The peri-germ cell membrane: poorly characterized but key interface for plant reproduction

Over the past 50 years, only a few studies have examined the peri-germ cell membrane. Nonetheless, it has been assigned multiple names. For example, the peri-germ cell membrane was identified in 1969 (ref. 1) as the ‘generative cell envelope’. Subsequent ultrastructural work in the 1980s led to new names, including ‘internal plasma membrane of the vegetative cell’, ‘inner plasma membrane of the pollen grain’ and ‘inner vegetative cell plasma membrane’2,3,4,5,6. Another study referred to it as the ‘pollen tube inner plasma membrane’ when considering the pollen tube7. When the Arabidopsis small GTPase RHO OF PLANTS 9 (AtROP9) was identified as the first protein reported to locate to this membrane, the authors named it the ‘invaginated pollen tube plasma membrane’8. More recently, when the maize NOT-LIKE-DAD protein (NLD; also known as MATRILINEAL (MTL) or PHOSPHOLIPASE-A1 (ZmPLA1)) was found to localize exclusively to the peri-germ cell membrane, the authors named it the ‘pollen endo-plasma membrane’9. This diverse nomenclature causes confusion and calls for the implementation of a consensus on terminology. For example, the term ‘generative cell envelope’ is too restrictive, as the peri-germ cell membrane also encloses the two sperm cells. Similarly, the use of ‘pollen tube’ in the nomenclature is not appropriate because this membrane is also present before pollen germination. Furthermore, using ‘plasma membrane’ could lead to confusion with the classical plasma membrane (PM) of the generative cell, sperm cells or vegetative cell. Additionally, the term plasma membrane is misleading, given that the peri-germ cell membrane may differ in protein and lipid composition from the classical PM.