Tip-localised H+-fluxes and the applicability of the acid-growth hypothesis to tip-growing cells: Control of chloronemal extension in Funaria hygrometrica by auxin and light

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Abstract
The role of tip-localised H+ secretion in regulating chloronemal tip growth in the moss Funaria hygrometrica Hedw. was investigated. pH was monitored with pH microelectrodes placed close to the cell surface while the rate of extension growth was manipulated by illumination and by the application of indole-3-acetic acid. Growth stimulations were accompanied by acidification of the external solution; this acidification was most pronounced at the growing tip. The timing and extent of acification external to the tip correlated well with the magnitude and time course of growth stimulations. The maintenance of both growth and H+ efflux under CO2-free conditions indicated that neither photosynthetic nor respiratory CO2 metabolism were involved. Artificially acidifying the nutrient solution rapidly but transiently stimulated elongation in both white light and darkness. Furthermore, the stimulation of elongation caused by white light was inhibited if the nutrient solution was buffered strongly near neutrality. We conclude that the “acid growth” hypothesis is applicable to tip growth in Funaria and that light and exogenous indole-3-acetic acid act at least in part by stimulating localised H+-ion efflux.