Mechanisms of plant defense
Plants defend themselves against pathogens and insects by several well-described mechanisms: i) innate (non-host) immunity; ii) localised race-specific resistance; iii) systemic resistance; iv) microbial-based mechanisms of defense (biological control). Microbial-based defense is especially important because plants lack genetic resistance to some of the most common pathogens and insects, especially organisms that are soilborne. For example in wheat production, the diseases Pythium root rot, Rhizoctonia root rot and Take-all cause billions of dollars in losses annually, yet no commercial variety has resistance. Thus, microbial-based mechanisms serve as the first line of defense against these and other diseases and insects. These mechanisms are modulated by the plant through processes of leaf exudation and rhizodeposition, which stimulate and support specific groups of antagonist microbes (Weller et al., 2007). Pathogen or insect suppression by antagonistic micro-organisms occurs through the mechanisms of competition/pre-emptive exclusion, parasitism/predation, induction of systemic resistance, and/or antibiosis/toxin production. Multiple mechanisms of antagonism can operate simultaneously, and in addition, a micro-organism may both suppress pathogens and/or insects and directly stimulate plant growth by enhancing the uptake of nutrients, producing phytohormones and/or degrading ethylene.