A team of researchers led by the University of California has demonstrated for the first time one way that a small molecule turns a single cell into something as large as a tree.
For half a century, scientists have known that all plants depend on this molecule, auxin, to grow. Until now, they didn't understand exactly how auxin sets growth in motion, but a new study published in the latest issue of the journal Nature explains how.
Plant cells are encased in shell-like cell walls, whose primary layer has three major components: cellulose, hemicellulose, and pectin. Cellulose works like rebar in a high rise, providing a broad base of strength. It's reinforced by hemicellulose chains and sealed in by pectin.
These components define the shape of plant cells. These shapes help tightly glue cells together and provide physical strength for plants against elements such as the wind. With everything locked so tightly by the cell walls, how is movement and growth possible?
One theory posits 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, but how auxin activates the acidification remained a mystery until now.
The researchers discovered 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 that breaks down links between cellulose and hemicellulose, allowing the cells to expand.
The pumping of 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.
"Like a balloon, expansion depends on how thick the outsides are, versus how much air you're blowing in. Lowering the pH in a cell wall can allow water outside of a cell to move in, fueling turgor pressure and expansion," explained UCR botany professor and research team leader Zhenbiao Yang in a report.
