Two genes; PEN2 and PEN3 are also necessary for resistance to pathogens and are required for both callose deposition and GSL activation suggesting that the pathogen-triggered callose response is required for resistance to microbial pathogens. The study shows that well- studied plant metabolites previously identified as important in avoiding damage by herbivores are also required as a component of the plant defense response against microbial pathogens (Clay et al. 2009)