A research team led by the University of California, Riverside have
discovered a way by which plants depend on auxin for growth.

Auxin uses two main pathways to orchestrate plant growth, one of them
has been discovered by Professor Zhenbiao Yang and his team. One theory
proposed a century ago suggests that when plants are ready to grow,
auxin causes their cells to become acidic, loosening the bonds between
components and allowing the walls to soften and expand. How auxin
activates acidification remained a mystery until now.

The team of Professor Zhenbiao Yang discovered that auxin creates that
acidity by triggering the pumping of protons into the cell walls,
lowering their pH levels. The lower pH activates a protein, expansin,
appropriately named because it breaks down links between cellulose and
hemicellulose, allowing the cells to expand. Pumping protons into the
cell wall also drives water uptake into the cell, building inner
pressure. If the cell wall is loose enough and there is enough pressure
inside the cell, it will expand.

Auxin not only "contributes" to plant growth, but it is essential to
nearly every aspect of a plant's growth and development, including
aspects that are important to agriculture such as fruit, seed and root
development, shoot branching, and leaf formation. Even the plant's
correct responses to gravity and light depend on auxin to ensure roots
head down while the shoots grow up toward the light.

Scientists solve 50-year-old mystery behind plant growth | News
(ucr.edu)
[news.ucr.edu]