Gene-for-gene interactions: bacterial avirulence proteins specify plant disease resistance

Resistance of plants to bacterial pathogens is often controlled by correspo nding genes for resistance and avirulence in host and pathogen, respectivel y. Fifty years after discovery of the genetic basis of gene-for-gene intera ctions, several avirulence and plant resistance genes have been isolated an d are being studied on the molecular level. Tremendous progress has been ma de due to a better understanding of type III secretion systems that are req uired for bacterial pathogenicity. We are beginning to grasp how the plant actually recognizes bacterial avirulence determinants. The current view is that the bacterium translocates avirulence proteins into the host cell by t he Hrp type III secretion system and that recognition occurs in the plant c ell.