Understanding How Fire Blight Resistance Proteins Work to Protect Plants

Greg Howard
20th July, 2024

Understanding How Fire Blight Resistance Proteins Work to Protect Plants

Image Source: Natural Science News, 2024

Key Findings

  • Researchers at Seoul National University identified a protein, MrMR5, from wild apple species Malus × robusta 5 that helps resist fire blight
  • MrMR5 detects a bacterial molecule called EaAvrRpt2, triggering the apple tree's immune response
  • A specific protein fragment, ACP3, produced by EaAvrRpt2, activates MrMR5, highlighting a potential path for developing fire blight-resistant apple varieties
Bacterial fire blight is a devastating disease affecting apple trees, caused by the bacterium Erwinia amylovora. Traditional methods to combat this disease have had limited success, prompting researchers to explore genetic resistance as a more effective solution. In a recent study conducted by Seoul National University, a specific protein from wild apple species Malus × robusta 5 (MrMR5) has been identified as a key player in conferring resistance to fire blight[1]. MrMR5 is an NLR (nucleotide-binding and leucine-rich repeat receptor) protein. NLR proteins are part of the plant's immune system and detect pathogen effectors, which are molecules secreted by pathogens to facilitate infection. The study revealed that MrMR5 recognizes a cysteine protease effector called EaAvrRpt2, secreted by Erwinia amylovora. This recognition is crucial for triggering the plant's immune response. Interestingly, the researchers found that MrMR5 is activated by a specific cleavage product of Malus domestica RIN4 (MdRIN4), a protein targeted by EaAvrRpt2. This cleavage product, termed ACP3, is produced through EaAvrRpt2-directed proteolysis. The study further explored the functionality of a closely related NLR protein, MbMR5, from another wild apple species, Malus baccata. MbMR5 shares 99.4% amino acid sequence identity with MrMR5 but shows auto-activity when expressed in Nicotiana benthamiana, a model plant used for studying plant-pathogen interactions. To understand the differences between MrMR5 and MbMR5, the researchers conducted domain swap and mutational analyses. They discovered that a single amino acid polymorphism in the MbMR5 CC domain is critical for its enhanced auto-activity. When MrMR5 was engineered to carry seven amino acid polymorphisms present in MbMR5, it was not activated by MdRIN4 ACP3 but could still recognize AvrRpt2 without MdRIN4 in Nicotiana benthamiana. These findings indicate that naturally occurring polymorphisms in MR5 variants can alter their cell death-inducing activity and effector recognition mechanisms, likely due to changes in compatibility with RIN4. This research builds on previous studies that have highlighted the importance of NLR proteins in plant immunity. For example, a study on Arabidopsis thaliana identified that NLRs like RPM1 and RPS2 are activated by RIN4-targeting effectors, triggering immune responses[2]. Another study demonstrated that NRG1, a subclass of RPW8-NLR proteins, is essential for the signaling of multiple TIR-NLRs, further emphasizing the role of NLR proteins in plant defense[3]. Moreover, the study on AvrRpt2 in Arabidopsis thaliana showed that this effector protein undergoes N-terminal processing and eliminates RIN4 through proteolytic cleavage, which is crucial for its virulence activity[4]. The current study extends this knowledge by demonstrating that MrMR5 can recognize EaAvrRpt2 and trigger an immune response, providing a new avenue for developing fire blight-resistant apple varieties. Overall, the research conducted by Seoul National University highlights the potential of leveraging naturally occurring polymorphisms in NLR proteins to enhance disease resistance in crops. By understanding the molecular mechanisms underlying NLR-mediated immunity, scientists can develop more effective strategies to combat plant diseases and improve agricultural productivity.

GeneticsBiochemPlant Science

References

Main Study

1) Comparative analysis on natural variants of fire blight resistance protein FB_MR5 indicates distinct effector recognition mechanisms.

Published 17th July, 2024

https://doi.org/10.1016/j.mocell.2024.100094

Related Studies

2) Ptr1 and ZAR1 immune receptors confer overlapping and distinct bacterial pathogen effector specificities.

https://doi.org/10.1111/nph.19073

3) Diverse NLR immune receptors activate defence via the RPW8-NLR NRG1.

https://doi.org/10.1111/nph.15659

4) Molecular characterization of proteolytic cleavage sites of the Pseudomonas syringae effector AvrRpt2.

Journal: Proceedings of the National Academy of Sciences of the United States of America, Issue: Vol 102, Issue 6, Feb 2005


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