Plant immunityswitched from bacteria to virus
Artemis Giannakopoulou1, Aleksandra Bialas1, Sophien Kamoun1 & Vivianne G. A. A. Vleeshouwers2
1The Sainsbury Laboratory, Norwich Research Park, Norwich, NR4 7UH, UK, 2Wageningen UR Plant Breeding, Wageningen University and Research Centre, Droevendaalsesteeg 1, Wageningen 6708 PB, The Netherlands. AG and AB contributed equally. e-mail: or
A plant immune receptor is engineered to recognize viruses rather than bacteria.
Each year, staple crops around the world suffer massive losses in yieldowing to the destructive effects ofpathogens. Improvingthe disease resistanceof crops by boosting their immunity has been a key objective of agricultural biotechnology ever since the discovery of plant immune receptors in the 1990s. Nucleotide-binding leucine-rich repeat (NLR) proteins, a family of intracellularimmune receptors that recognize pathogen molecules, are promising as targetsfor engineering pathogen resistance [AU:OK?]. In a recent paper published in Science, Kim et al.1switched the specificity of an NLR receptor from bacteria to viruses by modifying a pathogen effector target protein instead of the NLRreceptor itself (Fig. 1).
Severalstrategieshave emergedto improve diseaseresistance in crop plants. (Fig. 1).The efficient transfer of NLR genes between plant species in the laboratory has suggested opportunities for engineering broad disease resistance in the field2. However, the limited specificity of naturally occurring NLR proteins might hinder the success of this approach. NLR variants that recognizea broader range of pathogen effectorshave been generated through in vitro evolution and rational design (Fig 1b)3,4.A related approach is to exploit additional domains found on some NLRs that function as effector baits (Fig. 1c)5.
NLR proteins recognize pathogen effectors in two ways, either directly, by binding to them, or indirectly, by sensing host proteins that are modified by pathogen effectors. Kim et al.1focus on modification of the effector target in the host cell(Fig. 1d)rather than modification of the NLRprotein itself (Fig. 1). They take advantage of their earlier finding that proteolytic cleavage of an Arabidopsis thalianahost protein PBS1 by the Pseudomonas syringaeprotease effector AvrPphB activates the NLR protein RPS56. They have also shown7 that RPS5 can be bound and activated by a three amino-acid insertion at the cleavage site ofPBS1 protein in the absence of any effector.
These findings prompted Kim et al.1to substitute the AvrPphB cleavage site in PBS1 with a ‘decoy’ sequence that matches the cleavage sites of two different proteases [AU:OK?].The PBS1 protein was engineered into two forms that can be cleaved by protease effectors of Turnip mosaic virus (TuMV) in addition to a Pseudomonas syringae protease effector. This resulted in activation of RPS5 and a specificity switch of RPS5/PBS1 to the different pathogensin planta.
The strategy proposed by Kim et al.1has some limitations.First,PBS1functions inside plant cells, so it can only be engineered to confer resistance to pathogens that secrete a host-translocated protease, for example, barley powdery mildew fungus BEC1019 [REF. 8]. However, most agronomically important pathogens,including oomycetes and rust fungi, are not known to secrete host-translocated proteases.
Second,any mechanism of resistance must block pathogen colonization quickly and effectively. Although Kim et al.1switched RPS5/PBS1 specificity from Pseudomonas syringaeto Turnip mosaic virus, systemic spread of the virus was not prevented, and infection with the virus resulted in a trailing necrosis phenotype1.Such a phenotype would preclude commercial applications of the system.
Third, the RPS5/PBS1 proteins are present in the plasma membrane and thereforemight have to be redirected to other cellular compartments to respond to effectors that have different subcellular localizations.
Finally, plant pathogen effectors are typically functionally redundant and tend to be dispensable. Thus, the ability of a pathogen to overcome NLR-mediated resistance is not necessarily dependent on the mechanism by which a particular effector is detected, but is rather a productof the exceptional capacity of these pathogens to carry nimble and rapidly evolving effector repertoires9. Owing to these large repertoires, multiple versions of engineered PBS1 might haveto be combined to maximize the potential for durable resistance.
In general,plant biotechnologists have focused on engineering broad-specificity immune receptorsin order to keep pace withrapidlyevolvingplantpathogens. Engineered immune receptors have been used in the field 3,10 and have become important tools for the development of resistant crops. By manipulating the pathogen targets in the host, Kim et al.1have substantially expanded the options for engineering synthetic plant immunity.The importance of basic research into plant immunity for devising different approaches to subvert and expandplant immunity should not be underestimated.
COMPETING FINANCIAL INTERESTS
The authors declare competing financial interests: details are available in the online version of the paper.
Figure 1: Engineeringplant immune system.
Two phylogenetically unrelated pathogens secrete effectors into the plant cell cytoplasm. a. The wild-type NLR perceives secreted effectors (red circle) from apathogen with high specificity. Activation of the NLR upon effector binding triggers immune responses that result inplantresistance. Synthetic NLRs can be engineered to respond toeffectors from differentpathogens using two general approaches. b. Single amino acid changes in the NLR (light green line) or c. Integration of an effector target domain (light green oval shape) in the NLR. d. Modification of the effector target (dark and light green circle) results in a specificity switch to a different effector (brown shape). Red and brown shapes represent effectors secreted by pathogens. Light and dark green oval shapes represent effector targets in the host.
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