Natural Cellular Molecules that Drive Key Plant Immune Responses Discovered

Posted by Ilsa Miller on August 1st, 2022

Researchers have discovered natural cellular molecules that drive key plant immune responses. All of the features of these compounds are small messengers customized by factories to open key defense control centers. Using these insights, scientists and plant breeders can design molecules that make plants, including many important crop varieties, more resistant to disease.

By 2050, world food production must double to feed the projected increase of 2 billion people. Increasing food production requires increasing the yield of many of our staple crops. To this end, strategies need to be developed to ensure that we can make plants more resistant to microscopic infectious agents while ensuring that food production is environmentally safe. In turn, to do this requires a detailed understanding of the plant immune system, the plant’s defense system in the face of invading microbes. Now, in two landmark studies, scientists led by the Max Planck Institute for Plant Breeding in Cologne, Germany, and Chai Jijie and Jane Parker at the University of Cologne, in collaboration with Chang Junbiao's team at Zhengzhou University and Han Zhifu and colleagues at Tsinghua University in Beijing, China, have identified two types of molecules and determined their mode of action in mediating plant intracellular immune responses. Their findings pave the way for the design of bioactive small molecules that allow researchers and plant growers to manipulate, and thus improve plant resistance to harmful microorganisms.

At the molecular level, a major immunization strategy employed by plants involves a protein called nucleotide-binding leucine-rich repeat receptors, referred to as NLRs. nlr is activated by invading microbes and initiates motility to protect immune responses. These immune responses culminate in so-called hypersensitivity reactions, which include limiting the growth of pathogens and often strictly limiting the death of cells at the site of infection, similar to cutting toes to ensure physical survival.

A class of NLR proteins, with the so-called toll/interleukin-1 receptor (TIR) domain, known as TIR-nlrs (or TNLs), has been shown to transmit signals to downstream immune proteins enhancing disease susceptibility 1 (EDS1). Smaller tir-containing proteins also signal EDS1 to enhance disease resistance. EDS1 acts as a control hub, pushing plant cells to restrict pathogen growth or cause cell death depending on the other protein types that interact with it. Earlier studies showed that TNL receptors and TIR proteins are in fact pathogen-induced enzymes. Evidence suggests that these TIR enzymes generate one or more small messengers that signal EDS1 within the cell. However, the identity of the precise molecules produced by TNLs or TIRs that stimulate different immune responses remains difficult to determine.

Parker and colleagues demonstrated that TNL enzymes activated by pathogens within plant cells can trigger two functionalized EDS1 modules, leading to immunity or cell death. To identify small molecules produced by TNLs or TIRs and acting on EDS1, the Chai group recombined key components of signaling pathways in insect cells, a system that can produce and purify large numbers of molecules, which are then isolated and characterized. With this approach, the authors found two different classes of modified nucleotide molecules generated by TNLs and TIRs. These compounds preferentially bind to and activate the different EDS1 subcomplex. Thus, the authors demonstrate that different EDS1 subcomplexes recognize specific tir-producing molecules that act as information-carrying chemicals to promote immune responses.

TIR immune receptors and EDS1 central proteins are present in many important crop species, such as rice and wheat, and Chai Jijie pointed out: "The identified TIR catalytic small molecules can be used as general and natural immunostimulants to control crop diseases.” “Understanding the biochemical patterns of these small molecule actions opens a whole new chapter for plant immune signaling and disease management," Jane Parker further points out."

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