Why would a plant that doesn’t live in a wildfire zone need to be able to smell smoke?

Wildfires can sweep through a landscape bringing death en masse to plants. But when fire clears so many plants, an opportunity opens up for the survivors. Seeds protected from the flames can wait until they smell smoke to germinate, using a “smoke detector” protein called KAI2, but where did this skill come from? Angelica Guercio and colleagues examined plants that aren’t adapted for fire to see what KAI2 proteins do for them.

The KAI2 proteins are something that plants developed early on in their evolution. The proteins interact with chemicals called karrikins. Karrikins are a particularly useful chemical for plants to know about because they’re produced by burning vegetation. If a plant can smell karrikins, they know other plants have a problem, so karrikins can trigger germination in seeds for new plants to sprout.

So valuable is this ability that Flematti and colleagues have suggested that seeds first began to be able to detect smoke in the Cretaceous period, when angiosperms, plants that produce seeds, were evolving rapidly. These days karrikins can induce germination in some plants that are not otherwise fire-adapted, including the botanists’ favourite model plant, Arabidopsis.

The plant that Guercio and colleagues studied was Pisum sativum, the garden pea, another plant that doesn’t usually cope well with fire. The researchers looked at peas because they belong to the legumes, one of the largest families of flowering plants, including several crop species. Legumes can ‘fix’ nitrogen from the air through a symbiosis with microbes.

They found that the original KAI2 gene was duplicated early in the evolution of legumes, producing two genes, KAI2A and KAI2B. Using various advanced techniques from genetics, biochemistry and protein crystallography, they found that the two receptors react to distinct ligands. A ligand is the chemical that a receptor receives. A ligand irreversibly binds to a receiving protein molecule, the receptor triggering responses inside a cell.

Guercio and colleagues used microscopes to examine cells at the ångström scale. An ångström is just one ten-billionth of a metre, the kind of scale you’d use to measure the distance between atoms. Examining the cells so closely, they could see not just the ligand and the receptor but the process of how the two combine. The authors write: “Unlike the D14 α/β hydrolase, mass spectrometry analysis and structural examination reveal a mode of ligand perception and hydrolysis by PsKAI2B, that involves an intermediate step in which the catalytic serine is transiently bound to a moiety of the ligand and then forms a stable adduct with the catalytic histidine.”

The team found that KAI2B reacted to a wide range of ligands, including a class of plant hormones called strigolactones. Strigolactones affect a wide variety of processes in plants, including the growth of roots and shoots and how root networks react to fungi and microbes in the soil. Understanding how these strigolactones work in a plant could help develop more efficient crops that require less fertilizer.

RESEARCH ARTICLE

Guercio, A.M., Torabi, S., Cornu, D., Dalmais, M., Bendahmane, A., Le Signor, C., Pillot, J.-P., Le Bris, P., Boyer, F.-D., Rameau, C., Gutjahr, C., de Saint Germain, A. and Shabek, N. (2022) “Structural and functional analyses explain Pea KAI2 receptor diversity and reveal stereoselective catalysis during signal perception,” Communications Biology. https://doi.org/10.1038/s42003-022-03085-6