Reactive Oxygen Species and Nitric Oxide Mediate Actin Reorganization and Programmed Cell Death in the Self-Incompatibility Response of Papaver

Pollen-pistil interactions are critical early events regulating pollination and fertilization. Self-incompatibility (SI) is an important mechanism to prevent self-fertilization and inbreeding in higher plants. Although data implicate the involvement of reactive oxygen species (ROS) and nitric oxide (NO) in pollen-pistil interactions and the regulation of pollen tube growth, there has been a lack of studies investigating ROS and NO signaling in pollen tubes in response to defined, physiologically relevant stimuli. We have used live-cell imaging to visualize ROS and NO in growing Papaver rhoeas pollen tubes using chloromethyl-2', 7'-dichlorodihydrofluorescein diacetate acetyl ester and 4-amino-5-methylamino-2', 7'-difluorofluorescein diacetate and demonstrate that SI induces relatively rapid and transient increases in ROS and NO, with each showing a distinctive "signature" within incompatible pollen tubes. Investigating how these signals integrate with the SI responses, we show that Ca²⁺ increases are upstream of ROS and NO. As ROS/NO scavengers alleviated both the formation of Si-induced actin punctate foci and also the activation of a DEVDase/caspase-3-like activity, this demonstrates that ROS and NO act upstream of these key SI markers and suggests that they signal to these SI events. These data represent, to our knowledge, the first steps in understanding ROS/NO signaling triggered by this receptor-ligand interaction in pollen tubes.