The Mechanism of Auxin Action.

Plant growth hormones are known to produce a number of striking effects on various processes including cell enlargement, root initiation, bud inhibition, etc. It has also been shown that the auxins, and indole-3-acetic acid in particular, participate in some general protoplasmic processes. Support for this view has been most recently afforded in the demonstration by COMMONER and THIMANN (1) that this hormone is involved in part of the respiratory metabolism of plant cells. The mechanism of the action of indole-3-acetic acid in the various growth processes which it influences is as yet unclear, however, even in the most favorable case of cell enlargement. Two theories concerning this auxin effect have been put forth. One suggests that the hormone lowers the resistance of the cell wall to turgor pressure, thereby permitting cell enlargement to take place. The other theory, suggested by CZAJA (2), holds that cell enlargement is the result of an increase in the intensity of an osmotic driving force. Although it has been the practice to consider these as two opposing theories it must be pointed out that both agree that the actual driving force in cell enlargement is osmotic in nature for it is clear that the turgor pressure is itself a consequence of osmotic forces. No conclusive evidence in support of the wall-resistance theory has been presented thus far. The second theory has been objected to on the ground that growing plant cells do not show any increase in the osmotic pressure of their vacuole contents. The very failure of growing cells to exhibit a reduction in osmotic pressure while the vacuoles rapidly absorb water and itierease in volume indicates, however, that the absolute solute content of the sap must be increased during this process. Thus it becomes important to know whether indole-3-acetic acid influences the solute content of the cell sap and so by momentarily increasing the osmotic pressure, affects water uptake and cell enlargement. It would be expected that no observable increase in osmotic pressure would occur, since the uptake of water would counteract the increase in solute content. Increase in sap solute might result from the hydrolysis of stored material such as protein and starch or from the absorption of salts. Attention toward the latter alternative was drawn by the observation of THIMANN and SCHNEIDER