The Plant's Chemical Editors: Unlocking the Secrets of the MES Enzyme Family

Plants, like all living things, are masterpieces of chemical engineering. At the heart of their ability to grow, defend themselves, and respond to their environment are complex processes of adding and removing small chemical "tags" to molecules. One of the most important tags is a methyl group.

Imagine putting a small, temporary cap on a molecule. This "methylation" can change the molecule's function, make it easier to transport, or put it into storage. An entire family of plant enzymes, called SABATH, is dedicated to putting these caps on. But what happens when the plant needs to take the cap off to activate a hormone or use a molecule? That's where a different, equally important family of enzymes comes in: the Methylesterases, or MES family. These are the plant's chemical editors, precisely snipping off methyl groups to unleash the power of key compounds.

Who are the MES Enzymes?

First discovered in the early 2000s, the MES family was found to be the functional opposite of the SABATH family. While they share the "methylesterase" name with other enzymes like those that modify plant cell walls (PMEs), they are not closely related. MES enzymes belong to a massive and ancient group of enzymes called the α/β hydrolase superfamily, but they form their own distinct evolutionary branch.

Their specialty? While some enzymes can work on a wide variety of molecules, MES enzymes are typically specialists. The vast majority of the MES members we've studied so far are known to act on the methylated forms of three crucial plant hormones:

• Salicylic Acid (SA): The hero of plant defense, crucial for fighting off bacteria, fungi, and viruses.

• Jasmonic Acid (JA): A key player in defending against insect attacks and regulating fruit ripening.

• Indole-3-acetic Acid (IAA): The master architect of plant growth and development.

By removing the methyl cap from these hormones (turning MeSA into SA, MeJA into JA, etc.), MES enzymes activate them in the right place at the right time, allowing the plant to mount a defense or carry out a developmental program.

More Than Just Hormone Regulators

While hormone regulation is their most famous job, the MES family is full of surprises. Some members have evolved to perform other vital tasks:

• The Cleanup Crew: One MES enzyme in Arabidopsis is responsible for breaking down the toxic byproducts of chlorophyll degradation during leaf aging, a critical recycling and detoxification process.

• The Resource Recyclers: Another member is involved in recycling nicotinic acid to maintain levels of NAD, a coenzyme essential for metabolism in all living things.

• The Specialty Crafters: In wild tomatoes, a unique MES-like enzyme helps synthesize methylketones, compounds that are distasteful to insects and act as a natural pest repellent. Other MES enzymes are involved in making medicinally important compounds like ajmaline.

An Evolutionary Mystery

This is where the story gets truly intriguing. To understand the family's history, scientists built an evolutionary "family tree" using genes from across the plant kingdom—from mosses and ferns to pines and flowering plants. They found that MES genes are present in all land plants but are absent in the algae they evolved from, suggesting the family originated right around the time plants first conquered land.

The analysis revealed that all MES genes fall into three major groups (I, II, and III), and that these three groups likely existed very early in land plant evolution.

Here’s the mystery: every single MES enzyme that has been studied in detail so far belongs to Group III. This one group contains all the known hormone regulators, the chlorophyll recyclers, and the specialty crafters. The other two massive groups, Group I and Group II, are a complete black box. Members of Group I are found in every single land plant, hinting at a fundamental, ancient role we have yet to discover. Group II, on the other hand, seems to have been lost in certain lineages like flowering plants.

What are the enzymes in these mystery groups doing? Are they performing ancient, essential functions we haven't thought to look for? Or have some of them evolved entirely new, non-esterase abilities, like the methylketone synthase? Answering these questions is the next frontier for understanding this vital and versatile family of plant enzymes. It’s a powerful reminder of how much we still have to learn about the intricate chemical world inside the plants all around us.