THE SUCCESSFUL PICK-UP LINEHow Do Pollen Tubes and Ovule Get Together?

Goals and Purpose:

Fertility - and sterility - is important for everyone, including plants. How does the pollen tube in a plant find the ovule? How does it manage to be the successful pollen tube among the others? Following a gradient seems to be important for the pollen tube in the beginning, and ovules emit a diffusive attractant as a guide. We set out to figure out how the pollen tube and ovule are really communicating. Understanding these interactions might aid crop yields by increasing successful fertilization, or could help us ensure sterility in potentially invasive crops.

Big Questions:

The turning isn't random - the random components of the pollen tubes' motion is the same if ovules are present or not.

The turns pollen tubes make are well-described by a model where they respond to a gradient ina chemical emitted by the ovule. When the ovule had more time to emit the attractant, pollen tubes more frequently moved closer to the ovule. When the pollen tubes were closer to the ovules, this wasn't as noticeably because of the initial orientation in vitro. After getting within about 50 micron of the ovule, the pollen tubes grow more directly toward the micropyle (fig 1c), and cut their speed in half (fig 2). In simulations, reducing the growth rate increased the pollen tubes' ability to target ovule (fig 2c).

Shannon Stewman, Matthew Jones-Rhoades, Prabhakar Bhimalapuram, Martin Tchernookov, Daphne Preuss, Aaron R Dinner

*Figure 1 (fig 1) : Schematics of fertilization in vivo and in vitro. After a pollen grain contacts the stigma, it polarizes and develops a growing extension (the pollen tube) that traverses the pistil, eventually fertilizing an ovule by growing along its funiculus, entering through its micropyle, and releasing sperm cells into its embryo sac.

**Figure 2 (fig 2): Pollen tube formation…..

Figure 1* Figure 2**

Discoveries:

Pollen tubes seem to navigate by detecting changes in the attractant -- chemotrophically -- emitted continuously by the ovule from a distance - 100 to 150 microns away. When they get close, within about 50 micron, tube tips slow to make the turn into the ovule (probably also a response to the attractant's gradient), and the slowing enhances the ability of pollen tubes to successfully target the ovules (figs 1, 2).

How similar is this to navigation in vivo?

What molecules are actually providing the navigational cues, and how are they controlled in the plant?