The process of reproduction in flowering plants is well known for over 120 years. Yet, there exist many mysteries to uncover. Scientists from INRAE, ENS de Lyon, CNRS, and Limagrain distinguished a new membrane within the pollen grain that surrounds the two sperm cells.
Image of the NOT-LIKE-DAD protein on the membrane that encircles the two male sperm cells present in each pollen grain (image taken by confocal microscopy). Image Credit: National Research Institute for Agriculture, Food and the Environment.
The research published in the Journal of Cell Biology on July 29th, 2021, reveals that this membrane plays a vital role in ensuring that the male reproductive cells remain intact on the course of their journey towards the female flower, to eventually form viable seeds. These seeds are the primary food source for humankind and also a staple food for livestock. This basic understanding is beneficial for the creation of novel plant varieties.
The birth of a person begins with fertilization—the fusion of male and female cells. In contrast to animals, where a single fertilization event is mandatory for the formation of an embryo, flowering plants need a double fertilization event, where two male reproductive cells fuse separately and at the same time with two female reproductive cells.
Double fertilization is inevitable for the formation of viable seeds. These seeds supply the bulk of human’s plant-based diet and also feed for livestock.
However, in flowering plants, the two male sperm cells are not motile and hence rely on pollen grain to carry them to the maternal tissues embedded within the flower. In the current research, researchers from INRAE and their associates exposed a new membrane within the pollen grain that surrounds the two sperm cells, which is vital in ascertaining sperm cell integrity during their journey toward the female flower.
A key membrane that reveals its secrets, including the presence of a “NOT-LIKE-DAD” protein
The distinctive features of this membrane have been brought to light by microscopy technology and molecular and cellular biology techniques.
The first distinct feature of this membrane is enrichment for a particular negatively charged lipid. The second feature is the presence of a NOT-LIKE-DAD protein that has a stretch of positive charges, along with lipid anchors that enable NOT-LIKE-DAD to attach exclusively to this atypical membrane.
The scientists also demonstrated that this membrane surrounding the two sperm cells performs a vital role in a smooth double fertilization process, making sure that the two reproductive cells end up in the correct place at the correct time.
The discovery of a universal mechanism in the plant kingdom?
These discoveries enhance people’s knowledge about living organisms. Moreover, other specific membranes, those that surround some symbiotic and parasitic fungi, also seem to have a lipid signature like the one found in the pollen grain. This indicates that this kind of membrane might be a universal mechanism to delineate specific structures inside plant cells.
The specific localization of the NOT-LIKE-DAD protein on this specific membrane is the initial step in casting light on the mechanism by which the lack of this protein may result in degraded paternal chromosomes within the enclosed male reproductive cells. This phenomenon is regularly employed to create new varieties of maize and can also boost plant breeding for various other species.
Gilles, L. M., et al. (2021) Lipid anchoring and electrostatic interactions target NOT-LIKE-DAD to pollen endo-plasma membrane. Journal of Cell Biology. doi.org/10.1083/jcb.202010077.