Betacyanin synthesis in red beet (Beta vulgaris) leaves induced by wounding and bacterial infiltration is preceded by an oxidative burst

Red beet (Beta vulgaris) accumulates betacyanins, mainly betanin, in the store root. We report here that in leaves, the synthesis of betacyanins is induced by wounding or infiltration with an avirulent Pseudomonas syringae pv. tabaci or Agrobacterium tumefaciens. A red-violet pigmentation characteristic of betacyanin bordered the damaged tissue in wounded leaves. P. syringae infiltration induced a hypersensitive response (HR) and betacyanins accumulated in the epidermis of the region surrounding the dry tissue. In contrast, A. tumefaciens infiltration did not induce the HR. The tissue maintained a turgid appearance and betacyanins accumulated in the epidermis were uniformly distributed in the infiltrated area. It was also found that infiltration with A. tumefaciens elicited extracellular production of reactive oxygen species (ROS) prior to the accumulation of betacyanins. Infiltration with an H ₂O ₂-generating system (glucose/glucose oxidase) also induced betacyanin synthesis, whereas, the infiltration with diphenylene iodonium chloride (DPI), an NADPH oxidase inhibitor, reduced the A. tumefaciens-induced betacyanin accumulation. We suggest that ROS are signals inducing the betacyanin synthesis, and that betacyanins may act as ROS scavengers, limiting damage caused by wounding and bacterial infiltration.