Agrobacterium induces plant cell death in wheat (Triticum aestivum L.)

The symptoms of gall or hairy root do not occur in the interactions between wheat (Triticum aestivum L.) and other monocotyledonous plants, with Agrobacterium tumefaciens or Agrobacterium rhizogenes. However, both bacteria colonized wheat root surfaces at similar levels (2.0×107 colony forming Ug−1 root) and grew without inhibition in suspension with intact or wounded wheat embryos or root segments present. Suspension-cultured wheat embryo cells grown in 7.4m M O2 displayed 23% cell death after 1h exposure to Agrobacterium cells, while the extent of cell death with 2.1m M O2 averaged 8%. Cultured wheat embryo and root cells rapidly produced hydrogen peroxide (H2O2) when contacted with A. tumefaciens or A. rhizogenes. Production of H2O2 was lower at 2.1m M O2 than 7.4mM O2. Browning and autofluorescence of epidermal cells of callus derived from wheat embryos and wheat roots was observed after inoculation with Agrobacterium. An increase in ferulic acid was detected in the walls of roots exposed to Agrobacterium. However, neither lignin nor callose was detected by diagnostic staining methods. These findings suggest that Agrobacterium induced a resistance-like response in wheat that may reduce the efficacy of transformation and limit the normal symptom formation.