Tree Biology

Auxin is a plant hormone with multiple and confusing roles in plants. Mark Estelle, Nihal Dharmasiri and Sunethra Dharmasiri at Indiana University have figured out how auxin works. This is a monumental achievement and will help unravel many other mysteries of plant development. [Press releases from Indiana University and the National Science Foundation. Paper to be published in Nature] [UPDATE: The paper is in Nature now. Abstract is available here.]

Auxin is produced in shoot tips and moves downward, never up, in the plant body. Its roles range from inhibiting lateral branch growth to promoting root formation, from measuring gravity to helping fruits develop. I recall finding the roles of auxin, which had to be memorized in undergraduate plant physiology, endlessly confusing and conflicting. The reason for the confusion was clear: we knew the end effects of auxin but not how it did its work.

Estelle and his colleagues found that auxin attaches to a protein called TIR1. The auxin-TIR1 combination switches on growth by removing another protein that stops growth genes from being expressed. Once the growth genes are expressed, they make proteins that are used by the plant to produce new cells.

There is still, of course, much to be learned. One of the great things about a major discovery like this is that it opens up huge opportunities for scientists to probe the details of plant growth. It is safe to predict that, if the Estelle labs results are confirmed by other researchers, there will be a flood of new discoveries to come.

And finally, students of plant physiology will have a rational explanation for some of the mysteries that have bedeviled them.

The research is being published in Nature. It is supported by the National Science Foundation, National Institutes of Health and the US Department of Agriculture.

Update:  There are actually two papers in today's Nature regarding auxin attaching to TIR1 protein. The second paper, by Stefan Kapinski, University of York, and Ottoline Leyser, Umea Institute of Plant Biology, appears to come to the same conclusions as the work of Estelle and his colleagues.

When plants are wounded or infected by viruses, bacteria or fungi, they respond by walling off the infected or injured cells. This response is an important part of the plant immune system. In trees, this is known as "compartmentalization", but more generally is called "Programmed Cell Death" (PCD), or the "hypersensitive reaction." PCD is a critical immune response in plants. Understanding how PCD works will help plant breeders to improve the ability of plants to defend themselves against pests and pathogens. How do plants kill off some cells without killing others? The margins of killed areas are usually very discrete, with some cells dead and cells immediately adjacent appearing perfectly normal. You can see this in almost any plant leaf that has been injured, infected or infested - the margin between dead and living cells is usually a sharp line. Savithramma Dinesh-Kumar and his colleagues at Yale University have found that plant cells produce both "pro-survival" and "pro-death" signals. Without the pro-survival signals, the pro-death signal can move out of the infected area and cause further cell death. Dinesh-Kumar and his colleagues were able to silence the pro-survival gene, stopping the gene from expressing itself. When tobacco leaves with the pro-survival gene silenced were infected with a virus, the zone of death spread throughout the plant. The presence of both pro-survival and pro-death signals in plant cells illustrates the complexity and sophistication of the plant immune system. The experiments were also complex and sophisticated. Dinesh-Kumar first had to develop the techniques for silencing individual genes involved in PCD before he could separate the pro-survival and pro-death signals. The research, published in Cell, was funded by the National Science Foundation.
	An illustration of PCD in a leaf. The brown cells have been killed by the pro-death signal. Green cells outside the kill zone are protected by the pro-survival signal. The virus (purple) elicits the response, but cell death is an immune response of the plant. Illustration by Nicole Rager Fuller courtesy of the National Science Foundation