Plants spend most of their lives using photosynthesis to make energy. However, in the earliest phase after a seed begins to grow, they cannot yet capture light. During this short but critical window, they depend on stored fatty acids instead. To break down these fatty acids, plant cells use a specialized structure known as the peroxisome, a membrane-bound compartment also found in human cells. Because of their size and visibility, plant cells serve as a useful system for studying how peroxisomes work.
"The plant we use, Arabidopsis, has large cells and peroxisomes so large that we can see inside them with a light microscope," said Bonnie Bartel, the Ralph and Dorothy Looney Professor of Biosciences. "The peroxisome gets even larger during the seed to seedling stage, when the plant is relying on fatty acids for energy, before shrinking back down to its normal size once the plant can photosynthesize."
Protein PEX11 Helps Control Peroxisome Size
Bartel's team focuses on these enlarged peroxisomes, particularly a protein called PEX11. Scientists have long known that PEX11 plays a role in helping peroxisomes divide. In new research published in Nature Communications, the team found that this protein also helps control how peroxisomes expand and shrink during early plant development.
"Peroxisomes are implicated in some human diseases and used in bioengineering," said Nathan Tharp, first author of the paper and a Rice graduate student. "They can, however, be rather tricky to study."
Using CRISPR To Study a Complex Protein
A common strategy for understanding a protein is to disable the gene responsible for making it and observe the effects. In this case, the situation was more complicated. PEX11 is produced by five different genes. Disrupting just one of them had little effect, but removing all five caused the plant to die. This made it difficult to pinpoint the protein's function.
To get around this problem, Tharp used advanced CRISPR techniques to selectively disable different combinations of the five genes.
"I was able to use recent advances in CRISPR to go in and break specific combinations of the five genes," said Tharp, who recently defended his thesis. "It was only then that we were able to see that PEX11 is clearly involved in controlling the growth of the peroxisome during the seed to seedling stage."
Giant Peroxisomes Reveal Growth Control Mechanism
Tharp engineered two types of mutant plants, each missing a particular set of PEX11 genes. In both cases, peroxisomes expanded during the seed to seedling stage as expected. However, instead of shrinking back to their usual size, some continued growing far beyond normal limits. In extreme cases, the peroxisomes stretched from one end of the cell to the other.
These mutant cells also lacked vesicles, small membrane-bound compartments that typically form inside the peroxisome during fatty acid processing. Under normal conditions, these vesicles develop as the peroxisome grows and appear to remove portions of its outer membrane.
"The vesicles taking pieces of membrane as they form may help control the peroxisome's growth," Tharp said. "In our PEX11 mutants, these vesicles either don't form or are abnormally small and rare, and so we see these massive peroxisomes, way larger than normal."
Findings Extend Beyond Plants to Other Species
Although the research focused on plants, Tharp wanted to know whether the same mechanism might exist in other organisms. To test this idea, he introduced the yeast version of the protein, called Pex11, into the mutant plant cells.
"We put yeast Pex11 into our mutant plant cells to see if it could return the peroxisomes back to normal," Tharp said. "And it did."
This result suggests that Pex11 serves a similar function in yeast as it does in plants, despite the vast evolutionary distance between them. Because of this, the protein may also play a comparable role in other types of cells, including human cells.
"Finding that this protein fills the same role in yeast and plant cells suggests that it may be a highly conserved protein," Bartel said. "Our findings in plants, in this relatively easy-to-study model, may thus be applicable to human cells and cells used for bioengineering."
