Localized changes in apoplastic and cytoplasmic pH are associated with root hair development in Arabidopsis thaliana..
Tatiana N. Bibikova1, Tobias Jacob1,2, Ingo Dahse2, Simon Gilroy1
1Department of Biology, The Pennsylvania State University, 208 Mueller Lab, University Park, PA 16802, U.S.A.
2Friedrich Schiller University, Institute of Biochemistry and Biophysics, Department of Biophysics, Philosophenweg 12, 07743 Jena, Germany
Abstract.
Morphogenesis in plants is characterized by highly regulated cell enlargement. However, the mechanisms controlling and localizing regions of growth remain essentially unknown. Root hair formation involves the induction of a localized cell expansion in the lateral wall of a root epidermal cell. This expanded region then enters a second phase of localized growth called tip growth. Root hair formation therefore constitutes a model in which to study the cellular events involved in regulating localized growth in plants. Confocal ratio imaging of the pH of the cell wall revealed an acidification at the root hair initiation site. This acidification was present from the first morphological indications of localized growth, but not before, and was maintained to the point where the process of root hair initiation ceased and tip growth began. Preventing the wall acidification with pH buffers arrested the initiation process but growth resumed when the wall was returned to an acidic pH. Cytoplasmic pH was found to be elevated from approximately 7.3 to 7.7 at the initiation site, and this elevation coincided with the acidification of the wall. Preventing the localized increase in cytoplasmic pH with 10 mM butyrate however did not inhibit either the wall acidification or the initiation process. In contrast, there was no detectable gradient in pH associated with the apex of tip growing root hairs, but both elevated apoplastic pH and butyrate treatment irreversibly inhibited the tip growth process. Thus the processes of tip growth and initiation of root hairs show differences in their pH requirements. Apoplastic acidification is required for root hair initiation whereas the associated cytoplasmic alkalinization is not essential to this process. Tip growth appears to be disrupted when the dynamics of either cytoplasmic or apoplastic pH are altered. These results highlight the role of localized control of apoplastic pH in the control of cell architecture and morphogenesis in plants.
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