Function Follows Form in Plant Immunity – Genetics News

Scientists from the Max Planck Institute for Plant Breeding Research (MPIPZ) and the University of Cologne, Germany, have discovered a new biochemical mechanism explaining how immune proteins defend plants against invading microorganisms. Their findings are published in the journal Cell.

We humans rely on our immune system to protect us from diseases caused by harmful microorganisms. In the same way, plants also develop immune responses when invaded by harmful microbes. Key players in these plant immune responses are the so-called immune receptors, which detect the presence of molecules delivered by foreign microorganisms and trigger protective responses to fend off invaders.

A subset of these immune receptors house specialized regions called toll-interleukin-1 receptor (TIR) ​​domains and function as enzymes, special proteins that break down the molecule nicotinamide adenine dinucleotide (NAD+), a very abundant multifunctional small molecule present in all living cells. NAD Breakdown+, in turn, activates additional immune proteins, ultimately resulting in the so-called “hypersensitive response”, a protective mechanism that leads to the death of plant cells at sites of attempted infection as an effective means of protecting the plant in its entirety. However, studies have shown that the degradation of NAD+although essential, is not sufficient for plant protection, suggesting that additional mechanisms must be involved.

The authors, led by corresponding authors, Jijie Chai, affiliated with MPIPZ, University of Cologne and Tsinghua University in Beijing, China, Paul Schulze-Lefert of MPIPZ and Bin Wu of the School of Biological Sciences from Nanyang Singapore Technological University examined the function of TIR proteins and was able to show that these receptors not only break down NAD+, but curiously possess an additional function – the TIR domains also processed molecules with phosphodiester bonds, usually present in RNA and DNA, which are present in cells mainly as large linear single- or double-stranded molecules. Using structural analysis, the authors were able to show that TIR proteins form different multiprotein structures for NAD degradation+ or RNA/DNA, explaining how the same protein can fulfill two roles. To cleave RNA/DNA molecules, TIR proteins follow the contours of RNA/DNA strands and wrap tightly around them like beads on a thread. The ability of TIR proteins to form two alternative molecular complexes is a feature of the entire family of immune receptors. The exact shape of the TIR proteins thus dictates the respective enzymatic activity.

The authors then showed that this function itself was not sufficient for cell death, suggesting that specific small molecules generated by the degradation of RNA and DNA were responsible. Using analytical chemistry, scientists were able to identify the molecules as cAMP/cGMP (cyclic adenosine monophosphate/cyclic guanosine monophosphate), so-called cyclic nucleotides that are present in all kingdoms of life. Curiously, rather than the well-characterized 3′,5′-cAMP/cGMP, the authors’ analysis showed that the TIR domains triggered the production of the so-called non-canonical, enigmatic 2′,3′-cAMP/cGMP” cousins ”, whose precise roles have so far been unclear. When they reduced TIR-mediated 2′,3′-cAMP/cGMP production, cell death activity was impaired, demonstrating that 2′,3′-cAMP/cGMP molecules are important for the plant immune response.

If 2′,3′-cAMP/cGMP promotes cell death in plants in response to infection, it stands to reason that their levels would be tightly controlled. Indeed, the authors found that a known negative regulator of TIR function in plants, NUDT7, acts by depleting 2′,3′-cAMP/cGMP. Similar negative regulators are released by certain pathogenic microorganisms upon infection inside plant cells, and scientists could show that these pathogenic proteins also deplete 2′,3′-cAMP/cGMP. This suggests that invading microorganisms have evolved clever strategies to disarm the 2′,3′-cAMP/cGMP-dependent plant defense mechanism to their own advantage.

Dongli Yu, one of the three co-first authors of this study, along with Wen Song and Eddie Yong Jun Tan, summarizes the importance of his study thus:

“We have identified a novel role for the TIR domain of immune receptors in protecting plants against infection. In the future, the identification and characterization of 2′,3′-cAMP/cGMP targets will suggest new strategies to make plants more resistant to harmful microbes and thus contribute to food security. »

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Function Follows Form in Plant Immunity – Genetics News

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