Role of cytosolic pH in axis establishment and tip growth

The role of cytosolic pH (pHc) in determining the growth site and in tip elongation has been investigated by measuring and manipulating pHc. pHcacidifies by 0.1–0.2 units as the growth axis is established. Concomitantly, cells accumulate KCl, which increases the cellular osmotic pressure, resulting in the generation of turgor pressure. The K+taken up is apparently compartmentalized as the free cytosolic K+activity remains constant. At present, the relation between pHc, K+, and turgor pressure is not well understood. A small but statistically significant cytosolic pH gradient, acid at the future growth site, is also detectable during axis establishment. As growth is initiated the intensity of the gradient increases to approximately 0.3 pH units. The magnitude of the pH gradient correlates with the rate of tip elongation. The gradient may regulate tip elongation in a number of ways, including local control of the assembly and stability of cytoskeletal elements. Key words: cytosolic pH gradients, tip growth, turgor pressure, weak acids and bases, SNARF 1, pH-sensitive microelectrodes.