WESTERN PRODUCER — Root hairs function as an extension of a plant’s root tips. They increase the surface area of plant roots, helping them extract more water and nutrients from the soil.
Recently, scientists at Washington State University discovered a gene that drives the growth of root hairs. The gene causes faster-growing, denser webs of root hairs to find and use nitrates, a prime source of nitrogen essential to plant growth.
“We found that the phenotype on this root was a tiny root hairball,” said Karen Sanguinet, assistant professor at the university’s department of crop and soil sciences.
“On the outside, without looking at it under the microscope, it looked like it didn’t have any hairs. But the thing that was surprising was that the root grew twice as fast. We found the mutant, but we didn’t know what the gene was or what it was doing, or why it has the phenotype that it has. We found out that its unexpected role was nitrogen signalling. Everything kept leading us to look at nitrate and nitrate responsiveness.”
She said in a media release that nitrogen runoff and nitrogen-use efficiency are some of the pre-eminent issues facing agriculture. Understanding the genetic mechanisms that control nitrate uptake and signalling, as well as a plant’s better use of nitrate, is advantageous for the entire nitrogen cycle.
Sanguinet said that they called the new gene the buzz gene.
“It looks like the root has had a buzz cut. The week I found it, my little nephew had a buzz cut and shaved his head. At the same time, I found this mutant and when we looked at it under the microscope, the root had this stubble all over it. When you don’t know what the gene is, you name it after a phenotype (observable characteristics).”
She said that normally in wild-type plants and wild-type roots, there is a sensitive response to ammonium nitrate in localized and preferential growth.
“You see it where it grows preferentially in nutrient-rich patches. The phenotype is not suppressed under high nitrate conditions and the primary root grows faster while lateral roots are denser. Buzz is tightly regulated and, for such a sensitive response to nitrates, the plant needs a gene that is discreet and tightly regulated. That’s what made it so hard to find.”
The regulation of gene expression saves energy, and its expression happens when it takes encoded information and turns it into a function, in this case the search for nitrates. Rather than having genes expressing all the time, it is more efficient that they turn on only when they are required.
She said it is possible that the buzz gene is function specific and what makes it unique is that it is expressed in only a few specific cells.
“For this you need to do really basic molecular biology. We conducted experiments where we take the gene and make a fusion with a reporter gene.”
In molecular biology, a reporter gene is one that researchers attach to a regulatory sequence of another gene of interest.
“You try to record where it is localized and where it is expressed but this gene is expressed at such short, low levels that you can’t visualize it.”
She said that, because its expression is at such low levels, it had never been described before, which made finding it very challenging.
Understanding the gene is still a work in progress. Sanguinet said that when they found what type of gene it was, it didn’t seem to make sense given what the gene appeared to be.
“It is a kinase-like gene, and these genes typically regulate the cell cycle. But root hairs don’t grow by cells; they grow by polarized tip growth. All this is something we are trying to figure out.”
To study the gene, the research team used the plant Brachypodium distachyon, commonly called purple false brome. It is closely related on a genetic level to wheat and barley.
While the buzz gene initiates the growth of root hairs, the hairs do not necessarily elongate but grow twice as fast as wild-type roots, giving the root its fuzzy, buzz-style look. In addition, lateral roots showed more sensitivity to nitrate than primary roots.
Sanguinet studies crop and model species. The latter are valuable because they lay the groundwork for research in crops that can be difficult to transform and study specific gene functions. With the discovery of the buzz gene and validating its biological role, the team are looking deeper at the mechanism.
“Half the battle is getting to this point,” she said. “Now we’re finding the cool stuff about how plants use the gene that is very specific to nitrate and root systems. Figuring out how plants work is the joy of why we do this.”
She stressed that, given the concerns about climate change, it is important to understand how plant roots grow and seek out nutrients in the soil.
“Most of what we know about root systems and how they grow and develop is based on Arabidopsis, a mustard plant,” she said. “It has this primary tap root. But what we found working with Brachypodium is that the buzz gene phenotype is actually different. It is working in a different capacity to that of Arabidopsis, although it is still working in root cell development.
“But we are thinking it is really specific to grasses because the gene isn’t regulated in the same way by nitrate in Arabidopsis as it is in grasses. Just knowing that there are grass-specific nitrate responsive genes is a huge finding and hugely applicable to farmers. Nitrogen and inorganic nutrients are essential and any way you can improve on that is important. What we have tapped into is the nitrate signalling mechanism in grasses that occurs early and affects not only root hair growth but also root architecture, including primary root growth and lateral root growth.
“I can’t say too much as it’s not published yet but we are looking at applicability and translatability from Brachypodium to crops. Right now, we are trying to look at what other proteins buzz interacts with and how it reacts in a co-dependent manner. We are looking specifically at the primary nitrate response and genes that are known to be involved in early nitrate signalling. The second step is determining how well conserved this mechanism is in crops to see if there are the same growth responses in (agricultural) plants.”
The research was published in the journal New Phytologist.
