Researchers from Nagoya University have used a special microscopy technique to unlock the secrets of one-to-one pollen tube guidance.
Angiosperms, or flowering plants, sexually reproduce through pollination. This process involves transferring pollen grains from the anther of a flower’s stamen (the male reproductive part of a flower) to the stigma of a flower’s pistil (the reproductive part of a flower). Once pollen grains reach the stigma, they form tubes down the style toward the ovary. Then, the genetic material within the pollen grains travels down the tubes and fertilises the ovules within the ovary. The fertilized ovules develop into seeds, which can germinate and grow into new plants when planted.
However, did you know that there is more to this process than meets the eye? Angiosperms employ different mechanisms to prevent polyspermy, a phenomenon in which a single ovule gets fertilised by more than one pollen grain tube. Polyspermy can result in abnormal embryo development within seeds, which can compromise the viability of the offspring. To prevent this, some angiosperms, including thale cress (Arabidopsis thaliana), use a mechanism called one-to-one pollen tube guidance. This mechanism involves guiding pollen grain tubes inside the pistil to ensure each ovule gets fertilised only once.
To further investigate one-to-one pollen tube guidance, a group of researchers from Nagoya University in Japan has used two-photon imaging to directly observe it in a pollinated A. thaliana flower. Two-photon imaging is a microscopy technique used to visualise structures and processes within living tissues with high spatial resolution and minimal photodamage. This makes it ideal for studying delicate biological processes like pollen tube growth within the pistil.
The researchers found that ovules within the pistils of A. thaliana flowers secrete chemical signals during pollination. Initially, the ovules produce attraction signals to draw the tubes formed by pollen grains toward their direction. Then, once a pollen tube reaches an ovule, the ovule switches gears and sends out repulsion signals instead. This way, other pollen tubes on the way are diverted to unpaired ovules, preventing polyspermy.
“Our two-photon imaging revealed novel dynamics and spatiotemporal signaling in one-to-one pollen tube guidance. This study may accelerate the identification of signaling molecules driving this process in angiosperms, which exhibit a unique navigation system related to sexual reproduction,” said the researchers.
The full discussion of the study has been published in the peer-reviewed scientific journal EMBO Reports.