Researchers unravel mechanism of plant fertilization

An international group of researchers has unraveled the mechanism by which the cell that grows from the pollen, called the pollen tube, increases by up to a thousand times in size to reach the ovules of the flowers. Growth depends on the entry and exit of protons, which generates electrical activity in the cell membrane that makes it grow. The discovery paves the way for understanding everything from seed production in plants to the growth of fungi and neurons.

The work, supported by FAPESP, was published na Nature Communications. by researchers from Brazil, Denmark, Portugal and the United States.

“A pollen grain is made up of a single cell. When it comes into contact with the female sexual organ on the surface of the flower, it grows at a very high rate, forming what we call a pollen tube, until it reaches the base of the flower's ovary and releases the sperm cells. How this happened was something very little understood until then,” he explains. Maria Teresa Portes, who carried out the research during his postdoctoral studies at the University of Maryland, in the United States.

The growth of the pollen tube has always intrigued researchers because of its growth rate of up to a thousand times its original size, the highest recorded in living beings. The species used in the study was the Arabidopsis thaliana, herbaceous native to Europe and Asia widely used as an experimental model in research. From the same mustard family, its pollen tube grows up to three millimeters a day.

In the laboratory, the researchers produced mutant varieties of the plant, in which some genes are modified, and discovered that the inactivation of three genes at the same time, from a group known as AHA, causes the pollen tube to grow very little.

In the mutant plants, only the ovules closest to the surface were fertilized, causing them to produce the equivalent of only 5% of the seeds that the normal plant produces.

Using a series of experiments, the researchers observed that the proteins expressed by these genes work like “proton pumps”, placing or removing these electrical particles from the environment and making the pollen tube more or less acidic. The consequence is the generation of an electrical activity in the cell membrane, which causes it to grow towards the bottom of the ovary of the flower.

“We wanted to understand how the cell manages to organize this growth process. Then we observe that there is an ionic gradient, in which the distribution of protons is not homogeneous. There are more of them at the tip of the tube and fewer along the cell body. In addition, there are gradients of other elements, such as calcium and actin”, says Daniel Santa Cruz Damineli, another co-author of the study, who is undertaking a post-doctoral internship at the Faculty of Medicine of the University of São Paulo (FM-USP) with bag of FAPESP.

From seeds to neurons

Among the possible applications, the study paves the way for a deeper understanding of how seed production occurs. With that, in theory, in the future it could create improved varieties of plants that serve as food, such as legumes and cereals.

“It is not fully known how the pollen tube is guided and how communication between male and female occurs in plants. Therefore, this is a great topic of study, which culminates in seed production. The development of plants necessarily goes through this mechanism, which we are now beginning to understand better”, says Portes.

More broadly, however, the discovery provides evidence for understanding other cells that have so-called apical growth (at the tip), such as fungi and neurons, which is compromised in cancer cells.

“These are biologically poorly understood phenomena, in the sense of how growth is orchestrated. Now we can study them better”, says Damineli.