Sexual frustration: programmed cell death prevents plant inbreeding

Scientists have demonstrated the importance of programmed cell death in preventing inbreeding in plants, according to research published in Nature today. Researchers at the University of Birmingham School of Biosciences have found that self-incompatibility, an important mechanism in plants that prevents them fertilizing themselves with their own pollen, which is genetically controlled by products of the S locus, triggers programmed cell death in incompatible pollen coming into contact with the stigma.

The work, funded by the Biotechnology and Biological Sciences Research Council (BBSRC), was carried out on the field poppy, Papaver rhoeas and has shown the involvement of programmed cell death (PCD) in plant self-incompatibility for the first time. PCD is a mechanism used by many organisms to destroy unwanted cells in a precise and regulated manner and in various forms it plays a crucial role determining development in many things from embryos to tumours.

The researchers, using methods common in the study of animal cells but not widely used with plant cells, found that S proteins encoded by the stigma component of the S locus interacted with incompatible pollen to inhibit pollen tube growth and to trigger PCD. It has been known that plants use PCD to fight disease but it had not been proven that it was involved in self-incompatibility.

“Many researchers have been studying self-incompatibility in order to try to establish the mechanisms that plants use for this type of selective ‘contraception’ against unwanted ‘self’ or genetically identical pollen,’ says Dr Noni Franklin-Tong, who led the research group. ‘Our study, which provides the first demonstration of a self-incompatibility system using Programmed Cell Death, is a significant advance in our understanding of how plants inhibit ‘self’ pollen. By recognizing ‘self’ pollen and then causing it to commit ‘suicide’, this SI mechanism we have discovered in poppy provides a highly novel way to prevent self-fertilization.”

Most notably, the research provides evidence that a “caspase-like” activity is involved. Although caspases are known to be key enzymes involved in programmed cell death in animal cells this is a contentious finding because no caspase sequence has been identified in the Arabidopsis genome. Although it does not prove there is a caspase in plants, it does suggest a gene encoding a protein with a similar activity exists in plants.