Abstract
Plants possess different mechanisms to defend themselves against the continuous exposure to pathogenic attacks. The most elaborate defence responses are those that involve the activation of several specific antimicrobial reactions once the pathogen is detected. Thus, detection of a pathogen's component through plant receptors will unleash a defence response that will ultimately stop the pathogen spreading. Plants generally react to necrotrophic pathogens through the activation of jasmonic acid (JA)-dependent defence pathways, whereas defence responses to biotrophic pathogens are salicylic acid (SA)-dependent. Another plant immune pathway has been shown against viruses, in which the plant activates a virus-RNA degrading mechanism (RNA silencing) that is independent of the presence of receptors. To better understand the plant immune system, it is important to know how each of the previous defence mechanisms interacts with the other ones. Thus, the SA-dependent pathway regulates JA-dependent defence pathways and vice versa. Also, recent studies showed that detection of a pathogen through a plant receptor can induce RNA silencing of certain plant genes. This chapter will discuss the cross talk among some of the known plant defence pathways.