The mechanics of plant sex
Besides being pretty, flowers contain the plant’s reproductive organs: the male gametes are produced in the stamens, and female gametes are found in the carpel, topped by a style. To produce pollen, a ‘mother cell’ goes through a round of meiosis, followed by a round of mitosis. That means that each pollen cell contains two sperm cells and a vegetative cell.
As for the females, seven cells are found in the ovule: an egg cell, two synergid cells involved in signalling, a large central cell that nourishes the developing embryo, and three antipodal cells that eventually disintegrate.
The magic happens once the pollen grain lands on the style. That’s also when cell-cell communication starts.
LURE-ing them in
The sperm cells are carried to the ovule through the pollen tube. The pollen tube develops from the vegetative cell; guidance of the pollen tube to the ovule relies on the secretion of protein signals.
A group of cysteine-rich peptides, called LUREs, are the most well-known signalling proteins, known to attract compatible pollen tubes. With the help of these (and other) signals, the pollen tube eventually penetrates the ovule, with one sperm cell fertilizing the egg cell, and the second fertilizing the central cell.
While the mechanics of plant sex have been known for a long time, the proteins involved in male-female communication have remained a mystery until quite recently. Two recent studies, published in the Journal of Proteome Research and Genome Biology, have undressed flowering plant ovules and pollen tubes, respectively, to take a peek at the courting process in flowering plants.
Ovules before brovules
To study the proteins secreted by the ovule, a team of researchers from Université de Montréal played matchmaker with a few potato plants, then dissected the carpels to get to the ovules. The ovules also secrete a sugar-filled fluid that creates an optimal microenvironment for pollen tube growth, but the proteins found in this fluid had never been characterized.
They found 305 unique proteins in the ovary secretome, the bulk of which are involved in metabolic, cell growth, and stimulus response processes.
A comparative proteomic analysis found that 128 of these proteins are involved in pollen-ovule interactions. The majority of these are also not found at the mRNA level.
Almost one-third of the ovule-secreted proteins don’t contain the traditional signal peptides, showing the importance of the unconventional protein secretion pathway in plant reproduction. Liu et al. suggest that the sugary fluid secreted by ovules helps to regulate the expression of these unconventionally secreted proteins.
As the pollen tube develops, it also secretes a number of proteins. Hafidh and colleagues (from the Czech Academy of Sciences) recently published the first pollen tube secretome in Genome Biology. The authors identified 801 proteins secreted by the pollen tube after it penetrates through the female reproductive tissues. More than half of these proteins are secreted unconventionally, and the authors show this process at work for the first time.
It’s still unknown why the pollen tube would secrete its own proteins, but the authors suggest that the secretome could be involved in perceiving guidance signals released by the ovule.
In both of these articles, a large portion of proteins secreted during plant sex are subject to post-transcriptional and post-translational modifications, showing that the widely-applied gene expression studies only show a very limited picture.
However, until very recently, technical limitations kept researchers from being able to fully study the ovule and pollen tube secretomes in a way that closely mimics what’s really happening. The development of a semi-in vivo approach is paving the way for our understanding of the communication involved in flowering plant reproduction.