Animation 25.3 Auxin Affects Cell Walls

INTRODUCTION

Auxin is a plant hormone involved in many aspects of plant growth and development. One function of auxin is to trigger cell elongation in shoots. Auxin's action in shoots can be deduced from Arabidopsis thaliana plants that do not make auxin, because these plants are short, and supplying them with the hormone reverses this phenotype. Auxin also mediates phototropism, in which cells elongate more on one side of a shoot, causing the plant to bend toward light.

Plant cell elongation requires two elements: high turgor pressure inside the plant cell and a loosening of the cell wall so that the turgor pressure and additional incoming water can force the cell to expand. In this tutorial, we focus on how auxin loosens the cell wall—a process described as the acid growth hypothesis.

Textbook Reference: Key Concept 28.3 Responses to Temperature Help Clarify Homeostasis, p.694

CONCLUSION

The acid growth hypothesis explains auxin-induced cell expansion. The hypothesis holds that protons (H+) are pumped from the cytoplasm into the cell wall. The lower pH of the wall then activates cell wall enzymes called expansins that disrupt interactions between cell wall polymers. The disruption loosens the cell wall, making it easier to stretch.

Keep in mind that cells can expand only if they take up more water. Water increases a cell's turgor pressure and, therefore, drives cell expansion.

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