Too much tubulin flips cell wall flexibility

Cell wall composition is important for the proper functioning of guard cells in leaves, cells which control the opening and closing of stomata, which in turn regulates the supply of carbon dioxide into, and water out of, the leaf. In plant cells, including guard cells, the cytoskeleton forms a network of fibers and microscopic tubes (known as microtubules) that shuttle sugars and proteins around the cell. The cytoskeleton also helps coordinate changes in the structure and composition of the cell wall. How might that happen? Microtubules (which are made of long chains of a protein called tubulin) are almost constantly changing length, with individual tubulin molecules being either attached (to lengthen the microtubule) or removed (which shortens the microtubule). Changes in microtubule length can alter the flow of molecular traffic inside the cell, and thereby cause changes in cell wall composition and cell wall flexibility.

In a recent article in Tree Physiology, Scott Harding and colleagues sought to determine how excess tubulin interferes with guard cell function in poplar. They used poplar that had either regular or excess amounts of tubulin, and measured both the composition of the cell walls and changes in the gene expression of enzymes that manipulate cell wall structure over the course of the day. They found that poplars with excess tubulin had more galacturonic acid, a component of pectin (the same substance you add to jam to make it gel) that influences cell wall flexibility. But the effect of galacturonic acid on cell wall flexibility depends on whether or not the acid has methyl groups attached to it, and the researchers also found that these poplars had less pectin methylesterase, the enzyme that removes methyl groups from galacturonic acid. Reduced methylation of galacturonic acid decreases the flexibility of pectin and, by extension, the cell wall. The authors suggest that this is the mechanism impairing proper guard cell function in poplar with excess tubulin, since more rigid guard cell walls interfere with stomatal movement.

So what are the implications of these results? Understanding the molecular mechanisms by which guard cell function can be altered can provide us with a list of genetic targets that will affect plant carbon uptake, water loss, and, in the end, plant productivity. By understanding these mechanisms, we could then breed or genetically engineer trees for enhanced growth or drought tolerance, which would be particularly useful for the biofuel and forestry industries.