Pollen Tube Growth in Cross Combinations between Torenia fournieri and Fourteen Related Species

The precise guidance of pollen tubes is important for successful fertilization. Based on abnormalities in interspecific and intergeneric crosses, we investigated pollen tube guidance in Torenia fournieri, which is a model plant for studies on plant fertilization due to its protruding embryo sac. Critical abnormalities of pollen tube growth have never been recorded in interspecific crosses between either T. baillonii or T. concolor, and hybrids could be obtained. In contrast, abnormalities appeared in intergeneric crosses with twelve other species. The abnormalities could be observed for five phases of pollen tube guidance revealed in Arabidopsis mutants as follows: germination of the pollen grains, penetration of pollen tube through stigma cells, growth in the transmitting tract, arrival at the placenta and release of sperm into the micropyle. In the self-pollination of T. fournieri, in vivo, the pollen tubes grew straight from the placenta to the protruding embryo sac, omitting funiculus guidance. In addition, observation of an intergeneric cross between T. fournieri (female) and Mimulus hybridas revealed that 64% of the pollen tubes grew straight to the micropyles of ovules rather than to synergid cells, suggesting that the micropyle in T. fournieri transmits the attraction signal. In addition, the signal attracted pollen tubes of M. hybridas to the micropyle, unlike those of T. fournieri.