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First published online 1 November 2006
doi: 10.1242/dev.02683
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1 Howard Hughes Medical Institute, Department of Molecular Genetics and Cell
Biology, The University of Chicago, Chicago, Il 60637, USA.
2 Medical Scientist Training Program, The University of Chicago, Il 60637,
USA.
3 Department of Molecular Biology, Cell Biology and Biochemistry, Brown
University, Providence, RI 02912, USA.
* Author for correspondence (e-mail: Mark_Johnson_1{at}Brown.edu)
Accepted 4 October 2006
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 ß-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.
Key words: Arabidopsis, Pollen tube guidance, Double fertilization, Sperm, Gamete interactions
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