Ultrastructural analysis of the self-incompatibility response in Papaver rhoeas pollen reveal early and striking alterations to organelles

Self-incompatibility (SI) in Papaver rhoeas is accompanied by a cascade of signalling events that result in the rapid arrest and eventual death of the pollen tube. We have used rapid freeze fixation, freeze substitution and transmission electron microscopy to provide the first description of changes to pollen at the ultrastructural level during SI in this species. Our studies reveal that dramatic alterations to the morphology of mitochondria, Golgi bodies and ER occur within 1 h of SI induction. Similar symptoms have also been observed during programmed cell death (PCD) in some cell types. These include: the conspicuous condensation of the vegetative and generative nuclei, the swelling and loss of cristae in mitochondria and the disappearance of Golgi bodies. Some of the early alterations to the mitochondria and Golgi bodies observed at 1 h, almost certainly occur when cells are still alive. Other events, such as nuclear condensation, occur later and coincide with DNA fragmentation and the loss of cell viability. Our observations suggest that the SI response in P. rhoeas pollen may potentially involve a type of PCD.