Since Arabidopsis flowers do not produce a stigmatic lipid exudate, comparisons between the functions in tobacco and Arabidopsis flowers are difficult. 9, 10 The GRI protein has a signal peptide for secretion to the apoplast, and GRI-YFP localize partly to the apoplast. 3 The GRI gene encodes an Arabidopsis orthologue of the tobacco Stig1 gene which encodes a small protein secreted in stigmatic lipid exudates. One line with an O 3-sensitive phenotype was subsequently designated grim reaper ( gri) and characterized in more detail. For genes with fast responses to O 3 (within 30 min to 2 hours) the corresponding knock-out mutants were exposed to O 3 and screened for sensitivity. To identify novel components in ROS signaling in Arabidopsis thaliana we performed microarray gene expression profiling experiments to identify genes regulated by O 3. 2, 8 Detailed elucidation of how plants are able to use ROS for those opposing processes with such precision will answer many questions about signaling and signal integration during plant stress responses. 4, 5 The data available supports dual roles for ROS in lesion formation and spread as well as in the subsequent containment of cell death. However, the precise role of ROS during these processes is not well understood.
ROS are critical to the regulation of programmed cell death. Redox regulation is known for only few plant proteins, exemplified by heat-shock transcription factors (HSFs) 6 and NPR1, which confers redox regulation to the transcription factor TGA1 in response to salicylic acid (SA). 5 While it is clear that plants are able to distinguish different ROS with regard to type, timing and subcellular localization of ROS produced, the mechanisms of ROS perception are still largely unknown.
For example, pathogens and ozone induce ROS production in the apoplast while high light or the herbicide methyl viologen induce ROS production in the chloroplast.
4 ROS are produced in different subcellular compartments and play far more complex roles than acting simply as cytotoxic compounds. Reactive oxygen species (ROS) have been implicated in the response to many biotic and abiotic stresses for example during pathogen attack, osmotic stress, excess light, wounding and ozone (O 3).
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