  This study involved a lot of biochemistry and microscopy and uncovered interactions that are probably common to other fungal-corn interactions.
 
Living plant cell walls have microscopic holes, called plasmodesmata, that allow passage of sugars and proteins to adjacent cells.  These holes are smaller than normal fungal filaments (hyphae).  In this study in which the pathogen Magnaporthe orzae, cause of rice blast disease, initially invades the plant by forcing the outer leaf cells with an appresorium, to occupy a cell but maintaining the cytoplasm in the cell.  The hyphae of this fungus reduce the size of the hyphae to about 1/10th to squeeze through the plasmadesmata into the next cell.  The plant cell resistance includes reacting to the presence of the invader by depositing callose to close the plasmodesmata, and therefore restricting the fungus to the initially invaded cell.  The researchers found that a single fungal gene delays the resistance reaction until the pathogen has passed on the next cell.  Mutants of this gene in the fungus are not able to pass onto the next cell. Resistance is related to a quicker reaction in closing the plasmodesmata as well as repression of the fungus within the infected cells.
 
This study involving the interaction between a fungal pathogen and host is probably common to many leaf diseases.  Relatives of this fungal species attack other grass crops such as wheat and barley but apparently not corn. The study illustrates the evolution of methods of attack by pathogens and competing defense systems by plants.  It is also interesting that the study was done by several specialists employing multiple techniques and understandings of their science.