Whither chemotropism and pollen tube guidance?

Pollen tubes follow a well-defined path to deliver male gametes to female gametes, but the mechanisms they use to locate this path are poorly understood. The major hypothesis is (and long has been) that pollen tubes are guided by chemical gradients and/or physical structures. Recently, parallels have been drawn between chemical mechanisms of guidance in pollen tubes and other cells, such as axons. These comparisons highlight a problem with the current models for pollen tube guidance, namely the distance over which chemical guidance is proposed to occur. Based on this new perspective, some models are either invalid or pollen tubes are uniquely responsive to chemical guidance cues.