A gas, Viagra and sex in plants – researchers at the Instituto Gulbenkian de Ciência have found a link

Viagra affects growth of the male sex organ of plants, by intensifying the effect of nitric oxide during plant fertilization. This discovery, made by the Plant Development team at the Instituto Gulbenkian de Ciência (IGC), in Portugal, will be published in Development, in June. The study, led by José Feijó, takes a step further in understanding fertilization in plants, a complex process but an absolutely essential one for the survival and evolution of species.

Pollen grains, which contain the plants’ male gametes (sperm cells), are carried from the male organ of the flower (the stamen) to the female organ (the pistil). Here the pollen germinates and grows a pollen tube, which extends and is guided to the ovary, where it releases the sperm. The sperm fuse with the egg cells, giving rise to an embryo, part of the seed. For many years now, scientists have been trying to unravel the mechanisms that guide the pollen tube along the long route to reach the ovary.

The Plant Development group of the IGC, now shows, for the first time, that nitric oxide (NO), a well-known gas that animal cells use as a hormone, influences the speed and direction of growth of lily pollen tubes. Upon encountering a point source of NO, lily pollen tubes slow down, almost stop, make a 90 degree turn, and start growing again.

The IGC researchers bathed the pollen tubes in several enzyme inhibitors to identify the messenger molecule, inside the cell, that mediates the response to NO, sensed outside the cell. Of the inhibitors tested, only Viagra intensified the effect of NO, to the point where the pollen tubes made a 180 degree turn. Viagra is known to cause build up of cyclic GMP (cGMP) inside a cell; the researchers thus conclude that NO acts on the pollen tubes via this small messenger molecule.

These findings underscore how fundamental biological processes, such as fertilization, are conserved in their basic mechanisms, from plants to animals: Viagra, a drug that affects signalling inside the cell, has similar effects on male sex organs in animals and in plants.

There is general agreement amongst plant scientists that the pollen tube is guided by mechanical and chemical signals. The new findings of the IGC researchers take the search for a unifying theory of pollen tube guidance a step further. The scientists suggest that, in the plant, NO is released by the female organ and acts as a signpost to make the pollen tube change direction at the right place along the route.

According to José Feijó, there is still some way to go to fully understand how NO acts in plants. This gas is full of surprises, both in animals and in plants. In animals, amongst many other functions, NO controls blood pressure and acts as a messenger between cells of the nervous system. Nitroglycerine alleviates the symptoms of cardiac arrest because it causes cells to release NO.