Growing 'Up': Mechano-chemical aspects of anisotropic cell growth

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Human beings have become very skilled at building things; bridges, containers, houses, skyscrapers, auditoriums, cars, the list is endless. We can draw up a plan and manipulate materials to get most desired shapes and assemble those shapes into an object. However, this process is fundamentally different from how nature makes shapes, because nature grows things. A single cell can be multiplied, and its daughters altered in different ways, their subsequent growth rates and shapes changed, and those of their daughters, until a desired object is formed. Beyond even that, these shapes can be changed over time, responding to their environment and other cues, the ultimate smart-materials. We are only just beginning to understand the physical process of growth in plants, that is to say: what are the changes in the plant material that yield different shapes, and how are these changes regulated? To answer these questions, we will characterize a system involving a simple change in shape- the extension of a special organ, the hypocotyl, responsible for lifting the shoot tip out of the soil following germination.When a seed is planted in the soil, the young plant must extend itself out into the light and air to begin photosynthesis. This extension happens in a deceptively simple way: between the shoot tip and the root tip of a young plant there is a special organ- the hypocotyl. The hypocotyl has a fixed number of cells which begin as roughly cubed shapes, but who eventually increase in length over 200x, becoming very elongated rectangles, or cuboids. This type of growth is called anisotropic, where one direction of expansion exceeds the other; here length increase is greater than width. The result of anisotropic growth in the hypocotyl is the extension of the shoot tip up and out of the soil, very rapidly. An added bonus to the system is its dynamic response to light and gravity.What do we know about the control of cell growth direction in plants? A plant cell is basically a pressure filled box, where the pressure inside provides a non-directional force for growth and local changes in the wall provides the information about the direction of growth and hence any change in shape. The traditional view of anisotropic cell expansion, such as in the hypocotyl, is that each cell has a special organization in the wall, oriented cellulose fibers, that controls the direction of expansion. These fibers can be thought of as hoops around a barrel, limiting width expansion but capable of spreading apart to allow length increases. However, recent research points to a much more dynamic and complex picture. Within hypocotyl cells, cellulose orientation is more dynamic than previously thought- these are not just static hoops on barrels. In fact, true width-limiting orientations within the wall are only observed transiently after anisotropic expansion has begun, and even then not all the way around the cell surface, but only on their inner faces. Furthermore, another part of the cell wall, the pectin gel in which cellulose fibers are imbedded, is very important for regulating growth in plants.In this project we will characterize the biophysical and chemical changes in cell wall properties associated with anisotropic cell growth, and test the idea that these changes are coordinated by the plant hormone, auxin.