Acid Growth Hypothesis and Cell Elongation Responses

A classic response to auxin or auxinic herbicides is cell elongation. Within 10 to 15 minutes of exposure to auxin, cell elongation is induced because of acid-induced cell wall loosening. The model best describing this process is the Acid Growth Hypothesis. Cell walls become acidified by IAA turning on H+-ATPases via secondary messengers, or by IAA inducing, via changes in gene expression, more H+-ATPase production for incorporation into the cell membrane (Figure 11).

Fig. 11: H+-ATPase in the membrane pumps H+ into the cell-wall region (Image credit: Tracy Sterling, Deana Namuth, Jeremy Steele, and Smitha Kasinadhuni)

The H+-ATPases pump H+ out of the cell, expending ATP to move H+against its electrochemical gradient, further acidifying the cell-wall region. Wall loosening during acidification involves rearrangements of the load-bearing bonds of the cell wall and probably is mediated through special proteins called expansins. Once the cell wall is loosened, water moves into the cell, driven by higher internal osmotic pressure, thereby causing the cell to expand. Water uptake into the cells creates an internal turgor pressure that pushes against the cell membrane and extends the cell wall. Cell elongation after 30 to 60 minutes does not involve acid-induced elongation, but is due to auxin turning on genes which help cells elongate by other mechanisms (i.e. synthesis of new cell wall material). The animation ’Overall Picture of Auxinic Herbicide Action’ illustrates how auxin induces the H+-ATPases pump H+ out of the cell to cause cell wall loosening.

*this animation has no audio*