A novel biochemical mechanism is involved:
Key players in these plant immune responses are so-called immune receptors, which detect the presence of molecules delivered by foreign microorganisms and set in motion protective responses to repel the invaders.
A subset of these immune receptors harbors specialized regions known as toll-interleukin-1 receptor (TIR) domains and function as enzymes, special proteins that break down the molecule nicotinamide adenine dinucleotide (NAD+), a highly abundant, multi-functional small molecule found in all living cells. Breakdown of NAD+, in turn, activates additional immune proteins, ultimately culminating 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 way to protect the plant as whole. However, studies have shown that breakdown of NAD+, while essential, is not sufficient for plant protection, suggesting that additional mechanisms must be involved…
Using structural analysis, the authors could show that TIR proteins form different multi-protein structures for breakdown of NAD+ or RNA/DNA, explaining how one and the same protein can carry out two roles. To cleave the RNA/DNA molecules, the TIR proteins follow the contours of the RNA/DNA strands and wind tightly around them like pearls on a string. The ability of TIR proteins to form two alternative molecular complexes is a characteristic of the entire immune receptor family. The exact shape of the TIR proteins thus dictates the respective enzyme activity.
The authors went on to show that this function itself was not enough for cell death, suggesting that specific small molecules generated by the breakdown of RNA and DNA were responsible. Using analytical chemistry, the scientists could identify the molecules as cAMP/cGMP (cyclic adenosine monophosphate/cyclic guanosine monophosphate), so-called cyclic nucleotides that are present in all kingdoms of life. Intriguingly, rather than the well-characterized 3 ,5 -cAMP/cGMP, the authors analysis showed that the TIR domains were triggering the production of the so-called non-canonical 2 ,3 -cAMP/cGMP, enigmatic “cousins,” whose precise roles have thus far been unclear. When they reduced TIR-mediated production of 2 ,3 -cAMP/cGMP, cell death activity was impaired, demonstrating that the 2 ,3 -cAMP/cGMP molecules are important for the plant immune response. Max Planck Society, “Function follows form in plant immunity” at Phys.org (May 20, 2022)
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We keep discovering these novel mechanisms that all work together like an exquisitely designed system. And it is all supposed to have just happened by natural selection. Yet we never see that working in any situation where we try it.
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