Salicylic acid is vital for protecting plants from pathogens, but its synthesis remains unclear. A recent study by Shinshu University researchers has discovered that the protein HSR201 is key to its production. They found that HSR201 localizes to specific organelles called peroxisomes through a unique targeting signal. This discovery improves our understanding of how plants produce salicylic acid and could pave the way for developing engineered crops with improved disease resistance.
Plant hormones, or phytohormones, are vital for plant growth, adaptation, and defense. One key hormone, salicylic acid, is crucial for plant immunity and is produced through two main pathways: the isochorismate synthase (ICS) pathway and the phenylalanine ammonia lyase (PAL) pathway. Many plant hormones are produced through a process called β-oxidation in peroxisomes, including the pathway that makes salicylic acid. While the PAL pathway also involves β-oxidation, its exact mechanism remains unclear.
Researchers have recently identified the protein benzyl alcohol O-benzoyltransferase HSR201, which contributes to salicylic acid production in tobacco and may play a key role in the PAL pathway. To further investigate this, a team led by Dr. Shinpei Katou, Associate Professor at the Graduate School of Science and Technology, Shinshu University, along with Assistant Professors Dr. Yukako Tokutake and Dr. Akira Hosomi, and Professors Katsuharu Saito and Shinichi Yonekura, all from Shinshu University, studied how HSR201 localizes within plant cells. They discovered that HSR201 is located in peroxisomes, small cell compartments responsible for metabolic processes and that its localization is mediated by a unique targeting signal essential for salicylic acid production. Their study was published online in Plant and Cell Physiology on October 29, 2024.
The team traced the location of HSR201 in plant cells by tagging it with a yellow fluorescent marker called mVenus and comparing it with a red fluorescent marker for peroxisomes (mCherry-PTS1). "For the first time, we discovered that HSR201 predominantly localizes in the peroxisome, a finding that was quite unexpected," shares Dr. Katou. The team captured striking fluorescent images showing HSR201, tagged with mVenus, glowing in yellow as it accumulated specifically in peroxisomes, highlighted in red—sparking an investigation into the mechanism behind this precise localization.
To understand how HSR201 localizes in peroxisomes, the researchers created a series of HSR201 mutants with small changes in their amino acid sequence. They found that only specific sequences present in the protein structure of HSR201 allowed it to enter the peroxisomes, thereby identifying a unique targeting signal known as the peroxisomal targeting signal (PTS) at the protein's end or C-terminus, which was key to this process. "Unlike typical peroxisomal targeting signals, the PTS of HSR201 is unique and does not fit the usual consensus sequence. This was a surprising discovery and indicated that HSR201 uses a specialized pathway to reach the peroxisome," Dr. Katou explains.
They conducted more experiments which showed that a molecule called PEX5, guides HSR201 to the peroxisomes by recognizing its unique PTS signal. This confirmed that HSR201 uses the PTS1 pathway, rather than the alternative PTS2 pathway, both of which are involved in protein localization to peroxisomes. To confirm HSR201's role in salicylic acid production, the researchers introduced both the normal and mutated versions of the HSR201 protein into tobacco plants.
"Our main discovery was that only the wild-type HSR201, which was directed to the peroxisomes, increased salicylic acid levels when the plants were exposed to pathogens," says Dr. Katou. "This showed that for plants to effectively defend themselves, HSR201 must be located in the peroxisomes." The team found that HSR201's unique PTS was functional in yeast but not in human cells, revealing species-specific differences in protein transport. These results highlighted the potential role of this protein in creating disease-resistant crops by focusing on the salicylic acid pathway.
This study sheds light on how the HSR201 pathway and peroxisomal targeting contribute to salicylic acid production in plants. The findings offer new opportunities for developing disease-resistant crops, supporting food security and sustainable agriculture in line with the United Nation's Sustainable Development Goals.
Dr. Katou reflects on future directions stating, "We discovered that the unique non-canonical PTS1 pathway is crucial for the proper localization of HSR201 in peroxisomes, which is vital for salicylic acid biosynthesis. However, the exact role of this pathway, compared to the typical (canonical) PTS1 pathway, remains unclear. We aim to improve our understanding of this process, which could help develop new strategies to potentially enhance crop resilience and sustainability."
