The protection of Pseudomonas aeruginosa wheat seeds treated against powdery mildew and leaf blight correlates with up-regulated expression of a subtilisin-like gene in leaves

Here we present data on growth and a novel aspect of defence behaviour of wheat (Triticum aestivum, cv. Nkwazi) in response to seed treatment with the non-pathogenic root-colonizing bacterium Pseudomonas aeruginosa strain 7NSK2. Such treatment resulted in bacterial colonization (bacterization) of the wheat root. Bacterization did not adversely affect mean plant height, spike length or seed number, neither did it reduce total biomass of shoots, roots or seeds. However, when plants were challenged with Blumeria graminis f. sp. tritici or Chochliobolus sativus, the number of resulting lesions was significantly less in pre-treated than in untreated plants. Bacterization also prevented pathogen-induced total biomass reduction, reduction of crown and seed biomass and total seed number. A molecular investigation of the mechanisms that underlay these phenomena indicated that, in contrast to benzothiadiazole (BTH) treatment, both control and bacterized plants showed only low constitutive expression of the WCI-2 gene that is diagnostic for a systemic acquired response (SAR). Furthermore, differential display of RT-PCR products of leaf RNA from bacterized and unbacterized plants identified a gene encoding a subtilisin-like protease that was up-regulated in bacterized plants only. Since subtilisin-like proteases are induced both in compatible and incompatible interactions, we argue that the beneficial systemic response to root bacterization must share some similarity with plant defence responses and in particular induced systemic resistance.