A pollen tube can grow autonomously to some extent using its own nutrients; however, the resultant pollen tube is much shorter than the style. Pollen tubes perceive many molecules in the extra-cellular matrix (ECM) of the pistil, and no medium can produce pollen tube growth similar to that which occurs in the pistil. Thus, a semi-in vitro (also called semi-in vivo) system is sometimes used for pollen tube culture, wherein pollen tubes grow through a cut style. Pollen tubes germinate on the stigma, grow through the cut style, and enter the culture medium from the cut end of the style. Pollen tubes grown semi-in vitro show a higher growth rate and more normal morphology than those germinated on artificial medium. For example, in Torenia at 25 °C, pollen tubes grow at 2.3 mm/h in the pistil (in vivo), 0.6 mm/h in medium after germinating on the medium (in vitro), and 1.2 mm/h in the same medium after germinating on the stigma and passing through the style (semi-in vitro). Moreover, semi-in vitro growth is necessary for the capacitation-like mechanism of the pollen tube to respond to attractant from the synergid cell in Torenia (Higashiyama et al. 1998). Similar phenomena have been observed in lily (Janson 1993) and Aechmea fasciata (Bromeliaceae; Vervaeke et al. 2003); pollen tubes grown semi-in vitro more frequently penetrate the micropyle of the ovule than those grown in vitro. Among the biological models, semi-in vitro pollen tube growth has also been used in tobacco (Cheung et al. 1995), as described in (Johnson and Lord, this volume).
For semi-in vitro pollen tube growth, the medium must be prepared as carefully as that for in vitro pollen tube culture. It is important that the style tissue, including the cut end, is well-maintained on the medium so that pollen tube growth inside the tissue is supported. Any self-incompatibility of the plant species should also be noted before semi-in vitro culture is performed.
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