The Polarity of Plants

I’ve long been interested by the polarity of plants. Roots grow downwards and like the dark, shoots grow upwards and like the light. They have very different forms and yet, like the poles of a magnet, they are part of a greater whole. This polarity is established in their cells and tissues from the earliest stages of embryonic growth. What is the cellular basis of polarity? I started investigating this question in the 1970s by testing the hypothesis that the age of the two walls of a cell in a growing shoot confirmed a polarity upon them. In the tip of a growing plant when the cells divide, the upper cell goes on to divide again and its lower cell wall is older than its upper cell wall, produced by a subsequent division. But in the leaves of grasses and other monocotyledenous plants, where the leaves grow from the base the older cell walls are top of the cells not at the bottom. Yet they have the same polarity as other tissues as shown by their very strong polarity with which they transport the hormone auxin as I showed in this paper published in Nature. When stems of plants like tobacco and tomato and oak trees grow thicker, they do so by cells dividing longitudinally, producing new cells to the side rather than above or below and they too are polarized in the normal direction and transport auxin towards the root pole. So polarity seems to have nothing to do with the age of cell walls, nor can it be reversed by growing plants in an inverted position, with roots at what used to be their upper end and shoots at the lower end, as shown here. My investigations of polarity in one of the simplest systems possible, the thread-like file of cells that grows from a fern spore showed that the cells had a polarity connected with the way the cell membranes were attached to the cell walls at different ends of the cell, and this insight proved very relevant in understanding how polar auxin transport takes place, as discussed in another findings video. Finally, much remains unknown about electrical aspects of plant polarity, and I discuss simple experiments that shed light on this problem with the help of an electrostatic spray gun.