The Active Oxygen Response of Cell Suspensions to Incompatible Bacteria Is Not Sufficient to Cause Hypersensitive Cell Death

The inoculation of tobacco (Nicotiana tabacum L.) suspension cells with bacterial pathogens that elicit the hypersensitive response (HR) in leaves has been shown to elicit production of active oxygen. This response occurs in two phases, the second of which occurs 1 to 3 h after bacterial addition and is unique to HR-causing interactions. The relationship between the phase II active oxygen response and the HR was characterized using Pseudomonas syringae pv syringae and P. fluorescens (pHIR11), which contains a cosmid clone of the hrp/hrm region from P. syringae pv syringae. TnphoA mutations in complementation groups II through XIII of the hrp cluster blocked the phase II active oxygen response, whereas mutations in the group I hrmA locus did not affect phase II. Despite the normal active oxygen response, bacteria with mutations in the hrmA region did not cause the HR in intact tobacco leaves nor did they induce hypersensitive cell death in cell suspensions. The data indicate that the bacteria do not require the hrmA region to elicit active oxygen production, but a full and intact hrp/hrm region is required to elicit hypersensitive cell death. Therefore, the phase II active oxygen response does not directly cause hypersensitive cell death nor is the response itself sufficient to trigger the HR.