The horrors of nematodes may never cease, but they can teach us some surprising things, including a method of plant control that may completely alter our approach in dealing with the nematodes themselves.
Infestation and Takeover
The first step to explaining this new insight is to break down how the nematodes infect plants in the first place. Some of the biochemistry involved has already been explained in past Bioscription articles, but today’s focus will be on the tumor-like growth that the nematodes cause to form on the plant, commonly called a gall.
A prime target for root-knot nematodes is right in their name, they go after the roots of plants they parasitize. In the process, they form a gall known as a, you guessed it, root-knot. In their juvenile stage, the worms burrow into the roots until they reach the vascular system inside used for transferring water and nutrients up from the roots to other parts of the plant. Once arrived at their destination, the nematodes release a number of specialized molecules called effector proteins, which change the functionality of surrounding plant cells.
These former vascular cells begin to bloat and transform into what is termed giant cells, which are enormous cells with multiple nuclei inside of them and that have rapid metabolic processes for nutrient production. And it is these nutrients that the juvenile nematodes then live off of until adulthood.
Much of this has already been understood by biologists, but the questions they still have include just what genetic alteration these effector proteins enact in order to mutate normal cells into giant cells. The process in which they form is unlike normal cellular growth and seems to be as if they started the process of cellular division, without actually splitting into separate cells, staying one engorged mass.
Many different types of giant cells may develop in human beings during some sort of infection, but it is still not understood how or why this occurs. Meaning the process in plants is even more obscure. In addition, the nematodes induce irregular organogenesis that causes the roots they have infected to begin growing sideways during the final stages of gall formation. It is believed that this increases the feeding site availability for future infection, but just how it causes this to occur on a molecular level is not elucidated just yet.
Stem Cell Production
During the early embryo phase of a plant’s growth, the vascular system of the xylem and the phloem are produced via cells in an area called the procambium. It sits in between the two and produces cells that form into vascular system cells. In this manner, the cells of the procambium itself are considered to be multipotent stem cells and they don’t lose their potency over time. Even once the plant has grown, the procambium continues to retain a so-called “stem cell pool” for producing more vascular cells if needed, such as if the plant is damaged in some way.
Recent research has found that a number of phytohormones are involved in controlling the activity of the procambium during embryogenesis. Especially auxins, which are a specialized class of hormones involved in plant growth and development. A study earlier this year suggested that cyst nematodes, a closely related group to the root-knot nematodes, are able to control the procambium’s cell proliferation by using one of these hormonal pathways. Though the root-knot nematode variety don’t appear to have this ability.
Transcriptomics and Transgenics
But they may use something similar in their capacity to turn normal vascular cells into giant cells. In a collaboration between Kumamoto University, Universidad de Castilla, Tohoku University, and Nara Institute, researchers decided to probe this particular capacity of root-knot nematodes by using an extensive transcriptome sequencing. They used transgenic Arabidopsis as a model organism for the experiment, which they purposefully infected with root-knot nematodes from cucumber and mustard roots.
They used histology analyses in order to first confirm that the proliferation of cells and gall growth starts in the vasculature, then followed this up with a transcriptome profiling of different stages of gall growth to see what RNA signals are being expressed in the plant cells in order to cause this change. What they found is that the cells of the gall more closely resemble that of the stem cells in the procambium than they do the xylem or phloem.
Their investigation of the transcriptome included using GUS gene markers transgenically added to confirm the activation of certain genes. In this manner, they found that, indeed, procambium-related genes had been activated by the nematodes within the cells that formed the galls.
A New Way To Grow Plants
This suggests that both root-knot nematodes and cyst nematodes use protein activators and other hormonal hijacking in order to take control of stem cell production in order to form their galls for feeding. They may use different specific types of procambium stem cell control, but both are able to either force the procambium to produce more cells to cause a cell bloating to form a gall or cause existing vascular cells to begin turning into giant cells through procambium gene activation.
Figuring out that nematodes are capable of controlling stem cell growth and activation is a huge find. Since this isn’t an ability that scientists have been able to control themselves. By further studying the particular proteins and other molecules used by the nematodes, future research may allow us to control plant stem cell growth as well.
This would allow us to find ways to directly combat the nematodes and their infections, while also allowing a huge diversification of crop growth by controlling how they grow. This may prove critical for agriculture in the future.
Photo CCs: Root cyst nematode infection from Wikimedia Commons