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Negative regulation of a still to be discovered hormone

Often the messenger is discovered first and followed by the messenger recognising receptor and signalling pathway that is activated in response to this messenger. So, it appeared to be for karrakin, a messenger molecule in plant derived smoke, that activates responses in fire-adapted plants, like seed germination. Only after identifying the karrakin receptor, KAI2, in those fire-adapted plants it quickly became clear that those receptors could also be found in other plants.

Plants like Arabidopsis respond to karrakins by adjusting their growth and development. The processing of karrakin perception happens in a similar manner as the perception of the hormone strigolactone but using different protein family members and regulating a different set of genes.

This all was found out with help of karrakins. But it was the discovery that plants without KAI2 show the opposite phenotype to plants treated with karrakins, that made scientist to suggest that plants are producing karrakin-like (KL) molecules.

Negative feedback

Up till now the majority of the studies focussed on the response to karrakin(-like) and how plants regulate this. But hormone response is often held in check by negative feedback loops. Now a new study by a group of international researchers focused on this negative feedback loop.

They found a likely candidate to be part of this negative feedback loop in DLK2. DLK2 is a protein that looks a lot like KAI2 but without the ability to directly interact with the same proteins KAI2 interacts with. So, the question was how does DLK2 work.

First the researchers confirmed that DLK2 was indeed negatively regulating karrakin(-like) response. Having done that, they came up with two hypotheses. The first was that DLK2 breaks down KL molecules. The second was that DLK2 interfered with the interaction of KAI2 with its target protein, SMAX1, whose breakdown by KAI2 results in activating KL-response genes.

Hypothesis testing

Having no way of testing if DLK2 breaks down a molecule whose identity is not known, the researchers had to find a way around that. They did three tests. The first was to find out if DLK2 needs to be in the nucleus to do its job, because KAI2 and SMAX1 both need to be there to do their job.

Having created DLK2 proteins that were either localised in the nucleus or outside the nucleus. The researchers then combined those with a reporter system that is activated by SMAX1. This showed that DLK2 localised in the nucleus and localised outside the nucleus both prevented SMAX1 breakdown. Suggesting that DLK2 doesn’t need to be in the nucleus to do its job. Putting one strike against hypothesis number two.

Subsequently the researchers tested if DLK2 when in the vicinity of KAI2 and SMAX1 interfered with their interaction. Using again a reporter that lighted up when KAI2 and SMAX1 interacted the researchers noticed that with karrakin present DLK2 was not able to reduce the signal of the reporter. Suggesting that DLK2 was not interfering with the interaction between KAI2 and SMAX1. Putting a second strike against hypothesis number 2.

Breaking down karrakin like molecules

Lastly the researchers tested if DLK2 could break down karrakin-like molecules. Testing with a variety of molecules the researchers noticed that DLK2 broke down a KL-like molecule called Yoshimulactone Green, but not it closely resembling desmethyl Yoshimulactone Green. Suggesting that DLK2 has KL breakdown activity needed for the support of hypothesis number one.

Al together, this study shows that DLK2 functions as a negative regulator of karrakin-like signalling. It does this by breaking down karrakin-like molecules so they can no longer activate karrakin-like signalling. But that is not all this study shows. It also shows that even if the substrate or messenger molecule is not known, there is still a lot to be found out about how the plant regulates responding to that messenger molecule.

Literature

Q. Li, et al., 2026, Negative feedback regulation of karrikin signaling in Arabidopsis thaliana by an antagonistic paralog of karrikin receptors, Proc. Natl. Acad. Sci. U.S.A. 123 (37) e2525145123, https://doi.org/10.1073/pnas.2525145123.

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