Signaling in plant-microbe interaction

Analysis of viral and bacterial pathogenesis has revealed common themes in the ways in which plants and animals respond to pathogenic agents. Pathogenic bacteria use macromolecule delivery systems (types III and IV) to deliver microbial avirulence proteins and transfer DNA-protein complexes directly into plant cells. The molecular events that constitute critical steps of plant-pathogen interactions seem to involve ligand-receptor mechanisms for pathogen recognition and the induction of signal transduction pathways in the plant that lead to defense responses. Unraveling the molecular basis of disease resistance pathways has laid a foundation for the rational design of crop protection strategies. Since the onset of civilization, plant dis-eases have had catastrophic effects on crops and the well-being of human popu-lations. For example, the fungus Phytoph-thora infestans caused the epidemic that triggered the Irish potato famine of the 1840s. Infectious plant diseases continue to cause human suffering and enormous economic losses. An increasing human population and decreasing amounts of land available for agriculture make all ap-proaches to securing the world food supply critical. Protection of crops from disease can substantially improve agricultural pro-duction. Although pesticides have suc-cessfully controlled disease, their contin-ued and increasing use will have harmful effects on our health and the environ-ment. Use of high-yield crop varieties can also improve productivity, but carries a risk—such genetically uniform varieties cultivated over enormous areas are suscep-tible to devastating epidemics. Thoughtful application of the plant’s own defense mechanisms, combined with understand-ing of the complex ecology of real-world disease processes, can lead to more effec-tive protection against plant pathogens. In this review we analyze plant pathogen in-teractions, including microbial strategies for pathogenesis and key elements of host responses (Figs. 1 and 2). We focus on studies using the model plant Arabidopsis thaliana. Symbiotic interactions such as that of legume and Rhizobium are related and have been reviewed elsewhere (1, 2)