Durotropic growth of pollen tubes.

To reach the female gametophyte, growing pollen tubes must penetrate different tissues within the pistil, the female reproductive organ of a flower. Past research has identified various chemotropic cues that guide pollen tubes through the transmitting tract of the pistil, which represents the longest segment of its growth path. In addition, physical mechanisms also play a role in pollen tube guidance; however, these processes remain poorly understood. Here we show that pollen tubes from plants with solid transmitting tracts actively respond to the stiffness of the environment. We found that pollen tubes fromNicotiana tabacumand other plant species with a solid or semisolid transmitting tract increase their growth rate in response to an increasing matrix stiffness. By contrast, pollen tubes fromLilium longiflorumand other plant species with a hollow transmitting tract decrease their growth rate with increasing matrix stiffness, even though the forces needed to maintain a constant growth rate remain far below the maximum penetration force these pollen tubes are able to generate. Moreover, when confronted with a transition from a softer to a stiffer matrix, pollen tubes fromN. tabacumdisplay a greater ability to penetrate into a stiffer matrix compared with pollen tubes fromL. longiflorum,even though the maximum force generated by pollen tubes fromN. tabacum(11 mu N) is smaller than the maximum force generated by pollen tubes fromL. longiflorum(36 mu N). These findings demonstrate a mechano-sensitive growth behavior, termed here durotropic growth, that is only expressed in pollen tubes from plants with a solid or semisolid transmitting tract and thus may contribute to an effective pollen tube guidance within the pistil. Pollen tube growth in the style is guided by an active mechanoresponsive system that is sensitive to the stiffness and resistance of the growth path.