Hidden by soil and often ignored, the root is vital to the growing plant, exploring the soil to get water and nutrients and providing anchorage. Root form and function is regulated by the plant hormone auxin, which is produced and transported by specialized protein families in the plant. The activity of these proteins is understood in broad outline, but quantitative aspects remain unclear. The research goals of the project aim to advance knowledge of auxin transport by combining experimental and computational approaches. The experimental approaches of this multidisciplinary project include high-resolution measurements of the movement of auxin itself, as well as of the propagation of auxin-mediated responses, such as cell growth. The experiments will use the species Arabidopsis thaliana, the "fruit fly" of plant biology. The nearly transparent roots are a significant advantage for microscope work, and the available storehouse of auxin-related mutants is second to none. The computational aspect of the project will refine AuxSim, a model under development by Kramer that is already recognized as a powerful computational tool for auxin transport. The refined model and experimental results will fill in several blanks in the flow-chart for auxin transport within the root. This will synergize with contemporary efforts to understand the molecular biology of auxin action, and may also help applied efforts to enhance agricultural productivity by improving root efficiency. AuxSim will be made freely available. The award will partner undergraduates at Simon's Rock and UMass Amherst, and will provide training that is both biologically rigorous and computationally rich, a combination increasingly considered essential for tomorrow's workforce.

Project Report

PI: Eric M. Kramer, Bard College at Simon’s Rock, Great Barrington, MA. Co-PI: Tobias I. Baskin, University of Massachusetts, Amherst. Postdoc: Heidi Rutschow Project Outcomes One of the ways that plants differ from animals is that plant development is "plastic" – plants can sprout new roots or leaves in response to changes in their environment. Plants are capable of other, less obvious changes, too, like regulating the length of the root hairs that absorb nutrients from the soil. These macroscopic and microscopic changes in root form contribute to desirable traits like drought resistance and grain yield. One long-term goal of plant biology is the ability to predict what changes in root form will result under specific changes in the genes of the plant. In other words, can one predict phenotype from genotype? Currently, this is impossible for all but a handful of well-studied cases. Both root branching and root hair length are regulated by the movement and action of hormones within the plant, and the most important of these hormones is auxin. In the last decade, many plant scientists have used computers to model how auxin moves inside the root, as a way to understand root growth and branching, but the underlying numbers in these models are poorly known. To understand the magnitude of the problem, imagine trying to predict the trajectory of a baseball without knowing whether gravity accelerates objects at three feet per second per second, 300 feet per second per second, or somewhere in the middle. This is the state of knowledge regarding some parts of the auxin system. Thus, my research group set out to determine as many useful numbers as we could, to aid the design of more accurate and useful computer models. One of the quantitites we measured was the ability of auxin to move into plant cells. Although it has been known for about a decade that plant cells make proteins that can shuttle auxin across the cell membrane, their efficiency had never before been measured. In the lab, we dissolved plant roots into their constituent cells, and immersed these cells in a solution of labeled auxin molecules. By counting the number of auxin molecules that entered the cells per unit time, we were able to quantify this important process. We discovered that the carrier protein AUX1 can pump auxin into a cell an order of magnitude faster than it would otherwise enter in the absence of carriers. This will be important for future modeling efforts, as most computer models of auxin in the root have so far ignored the role of these carriers. A second quantity we measured was the ability of small molecules like auxin to move between plant cells via microscopic bridges called plasmodesmata. We achieved this goal by using a confocal microscope to track the movement of fluorescent dye molecules from one plant cell to the next. This revealed that cells in the root apex can exchange small molecules much more rapidly than previously suspected – on a time scale of just a few seconds. The observation applies equally well to the movement of hormones, carbohydrates, and salts. This discovery helped resolve a longstanding point of confusion – how can root tips grow as fast as they do, considering that the phloem elements that transport carbohydrates from the leaves to the roots don’t reach all the way to the root tip? The answer seems to be that plasmodesmata provide a post-phloem pathway for sugars to reach rapidly growing and dividing cells. We also discovered that hydrogen peroxide (H2O2) – a compound manufactured by plant cells in response to infection and other stresses – can regulate the plasmodesmata in surprising ways. Low levels of H2O2 dilate the plasmodesmata and increase transport, while higher levels close the plasmodesmata entirely. The mechanism of this regulation remains unknown. These discoveries are already changing the way scientists think about transport in the plant, suggesting improvements to computer models of how plants develop and grow, and ultimately opening the door for improved crops.

Agency
National Science Foundation (NSF)
Institute
Division of Integrative Organismal Systems (IOS)
Application #
0815453
Program Officer
Sarah Wyatt
Project Start
Project End
Budget Start
2008-09-01
Budget End
2013-10-31
Support Year
Fiscal Year
2008
Total Cost
$337,293
Indirect Cost
Name
Simon's Rock of Bard College
Department
Type
DUNS #
City
Great Barrington
State
MA
Country
United States
Zip Code
01230