Department of Chemistry, The University of Chicago, 929 E 57th St, Chicago, IL 60637, USA.
BMC Plant Biol. 2010 Feb 22;10:32. doi: 10.1186/1471-2229-10-32.
Plant biologists have long speculated about the mechanisms that guide pollen tubes to ovules. Although there is now evidence that ovules emit a diffusible attractant, little is known about how this attractant mediates interactions between the pollen tube and the ovules.
We employ a semi-in vitro assay, in which ovules dissected from Arabidopsis thaliana are arranged around a cut style on artificial medium, to elucidate how ovules release the attractant and how pollen tubes respond to it. Analysis of microscopy images of the semi-in vitro system shows that pollen tubes are more attracted to ovules that are incubated on the medium for longer times before pollen tubes emerge from the cut style. The responses of tubes are consistent with their sensing a gradient of an attractant at 100-150 mum, farther than previously reported. Our microscopy images also show that pollen tubes slow their growth near the micropyles of functional ovules with a spatial range that depends on ovule incubation time.
We propose a stochastic model that captures these dynamics. In the model, a pollen tube senses a difference in the fraction of receptors bound to an attractant and changes its direction of growth in response; the attractant is continuously released from ovules and spreads isotropically on the medium. The model suggests that the observed slowing greatly enhances the ability of pollen tubes to successfully target ovules. The relation of the results to guidance in vivo is discussed.
植物生物学家长期以来一直在推测引导花粉管到达胚珠的机制。虽然现在有证据表明胚珠会发出可扩散的引诱剂，但对于这种引诱剂如何介导花粉管与胚珠之间的相互作用知之甚少。
我们采用半离体测定法，即将拟南芥的胚珠从胚珠中分离出来，围绕在人工培养基上的切口花柱排列，以阐明胚珠如何释放引诱剂以及花粉管如何对其做出反应。对半离体系统的显微镜图像分析表明，与从切口花柱中出现的花粉管相比，在培养基上孵育更长时间的胚珠对花粉管更具吸引力。管的反应与其在 100-150 微米处感应到引诱剂梯度的情况一致，这比以前的报道更远。我们的显微镜图像还显示，花粉管在功能胚珠的珠孔附近减缓生长，其空间范围取决于胚珠的孵育时间。
我们提出了一个可以捕捉这些动态的随机模型。在该模型中，花粉管感知到与引诱剂结合的受体的分数差异，并相应地改变其生长方向；引诱剂不断从胚珠中释放出来，并在培养基上各向同性地扩散。该模型表明，观察到的减缓大大增强了花粉管成功靶向胚珠的能力。讨论了结果与体内引导的关系。
