Signals for local and systemic responses of plants to pathogen attack

Activation of plant defences following recognition of pathogen attack involves complex reiterative signal networks with extensive signal amplification and cross-talk. The results of two approaches that have been taken to analyse signalling in plant-microbe interactions are discussed here. Activation tagging with T-DNA harbouring multiple 35S enhancer elements was employed as a gain-of-function approach to dissect signalling related to bacterial pathogen resistance in Arabidopsis thaliana. From a screen of approximately 5000 activation tagged lines, one line was identified as harbouring a T-DNA leading to over-expression of an apoplastic aspartic protease (CDR-1), that resulted in resistance to normally virulent Pseudomonas syringae. The second approach was to screen for loss-of-function mutants in T-DNA tagged populations. From a screen of 11 000 lines, one line, defective in induced resistance-1 (dir-1) lost resistance to normally avirulent P. syringae. Models for action of the products of the CDR-1 and DIR-1 genes suggest involvement of peptide and lipid signals in systemic disease resistance responses in A. thaliana.