von Besser Kiera, Frank Aubrey C, Johnson Mark A, Preuss Daphne
Howard Hughes Medical Institute, Department of Molecular Genetics and Cell Biology, The University of Chicago, Chicago, Il 60637, USA.
Development. 2006 Dec;133(23):4761-9. doi: 10.1242/dev.02683. Epub 2006 Nov 1.
In flowering plants, sperm cells develop in the pollen cytoplasm and are transported through floral tissues to an ovule by a pollen tube, a highly polarized cellular extension. After targeting an ovule, the pollen tube bursts, releasing two sperm that fertilize an egg and a central cell. Here, we identified the gene encoding Arabidopsis HAP2, demonstrating that it is allelic to GCS1. HAP2 is expressed only in the haploid sperm and is required for efficient pollen tube guidance to ovules. We identified an insertion (hap2-1) that disrupts the C-terminal portion of the protein and tags mutant pollen grains with the beta-glucuronidase reporter. By monitoring reporter expression, we showed that hap2-1 does not diminish pollen tube length in vitro or in the pistil, but it reduces ovule targeting by twofold. In addition, we show that the hap2 sperm that are delivered to ovules fail to initiate fertilization. HAP2 is predicted to encode a protein with an N-terminal secretion signal, a single transmembrane domain and a C-terminal histidine-rich domain. These results point to a dual role for HAP2, functioning in both pollen tube guidance and in fertilization. Moreover, our findings suggest that sperm, long considered to be passive cargo, are involved in directing the pollen tube to its target.
在开花植物中，精子细胞在花粉细胞质中发育，并通过花粉管（一种高度极化的细胞延伸结构）运输穿过花组织到达胚珠。在靶向胚珠后，花粉管破裂，释放出两个精子，分别使卵细胞和中央细胞受精。在这里，我们鉴定了拟南芥HAP2的编码基因，证明它与GCS1等位。HAP2仅在单倍体精子中表达，是花粉管有效导向胚珠所必需的。我们鉴定了一个插入突变（hap2-1），它破坏了该蛋白质的C末端部分，并用β-葡萄糖醛酸酶报告基因标记突变花粉粒。通过监测报告基因的表达，我们发现hap2-1在体外或雌蕊中并不减少花粉管长度，但它使胚珠靶向率降低了两倍。此外，我们表明输送到胚珠的hap2精子无法启动受精。预测HAP2编码一种具有N末端分泌信号、单个跨膜结构域和C末端富含组氨酸结构域的蛋白质。这些结果表明HAP2具有双重作用，在花粉管导向和受精过程中均发挥功能。此外，我们的研究结果表明，长期以来被认为是被动货物的精子，参与引导花粉管到达其靶标。
