Signals controlling the expression of cytosolic ascorbate peroxidase during pathogen-induced programmed cell death in tobacco

In plants ascorbate peroxidase (APX) is an important H2O2-detoxifying enzym e. The expression of APX is rapidly induced in response to stresses that re sult in the accumulation of reactive oxygen species (ROS). We have recently reported that the steady-state level of transcripts encoding cytosolic APX (cAPX) is dramatically induced during the hypersensitive response (HR) of tobacco plants infected with tobacco mosaic virus (TMV). Because cAPX expre ssion is closely linked to the production of ROS in plant cells, studying t he regulation cAPX mRNA can reveal some of the signal transduction events a ssociated with ROS metabolism during the HR. Analysis of cAPX mRNA inductio n during the HR suggested that the expression of cAPX is under the control of the HR signal transduction pathway. The activation of cAPX expression fo llowed signaling events such as changes in protein phosphorylation and indu ction of ion fluxes. Expression of cAPX was suppressed under conditions of low oxygen pressure, and could only be mimicked by enhancing the intracellu lar generation of ROS. Interestingly, salicylic acid (SA), which is thought to be involved in ROS metabolism during the HR, did not affect the inducti on of cAPX mRNA during TMV-induced KR. Using cAPX expression as a marker fo r the production of ROS, it is suggested that SA may not be involved in the formation of ROS during the HR of tobacco to TMV, and that ROS may not be involved in the induction of the pathogenesis-related protein, PR-1, during this process.