A newly discovered pathway in a plant process could help farmers grow more successful crops, particularly in places where harsh, high light stresses plants.
The pathway complements the main workflow of photorespiration, indicating photorespiration is more flexible than it seems. Xiaotong Jiang, a post-doctoral fellow in Jianping Hu's lab at the Michigan State University-Department of Energy Plant Research Laboratory, and colleagues recently published their results in the journal Nature Communications.
Photorespiration works in association with photosynthesis, acting like a cleanup crew when the precursor to photosynthesis creates harmful byproducts. Typically, the primary enzyme of photosynthesis, Rubisco, acts as a carboxylase carbon dioxide to make sugar. But it is also an oxygenase that can oxygen. When that happens, the process spits out a chemical that is harmful to the cell. To prevent this chemical from building up, photorespiration kicks in to process it into something less volatile that can be reused in photosynthesis.
Jiang and colleagues discovered that under stressful conditions, plants can take a different approach. While doing research on photorespiration for her Ph.D. thesis in the Hu lab, Jiang experimented with a lab-made mutant of the common research plant Arabidopsis thaliana. The mutant lacked the ability to produce a key photorespiration enzyme, hydroxypyruvate reductase 1, or HPR1. When grown in a high-light environment, those plants struggled to keep pace with non-mutant plants.
To figure out what was happening inside those plants, the team worked backwards by introducing random mutations in addition to the broken HPR1 and observed which plants improved. Then, they determined which genes — and the enzymes they code for — could make up for the faulty HPR1 enzyme.
They found that breaking the enzyme glyoxylate reductase 1, or GLYR1, can activate a parallel pathway involving HPR1's relative HPR2, which the researchers call a cytosolic glyoxylate shunt. GLYR1 converts glyoxylate to glycolate, but when it's turned off, glyoxylate builds up and is converted into hydroxypyruvate then glycerate by another enzyme and then glycerate by HPR2, two enzymatic steps also happening in the main photorespiration process. The shunt takes place in the cytosol of the cell, whereas similar steps of main photorespiration are in peroxisomes.
Jiang describes the shunt like a highway detour, where cars (cytotoxic chemicals) are moving along a damaged road (the photorespiration pathway). "If the road is broken, a lot of cars may get stuck in their parking lots," she said. "If there's an alternative way to go, then the cars can keep moving."
"A really important takeaway for this work is that photorespiration is quite flexible," said Amanda Koenig, a post-doctoral fellow in the Hu lab at MSU-DOE Plant Research Lab and co-author on the paper. When the main pathway is compromised for some reason, the complementary pathway can aid in processing cytotoxins. "This parallel pathway may have a lot of potential for improving energy efficiency and crop yield without compromising their resilience to stress conditions."
Although the team looked at high light conditions specifically, Jiang said the pathway could be helpful in other stressful situations — but that needs to be determined in the future.
By Jude Coleman
