By Victoria G. Pook, PhD
Postdoc in the Department of Horticulture, College of Agriculture, Food and Environment
If you’ve ever made cookies, cakes or pastries, you’ll know one thing for sure – the combination of fat and sugar is irresistible! It turns out that this gastronomical wonder is also present in plentiful supply within the very cells that make up our bodies. Known as “sterol glucosides”, these molecules of fat and sugar are even more common in plants. But why are they there? Researchers are studying genetic mutants to find out.
Thale cress, also known as Arabidopsis, is a flowering plant related to cabbage and mustard. Due to the small size of its genome and the ease with which it can be grown, thale cress has become the “lab rat” of the plant science world. It is particularly popular for questions that involve genetics, and fabulous resources are available to researchers in this field.
The Salk Institute Genome Analysis Laboratory specializes in the production of mutant thale cress. Their entire catalog comprises plants in which particular genes have been switched off. Unfortunately, these mutants don’t have charismatic names such as Leonardo, Michelangelo, Donatello, or Raphael. The mutant that plays the starring role in this story is lovingly referred to as “ugt80B1”. In this mutant, the gene coding for the enzyme that makes sterol glucosides is switched off. Therefore, ugt80B1 mutants lack the molecules of fat and sugar that we are curious about.
The first step in figuring out the function of sterol glucosides is to identify distinctive mutant characteristics. One that stood out immediately was that the roots of ugt80B1 mutants are much less hairy than those of the regular, “wild type” plants – think Luke Skywalker versus Chewbacca. This was a great find! If mutants that lack sterol glucosides have roots that are less hairy, these molecules may play a role in root hair growth.
The cylindrical extensions known as root hairs increase the surface area of the root, and aid in the acquisition of water and nutrients – very important for growing plants! However, even in wild type plants, not all root cells share the same fate, only a selection will one day become hair cells. The fate of each root cell is determined by the following simple rule: if the cell contacts the boundary of two other cells, it must grow a hair; if the cell does not contact such a boundary, it must accept its destiny and remain hairless. But how does the cell know who its neighbors are?
In regular, wild type plants, cell membranes contain protein receptors that receive information about their surroundings. These proteins can be thought of as the “sensory system” of the cell, helping it to perceive its neighbors. The lack of root hairs in the mutant plants indicates that there could be a communication break down among cells. Researchers theorized that this could be due to an abnormality in the cell membrane receptors, and promptly got out their high-tech microscopes to observe the world inside the living cell.
Confocal laser scanning microscopy is powerful enough to reveal the machinery within individual cells and gentle enough to do so while the cells are still alive. This technique revolutionized biological research and plays a fundamental role in today’s discoveries. In our story, researchers used this technique to take a look at the receptor proteins inside ugt80B1 mutants. What they found was telling – the proteins were not in the membrane, rather they were drifting about aimlessly inside the cells. These mutant cells effectively had their eyes closed. It is, therefore, not surprising that they were a little disoriented and couldn’t identify their neighbors!
Why does a lack of sterol glucosides result in the loss of these receptor proteins from the membrane? We’re still trying to figure that out. What we do know is that without these molecules of fat and sugar, root cells are resigned to a fate of being both bald and blind.
This article is based on the following research paper: