The growth of multicellular organisms is fuelled not only by cell division but also by cell growth. Normally cells enlarge all over the surface. However, in many organisms, there are also specialized cells that grow only at their tip. How the necessary materials are delivered to the growing tip, is largely unknown. A new study of a scientific team led by Dr. Sandra Richter and Prof. Gerd Jürgens from the Center for Plant Molecular Biology now suggests that tip growth is not mediated by targeted trafficking to the growing site of the cell surface but rather depends on a specific recycling pathway.
These results have just been published in Nature Cell Biology.
Cells usually grow by the fusion of small transport containers -- so-called membrane vesicles -- with the cell membrane. These vesicles are not only important for the expansion of the cell surface but also transport molecules to different destinations within the cell and out of the cell. These intracellular transport processes are crucial for the development and the well-being of organisms and are therefore tightly regulated. The research team of Prof. Gerd Jürgens from the University of Tübingen is studying the regulation of such trafficking pathways in the flowering plant Arabidopsis thaliana (mouse-ear cress). They mainly focus on a small group of proteins, so-called guanine-nucleotide exchange factors (ARF-GEF), that are crucial regulators of vesicle formation.
In their previous publications, the authors had shown that different intracellular trafficking pathways are regulated by different ARF-GEFs. The ARF-GEF GNOM, for example, mediates a specific polar recycling pathway that transports proteins back to the cell suface and is thus important for the development of Arabidopsis. Another essential trafficking pathway required in each cell is jointly regulated by GNOM and GNOM-LIKE1.
In the new publication, the closely related ARFGEF GNOM-LIKE2 was identified as an essential regulator of tip growth in pollen. Surprisingly, GNOM-LIKE2 is not only required for polar germination of the pollen grain but also for the tip growth of the pollen tube. Pollen tubes transport the sperm cells to the egg cells. Interestingly, GNOM-LIKE2 can replace GNOM in polar recycling. Thus, the polar recycling pathway is essential for tip growth. As GNOM-LIKE2 appears to have evolved with the origin of the flowering plants, its raison d´être might be its ability to support the fast growth of the pollen.
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