They
 trigger increased efflux of protons across the cell wall, causing the activation of expansins; that mechanism
 is known as the acid-growth hypothesis.
Overexpression of YUCCA genes increase the synthesis of these compounds by increasing
 hydroxylation of tryptamine, and another step of their synthesis is catalyzed by nitrilase. The toxin
 fusicoccin mimics some of the effects of these compounds, which binds to the TIR1-SCF complex. They
 trigger increased efflux of protons across the cell wall, causing the activation of expansins; that mechanism
 is known as the acid-growth hypothesis. This hormone moves from shoots downward in a process known
 as polar transport, and it helps establish apical dominance. Experiments with blocks of agar also showed
 that its responsible for phototropism. For 10 points, name this plant hormone, examples of which include
 indole-3-acetic acid.
Overexpression of YUCCA genes increase the synthesis of these compounds by increasing
 hydroxylation of tryptamine, and another step of their synthesis is catalyzed by nitrilase.
{auxin}