CELL BIOLOGY OF EARLY EVENTS IN THE PLANT-RESISTANCE RESPONSE TO INFECTION BY PATHOGENIC FUNGI

In addition to passive (or constitutive) defence mechanisms, plants ha ve evolved a range of active (or inducible) responses that occur rapid ly on infection with an incompatible (avirulent) pathogen and that are thought to play a major role in the expression of resistance. These d efence reactions are only induced if the plant possesses the ability t o recognize and respond to the pathogen. Signal reception by the host must initiate a cascade of events that lead to the expression of resis tance. Some resistance responses, such as callose deposition, do not r equire the expression of new genes. Many responses, for example the sy nthesis and secretion of toxic compounds or molecules that enhance the strength of physical barriers, result from changes in the pattern of gene transcription. Other defence phenomena include hypersensitive cel l collapse, intercellular signalling, and the induction of defence gen e transcripts in surrounding cells. Changes in cell biochemistry and p hysiology are accompanied by characteristic structural modifications i n the infected cells, such as the redeployment of selected organelles and dramatic modifications of the host cell wall. Recent evidence indi cates that microtubules and microfilaments of the plant cytoskeleton f acilitate the rapid localization of these and other plant defence resp onses to the region of infection.