Pathogen- plant warfare

The chemical warfare between the host plant and pathogen occurs without much of our attention.  Differences in resistance to different pathogens among corn hybrids can be visible and we attempt to characterize these differences but the cellular interactions have required careful lab studies.  Plants preserve energy by delaying the pathogen defense until the pathogen has invaded.  With fungi, the initial reaction is to a common component of nearly all fungal cell walls (chitin).   With that detection, signal hormones, such as salicylic acid is produced.  The fungus produces enzymes to attack the host cells, as the signal hormones activate the resistance genes to produce the proteins to limit the fungus.
 
Most corn pathogens feed on the dead cell tissue, even after the progress of the pathogen in the leaf tissue is stopped.  From the limited, dead tissue the fungus produces spores and spreads to fresh leaf tissue on the same or different plants.
 
A few corn pathogens, however, can only reproduce when feeding on living cells.  Smuts and rusts are these sort of pathogens that are called biotrophs.  These fungi invade living cells without killing the cells, while feeding on the cell and then spreading to adjacent living cells.  Resistance to this type of pathogen can involve a single gene system in which the host plant detects the presence of this type of pathogen and then produces the signal molecule at such concentrations that the host cell dies and, consequently, so does the pathogen, stopping the spread of the pathogen to adjacent cells.  This resistance system is generally inherited by a single gene so genetic diversity in the fungal population often includes single gene mutant variants (races) that overcome this sort of resistance system.  Common smut fungus (Ustilago maydis) includes a race that produces an enzyme that digests the signal molecule salicylic acid before it can cause the host cell to die and therefore the fungus now can spread to adjacent cells. Single gene resistance to rusts for many crops, including corn, are commonly overcome by single gene differences in the rust pathogen that suddenly makes a variety susceptible. Genetic diversity works for both parties.