role of stigma peroxidases in flowering plants: insights from further characterization of a stigma-specific peroxidase (SSP) from Senecio squalidus (Asteraceae)

Angiosperm stigmas have long been known to exhibit high levels of peroxidase activity when they are mature and most receptive to pollen but the biological function of stigma peroxidases is not known. A novel stigma-specific class III peroxidase gene, SSP (stigma-specific peroxidase) expressed exclusively in the stigmas of Senecio squalidus L. (Asteraceae) has recently been identified. Expression of SSP is confined to the specialized secretory cells (papillae) that compose the stigma epidermis. The literature on stigma peroxidases and hypotheses on their function(s) is reviewed here before further characterization of SSP and an attempt to determine its function are described. It is shown that SSP is localized to cytoplasmic regions of stigmatic papillae and also to the surface of these cells, possibly as a component of the pellicle, a thin layer of condensed protein typical of 'dry' stigmas. Enzyme assays on recombinant SSP showed it to be a peroxidase with a preference for diphenolic substrates (ABTS and TMB) and a pH optimum of ~4.5. In such assays the peroxidase activity of SSP was low when compared with horseradish peroxidase. To explore the function of SSP and other stigmatic peroxidases, levels of reactive oxygen species (ROS) in stigmas of S. squalidus were investigated. Relatively large amounts of ROS, principally H₂O₂, were detected in S. squalidus stigmas where most ROS/H₂O₂ was localized to the stigmatic papillae, the location of SSP. These observations are discussed in the context of possible functions for SSP, other peroxidases, and ROS in the stigmas of angiosperms.