A joint study conducted by researchers at University of Montreal and Concordia University has found that tightening the grip around the delivery organ can trigger the release of sperm cells.
Concordia's nanobiotech team devised a microchip that enabled the University of Montreal biologists to observe what happened when pollen tubes - the sperm delivery tools used by plants - tried to negotiate a microscopic obstacle course. The pollen tubes were exposed to a series of narrow, elastic openings resulting in a variety of cellular responses. When the opening was too narrow or tight, pollen tube growth stalled. However, the elongating tubes successfully penetrated slightly larger openings. Curiously, the pollen tubes burst and released the sperm cells when passing openings of a particular size relative to the pollen tube width.
The microchip was designed to imitate the mechanical challenges that the female flower tissues place in the path of the rapidly growing pollen tube on its way to the egg cell. Unlike its human counterpart, a microscopic single-cell organ undertakes sperm delivery in plants: a cylindrical protuberance formed by the male gametophyte, the pollen grain. "Similarly to elongated human cells such as neurons, the pollen tubes are tip growing cells that invade other tissues, in this case those of the female flower organs. Unlike those found in humans or other animals, the invasive ability of tip growing cells in plants remain largely unexplored. Our goal was to address this lack of knowledge using pollen tubes, whose invasive life style is the fundamental underpinning of sexual reproduction in flowering plants," said senior co-author Anja Geitmann of the University of Montreal. "Since they are encased in a stiff cellular envelope, plant cells grow and invade differently from animal cells," explained Concordia University senior co-author Muthukumaran Packirisamy. "From a mechanical point of view, the process of pollen tube elongation is similar to that of a balloon catheter used for angioplasty - forces are generated based on the principle of a hydroskeleton, or fluid under pressure. We designed microchannels through which the pollen tubes had to forcefully squeeze in order to continue their elongation."