75 How Do the Cells of a Growing Plant Know in Which Direction to Elongate? Sometimes questions that seem simple can be devilishly dif- ficult to answer. Imagine, for example, that you are holding a green blade of grass in your hand. The grass blade has been actively growing, its cells dividing and then stretching and elongating as the blade lengthens. Did you ever wonder how the individual cells within the blade of grass know in what direction to grow? To answer this deceptively simple question, we will first need to provide answers to several others. Like Sherlock Holmes following a trail of clues, we must approach the an- swer we seek in stages. Question One. First, we need to ask how a blade of grass is able to grow at all. Plant cells are very different from ani- mal cells in one key respect: every plant cell is encased within a tough cell wall made of cellulose and other tough building materials. This wall provides structural strength and protection to the plant cell, just as armor plate does for a battle tank. But battle tanks can’t stretch into longer shapes! How is a plant cell able to elongate? It works like this. A growing cell first performs a little chemistry to make its wall slightly acidic. The acidity acti- vates enzymes that attack the cell wall from the inside, rear- ranging cellulose cross-links until the wall loses its rigidity. The cell wall is now able to stretch. The cell then sucks in water, creating pressure. Like blowing up a long balloon, the now-stretchable cell elongates. Question Two. In a growing plant organ, like the blade of grass, each growing cell balloons out lengthwise. Stating this more formally, a botanist would say the cell elongates parallel to the axis along which the blade of grass is extend- ing. This observation leads to the second question we must answer: How does an individual plant cell control the di- rection in which it elongates? It works like this. Before the stretchable cell balloons out, tiny microfibrils of cellulose are laid down along its inside sur- face. On a per weight basis, these tiny fibrils have the tensile strength of steel! Arrays of these cellulose microfibrils are organized in bands perpendicular to the axis of elongation, like steel belts. These tough bands reinforce the plant cell wall laterally, so that when the cell sucks in water, there is only one way for the cell to expand—lengthwise, along the axis. Question Three. Now we’re getting somewhere. How are the newly made cellulose microfibrils laid down so that they are oriented correctly, perpendicular to the axis of elongation? It works like this. The complicated enzymic machine that makes the cellulose microfibrils is guided by special guiderails that run like railroad tracks along the interior sur- face. The enzyme complex travels along these guiderails, laying down microfibrils as it goes. The guiderails are con- structed of chainlike protein molecules called microtubules, assembled into overlapping arrays. Botanists call these ar- rays of microtubules associated with the interior of the cell surface “cortical microtubules.” Question Four. But we have only traded one puzzle for another. How are the cortical microtubules positioned cor- rectly, perpendicular to the axis of elongation? It works like this. In newly made cells, the microtubule assemblies are already present, but are not organized. They simply lie about in random disarray. As the cell prepares to elongate by lessening the rigidity of its cell wall, the micro- tubule assemblies become organized into the orderly trans- verse arrays we call cortical microtubules. Question Five. Finally, we arrive at the question we had initially set out to answer. How are microtubule assemblies aligned properly? What sort of signal directs them to ori- ent perpendicular to the axis of elongation? THAT is the question we need to answer. Part II Biology of the Cell Seeing cortical microtubules. Cortical microtubules in epidermal cells of a fava bean are tagged with a flourescent protein so that their ordered array can be seen. Real People Doing Real Science