The Hidden Dance of Plant Fertilization

In the flowering plant world, reproduction means an intricate succession of events. It begins when a pollen grain that carries the sperm cells lands on the top of the pistil, the female reproductive structure of a flower. The pollen grain then germinates into a tube that guides the male reproductive cells to the depths of the ovary, picking up signals along the way that reveal the location of an ovule. Blocking signals prevent polytubey—the fertilization of an ovule by more than one pollen tube—to maximize offspring formation.¹  

Yoko Mizuta, a plant biologist at the Nagoya University, became fascinated by this attraction-repulsion dance over a decade ago. Yet the available imaging techniques did not allow Mizuta to visualize pollen tube guidance over time. Determined to solve this problem, Mizuta and her colleagues developed a new method for live imaging pollen tube dynamics in plant ovaries.² “With this live imaging technique, we can analyze each pollen tube journey to the ovule,” she said.  

The team hand-pollinated Arabidopsis thaliana plants using a mixture of fluorescently labeled pollen grains. Using a tissue clearing method that they previously developed, the researchers turned the carefully-dissected pistils transparent while preserving the fluorescent labels in the pollen grains and ovary tissue.³ Using time-lapse imaging microscopy, they captured snapshots of the structure, which were stacked for 3D reconstruction.

After years of optimization, the researchers obtained images that reveal the complexity of pollen tube guidance. As shown above, fluorescently tagged pollen grains elongate their pollen tubes and form a colorful bundle that penetrates the ovary in search of one of the hanging ovules. Using their method, the team discovered that polytubey blocking first employs weak repulsion signals, followed by strong ones.² Next, Mizuta hopes to identify the molecular mediator of this repelling system, unveiling another mystery of plant fertilization.