ROLE OF CYTOSOLIC PH IN AXIS ESTABLISHMENT AND TIP GROWTH

The role of cytosolic pH (pH(c)) in determining the growth site and in tip elongation has been investigated by measuring and manipulating pH (c). pH(c) acidifies by 0.1-0.2 units as the growth axis is establishe d. Concomitantly, cells accumulate KCl, which increases the cellular o smotic pressure, resulting in the generation of turgor pressure. The K + taken up is apparently compartmentalized as the free cytosolic K+ ac tivity remains constant. At present, the relation between pH(c), K+, a nd turgor pressure is not well understood. A small but statistically s ignificant cytosolic pH gradient, acid at the future growth site, is a lso detectable during axis establishment. As growth is initiated the i ntensity of the gradient increases to approximately 0.3 pH units. The magnitude of the pH gradient correlates with the rate of tip elongatio n. The gradient may regulate tip elongation in a number of ways, inclu ding local control of the assembly and stability of cytoskeletal eleme nts.