Plants spend most of their lives making energy through photosynthesis. However, within the initial stage after the seeds have germinated, they can’t yet absorb the sunshine. During this short but critical window, they depend on stored fatty acids. To break down these fatty acids, plant cells use a specialized structure called a peroxisome, a membrane-bound compartment also present in human cells. Because of their size and visibility, plant cells function a useful system for studying how peroxisomes function.
“The plant we use, Arabidopsis, has giant 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. “During the seed to seedling stage when the plant is relying on fatty acids for energy the peroxisome becomes even larger, before shrinking back to its normal size when the plant can photosynthesize.”
The protein PEX11 helps control peroxisome size.
Bartel’s team focuses on these enlarged peroxisomes, specifically a protein called PEX11. Scientists have long known that PEX11 plays a task in helping peroxisomes divide. In the brand new study, published in Nature Communications, the team found that this protein also helps control how peroxisomes expand and contract during early plant development.
“Peroxisomes are involved in some human diseases and are used in bioengineering,” said Nathan Tharp, first writer of the paper and a Rice graduate student. “However, they can be rather difficult to study.”
Using CRISPR to review a fancy protein
A typical strategy for understanding a protein is to knock out the gene answerable for making it and observe the consequences. In this case, the situation was more complicated. PEX11 is produced by five different genes. Disrupting just one among them had little effect, but removing all five caused the plant to die. This made it difficult to pinpoint the protein’s function.
To solve this problem, Tharp used sophisticated CRISPR techniques to selectively inactivate different combos of 5 genes.
“I was able to use recent advances in CRISPR to break down specific combinations of five genes,” said Tharp, who recently defended his thesis.
Giant peroxisomes exhibit growth control mechanisms.
Tharp engineered two forms of mutant plants, each missing a particular set of PEX11 genes. In each cases, peroxisomes proliferated during seeding until the seedling stage as expected. However, as an alternative of shrinking back to their normal size, some continued to grow far beyond normal limits. In extreme cases, peroxisomes extend from one end of the cell to the opposite.
These mutant cells also lacked vesicles, small membrane-bound compartments that normally form inside peroxisomes during fatty acid processing. Under normal conditions, these vesicles develop because the peroxisome grows and appear to remove parts of its outer membrane.
“The membrane fragments that form may help control peroxisome 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, which are larger than normal.”
The results extend beyond plants to other species.
Although the research focused on plants, Tharp desired to know if the identical mechanism might exist in other organisms. To test this concept, he introduced a yeast version of the protein, called Pex11, into mutant plant cells.
“We put yeast Pex11 into our mutant plant cells to see if it could restore peroxisomes to normal,” Tharp said. “And it happened.”
This result suggests that Pex11 has an identical function in yeast because it does in plants, despite the vast evolutionary distance between them. Because of this, the protein may play a comparable role in other forms of cells, including human cells.
“Finding that this protein plays 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 simple study model, may thus be applicable to human cells and cells used for bioengineering.”