During the course of stigma development in sunflower (bud, staminate and pistillate stages), correlation is evident among the accumulation of reactive oxygen species (ROS), nitric oxide (NO), and the activities of ROS scavenging enzymes [superoxide dismutase (SOD) and peroxidase (POD)]. Confocal image analysis shows a gradual increase in ROS and NO accumulation in the stigmatic papillae, which may provide immunity to the developing stigma and function as signalling molecules. A novel, NO-specific probe (MNIP-Cu) has been employed for the detection and quantification of intracellular NO. Mn-SOD (mitochondrial) and Cu/Zn-SOD (cytoplasmic) exhibit differential expression during the staminate stage of stigma development. An increase in total SOD activity at the staminate stage is followed by a peak of POD activity during pistillate stage, thereby indicating the sequential action of the two enzymes in scavenging ROS. An increase in the number of POD isoforms is observed with the passage of stigma development (from three at bud stage to seven at pistillate stage), and two POD isoforms are unique to pistillate stage, thereby highlighting their role in ROS scavenging mechanism. ROS and NO accumulation exhibit reverse trends during pollen–stigma interaction.
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