A Washington State University biologist has found what he calls "very strong support" for an 86-year-old hypothesis about how nutrients move through plants. His two-decade-long analysis of the phenomenon has resulted in a suite of techniques that can ultimately be used to fight plant diseases and make crops more efficient.

Washington State University biologist Michael Knoblauch measured and counted thousands of microscopic holes to test an 86-year-old hypothesis of how nutrients move in a plant. He is pictured with a microscope photo of a sieve plate that separates the elongated cells of a plant's vascular system. Photo: Washington State University.

Some 90% of the food people consume at one time went through a plant's phloem, the vascular system that carries sugars and other nutrients from leaves, where they are produced by photosynthesis, to the roots and fruits. Scientists know so little about how this works, though, that they're like cardiologists who haven't learned about the heart, according to Michael Knoblauch, professor in the Washington State School of Biological Sciences.

"If you have a little-supported hypothesis that is central to plant function, it's a problem," he said. "For example, take plant-insect interactions. Aphids feed on the system. If we don't understand how the system works in detail, we cannot find new strategies to kill aphids. Plant viruses also move through the system."

The fundamental principle of phloem transport was published by Ernst Munch in 1930. While Munch's hypothesis is intuitive and elegant, it does not appear to account for the extreme pressure needed to move fluid in something as large as a tree.

"He came up with the hypothesis because he knew how solute-driven flow could work, but he was not into measuring all these things or finding evidence for his hypothesis," Knoblauch explained.

To make his finding, published in the journal eLife, Knoblauch spent more than 20 years devising ways to look inside a living plant without disrupting the processes he was trying to measure and describe.

"It's super tough to work with this tissue," he said. "It's a technical question. It's really difficult to access it, and this has always fascinated me."

He measured flow velocities with fluorescent dies and radioactive isotopes. With his son, Jan Knoblauch, a Washington State sophomore and second author on the paper, he developed a "picogauge" that could measure extremely sensitive phloem pressures.

He looked at tomatoes, fava beans, kelp off the British Columbia coast and a red oak from the Harvard Forest in central Massachusetts. As director of Washington State University's Franceschi Microscopy & Imaging Center, Knoblauch used various microscopes to measure the circumferences of not only plant stems but the ciabatta-like holes of sieve plates that separate elongated cells in the phloem tissue.

The cell geometries were particularly critical, as an order-of-magnitude change in the diameter of a tube or hole creates a four-order change in the volume delivered to the roots or fruits.

He made roughly 100,000 measurements in each of three morning glory plants he grew alongside the university's five-story Abelson Hall.

In addition to building the evidence for a long-held hypothesis, Knoblauch hopes his work can find new ways to protect plants. It might also lead to ways of making the energy in biofuels easier to concentrate and access.

"If we can tell the phloem, 'Okay, store it here, where we can easily harvest it,' it will be a big step forward," he said.

Knoblauch's co-authors include post-doctoral researcher Daniel Mullendore and doctoral student Sierra Beecher of Washington State University, Jessica Savage and Michele Holbrook of Harvard University, Benjamin Babst of Brookhaven National Laboratory, Kaare Jensen of the Technical University of Denmark and undergraduate lab tech Adam Dodgen.