Scientists from the Max Planck Institute for Breeding Research (MPIPZ) and the University of Cologne, Germany, have discovered a new biochemical mechanism that explains how immune proteins protect plants from the invasion of 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. Similarly, plants also strengthen the immune response to the invasion of 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 repel invaders.
A subset of these immune receptors contains specialized regions known as interleukin-1 (TIR) receptor domains, and function as enzymes, special proteins that cleave the nicotinamide adenine dinucleotide (NAD) molecule.+), a multifunctional small molecule found in all living cells. Breakdown of NAD+, in turn, activates additional immune proteins, which eventually culminates in the so-called “hypersensitivity reaction”, a protective mechanism that leads to the death of plant cells at the sites of attempted infection as an effective way to protect the plant as a whole. However, studies have shown that the destruction of NAD+although necessary, but not enough to protect plants, which indicates the need to use additional mechanisms.
Authors led by relevant authors Ji Chai, who is affiliated with MPIPZ, Cologne University and Tsinghua University in Beijing, China, Paul Schulze-Lefert of MPIPZ and Bin Wu of the School of Biological Sciences, Nanyang Technological University, Tsingga show that these receptors not only destroy NAD+, but intriguingly have an additional function – TIR domains have also processed molecules with phosphodiester bonds, which are commonly found in RNA and DNA, which are present in cells mainly in the form of 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 decay.+ or RNA / DNA, explaining how the same protein can play two roles. To break down RNA / DNA molecules, TIR proteins follow the contours of RNA / DNA strands and tightly wind them like pearls on strands. The ability of TIR proteins to form two alternative molecular complexes is a characteristic of the whole family of immune receptors. Thus, the exact form of TIR proteins dictates the activity of the corresponding enzyme.
The authors further showed that this function alone is not enough for cell death, suggesting that certain small molecules formed by the breakdown of RNA and DNA are responsible for this. Using analytical chemistry, scientists have been able to identify molecules like cAMP / cGMP (cyclic adenosine monophosphate / cyclic guanosine monophosphate), so-called cyclic nucleotides present in all kingdoms of life. Interestingly, the analysis of the authors, instead of the well-characterized 3 ‘, 5′-cAMP / cGMP, showed that TIR domains trigger the development of so-called non-canonical 2′, 3’-cAMP / cGMP, the mysterious “cousins”, whose exact roles When they reduced TIR-mediated 2 ‘, 3′-cAMP / cGMP production, cell death activity was disrupted, indicating that 2′, 3’-cAMP / cGMP molecules are important for plant immune responses.
If 2 ‘, 3’-cAMP / cGMP contribute to cell death in plants in response to infection, it is clear that their levels will be kept under control. 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 some pathogens during infection inside plant cells, and scientists have been able to show that these pathogenic proteins also deplete 2 ‘, 3’-cAMP / cGMP. This suggests that the invading microorganisms have developed clever strategies to disarm the 2 ‘, 3’-cAMP / cGMP-dependent plant defense mechanism for their own benefit.
Dongli Yu, one of the three co-authors of this study, along with Wen Song and Eddie Yong Jun Tang, summarizes the significance of his study as follows:
“We have identified a new role for TIR immune receptors in protecting plants from infection. Identification and characterization of 2 ‘, 3’-cAMP / cGMP targets will propose new strategies to make plants more resistant to harmful microbes. and thus promote food security ”.