One Way Traffic: Max Planck Scientists Idendified Genes Involved In Transporting An Important Plant Growth Factor

Scientists have identified important genes for plant growth whose products help to distribute an vital growth factor throughout the plant. The discovery of these genes means that scientists will be able to design better crops and herbicides. The scientist's findings, reported recently in Science (Vol 282, 2226-2230) and in the December 1998 issue of EMBO Journal (Vol 17, 6903-1911) are regarded as a major breakthrough.

Figure 1: The mutation of the AtPIN1 gene alters the outer appearance of arabidopsis plants. The pin-formed, naked inflorescence with no or defective flowers is the main characteristic aspect of the mutants. Full size image available through contact

Auxin, an important plant growth factor, plays a key role in determining how and in what direction plants grow. Like many animal hormones, plant growth factors are transported and so have long been regarded to act in a similar fashion to animal hormones. One key difference, though, is that animals have a blood system to transport hormones to target sites around every part of the body, but plants don't. Instead, plants use a specialised delivery system to transport auxin from cell to cell. This delivery system, called the polar auxin transporter, has eluded scientists for a long time, even though its existence had already been postulated in the 1930s. Now, a group headed by Klaus Palme at the Max Delbrück Laboratory in Cologne (a part of the German Max Planck Society) has identified the major parts of this system and provided important pieces in the puzzle of how plant cells communicate with each other.

Plant cells need to communicate with one another to coordinate development of the plant as a whole. For decades, scientists have known that plants move auxin from its site of synthesis in the shoot tips to sites of action elsewhere in the plant. The direction of transport is from the shoot tip down to the tip of the root. Scientists believe it is the movement of auxin that determines the shape of the plant.

The transport of auxin within the plant is a process involving tiny molecular gates that span the membranes surrounding living plant cells. These gates selectively allow auxin to flow from one side of the membrane to the other. Until now the nature of these doors was unknown. The only thing known was that they must be located at only one end of cells and that the other side of the cell must lack them. This would ensure that auxin could only flow in one direction through the cell.

Scientists found a mutant of an arabidopsis plant that was shaped strangely. Since the plant looked like a knitting needle, researchers called it "pin" (Fig. 1). When they looked to see what had caused this, they noticed that the plant could not transport auxin properly.

Figure 2: The AtPIN1 protein was localized at the basal end of cells. The yellow-green signals are marking the AtPIN1 protein in longitudinal sections of inflorescence axes. Full size image available through contact

Modern methods of plant biotechnology make it possible to tag genes. By inserting a foreign piece of DNA into a gene, the gene is interrupted and cannot function normally. Leo Gälweiler and other collaborators in Klaus Palme's group at the MDL cooperated with scientists from the Max Planck Institute for Plant Breeding Research in Cologne to use this method to produce arabidopsis mutants that looked like the pin mutants. Since the gene was tagged, the researchers could distinguish it from the many other plant genes.

Two genes were identified that could code for auxin gates. Both make their products in different cells of the plant. While one is for transport of auxin in the outer cell layers of the root, the other one is for transport in the vascular system, the plant tissue through which sugars, water and nutrients are carried. The proteins made by the two genes were found located at only one end of a cell, suggesting their genes are blueprints for different types of the long sought molecular gates (Fig. 2).

The one way traffic of auxin, called polar auxin transport, is fundamental for normal plant growth and development, and hence these findings are important both for basic and applied plant science. The recent discoveries shed new light on how plant growth is regulated and have opened up a whole new field of molecular plant physiology. This discovery is also likely to have great significance to the agrochemical/ biotechnology industries. Auxin-transport inhibitors are known to inhibit plant growth; some herbicides on the market are based on this. Therefore, these proteins will probably serve as powerful new targets for herbicide-screening programmes that improve old herbicides by rational design.

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