Decline in leaf growth under salt stress is due to an inhibition of H+-pumping activity and increase in apoplastic pH of maize leaves

This article will cover historical and recent aspects of reactions and mechanisms involved in the auxin-induced signalling cascade that terminates in the dramatic elongation growth of cells and plant organs. Massive evidence has accumulated that the final target of auxin action is the plasma membrane H(+)-ATPase, which excretes H(+) ions into the cell wall compartment and, in an antiport, takes up K(+) ions through an inwardly rectifying K(+) channel. The auxin-enhanced H(+) pumping lowers the cell wall pH, activates pH-sensitive enzymes and proteins within the wall, and initiates cell-wall loosening and extension growth. These processes, induced by auxin or by the "super-auxin" fusicoccin, can be blocked instantly and specifically by a voltage inhibition of the H(+)-ATPase due to removal of K(+) ions or the addition of K(+)-channel blockers. Vice versa, H(+) pumping and growth are immediately switched on by addition of K(+) ions. Furthermore, the treatment of segments either with auxin or with fusicoccin (which activates the H(+)-ATPase irreversibly) or with acid buffers (from outside) causes an identical transformation and degradation pattern of cell wall constituents during cell-wall loosening and growth. These and other results described below are in agreement with the acid-growth theory of elongation growth. However, objections to this theory are also discussed.

The ability of low pH and CO(2) to induce rapid cell elongation and wall loosening in the Avena coleoptile has been examined with the use of a continuous growth-recording technique and an Instron extensometer, respectively. In particular, the properties of the response to hydrogen ions have been examined in detail and have been compared with the responses initiated by CO(2) and auxin. The optimal pH for growth is about 3.0, and both the maximal growth rate and wall extensibility are similar to that produced by optimal auxin. The timing (initiated in less than 1 minute) and duration (up to 2 hours) of the response to hydrogen ions, as well as certain other aspects of the growth and wall-loosening responses, are described. It is shown that the pH response can be clearly separated from the CO(2) response. Possible mechanisms for the initiation of the growth response to low pH are briefly discussed.