Rice plants release momilactones, a group of so-called allelopathic compounds that inhibit the growth of nearby competing plant species. If other plants could make these compounds, they could serve as biological herbicides.
Now, Elizabeth S. Sattely and Ricardo De La Peña of Stanford University have figured out the complete momilactone biosynthetic pathway, which could pave the way to engineering other plants to make the compounds (Nat. Chem. Biol. 2020, DOI: 10.1038/s41589-020-00669-3).
“Momilactones, I thought, were a really cool case because they had this known function. We thought if we could move the pathway, maybe we can move the function as well,” Sattely says. “We haven’t made stable lines or anything like that, but our work demonstrates that you can take the whole metabolic pathway, put it in another plant, and then isolate a compound that has the same [herbicidal] activities.”
To identify the missing pieces of the pathway, the researchers transiently expressed candidate genes in tobacco plants. To boost yields in tobacco plants, they needed to redirect some reactions from chloroplasts to the cytosol.
“You need to analyze whether your enzyme of interest has a chloroplast localization signal,” De La Peña explains. “If it does, then the only thing you have to do is truncate this peptide sequence, and the enzyme will no longer be able to relocate from the cytosol to the chloroplast.”
With this redirection, tobacco plants were able to synthesize 10 times as much momilactone as rice plants. Momilactone B extracted from tobacco plants inhibited the growth of Arabidopsis thaliana, verifying that the compound works even when made by a plant other than rice.
“Elucidation of this pathway has been a target for some time due to the import of the momilactones as the only really bona fide plant allelochemical. Identification of the relevant tailoring enzymes allows more precise dissection of this natural product, which may provide structure-function relationships for such allelochemical activity,” Reuben J. Peters, a biologist at Iowa State University who previously elucidated part of the momilactone pathway, says in an email. “Perhaps more importantly, it is now possible to consider importing the entire pathway into other plants to impart allelopathic properties that could serve as a natural weed suppression mechanism.”