The pattern of systemic acquired resistance induction within the Arabidopsis rosette in relation to the pattern of translocation

Local leaf infections by a necrogenic pathogen can lead to systemic acquired resistance (SAR) in untreated leaves. We reasoned that, whatever the nature of the long-distance signal, if it is transported in the phloem, the pattern of SAR induced within the plant by treatment of a single leaf should match the pattern of translocation out of that leaf. The source-sink relationships (orthostichies) in the Arabidopsis rosette were established with [C-14]Suc or phloem-mobile 3-aminotriazole at herbicidal concentrations. SAR was activated by infiltrating a single Columbia leaf with Pseudomonas syringae pv maculicola DC3000/avrRPM1, which causes a hypersensitive response. The pattern of SAR in the rosette was monitored by assessing the growth of wild-type DC3000 and by measuring the SAR markers salicylic acid and PR1 transcripts. Although the orthostichy of a single leaf was clearly limited to a row of vertically aligned leaves, SAR and SAR markers were also found outside the orthostichy. This indicates that, whatever the nature of the long-distance signal from the treated leaf to the upper responding leaves, its transport is either not limited exclusively to the phloem or the minor proportion of translocate that is not confined to the orthostichy contains enough of the SAR systemic signal to set in motion events leading to the establishment of the SAR state in the upper leaves.