Pathogenic infection and the oxidative defences in plant apoplast

The structural and functional continuum of the plant apoplast is the first site of contact with a pathogen and plays a crucial role in initiation and coordination of many defence responses. In this paper, we present an overvi ew of the involvement of the plant apoplast in plant-pathogen interactions. The process of infection of French bean (Phaseolus vulgaris L.) plants by Colletotrichum linde-muthianum is analysed. The ultrastructural features of plant defence responses to fungal infection are then compared with those o bserved in plants or cell suspensions treated with various elicitors. Chang es in cell walls and in whole plant calls responding to infection seem to b e highly similar in all systems used. Model systems of French bean and whit e lupin (Lupinus albus L.) are then utilised to provide some biochemical ch aracteristics of oxidative reactions in the apoplast evoked by elicitor tre atment. The species specificity of various mechanisms generating reactive o xygen species is discussed, and some details of pH-dependent H2O2-generatin g activity of peroxidases are demonstrated. As its exocellular nature is an important feature of the oxidative burst, the major consequence of this ev ent, i.e. the oxidative cross-linking of wall components during the papilla formation and strengthening of the walls, is analysed. Finally, the possib le involvement of other wall-associated and developmentally regulated H2O2- generating mechanisms, like amine and oxalate oxidases, in plant defence is demonstrated. It is concluded that under stress conditions, such apoplasti c mechanisms might be employed to increase plants' chances of survival.