Plant disease resistance: Chloroplast protein gets guarded

signaling update home

SOURCE:

It has been found that following the infection of a plant with the tobacco mosaic virus (TMV), the viral helicase protein and a chloroplast protein form a complex that is recognized by a plant immune receptor.

After tobacco mosaic virus (TMV) infects plants, the viral helicase protein and a chloroplast protein form a complex that is recognized by a plant immune receptor, report Caplan et al. in Cell. Moreover, the chloroplast protein is required for defence against TMV. This is the first time that a chloroplast protein has been shown to have a direct role in pathogen recognition by a plant innate immune receptor.

Plants lack an adaptive immune system, so each cell must mount an individual defence upon infection. A successful defence against pathogens limits damage to a few cells at the site of infection, but if defences fail, pathogens can spread throughout the plant. There are two defence systems in plant cells. The first line of defence comprises transmembrane pattern-recognition receptors, which respond to conserved pathogen signals, such as flagellin. The second line of defence involves cytoplasmic nucleotide-binding leucine-rich-repeat (NB-LRR) proteins, which are related to animal Nod (nucleotide-binding oligomerization domain) proteins and are encoded by plant R (resistance) genes. There are two main classes of NB-LRRs: CC-NB-LRRs have a coiled–coil domain and TIR-NB-LRRs have a Toll-interleukin-1 domain. Although both classes of NB-LRRs can interact directly with pathogen proteins, many NB-LRR proteins 'guard' crucial host proteins and detect interactions that are made between these proteins and pathogen virulence proteins.

Previous research from the Dinesh-Kumar laboratory indicated that TIR-NB-LRR immune-receptor-protein N interacts indirectly with the TMV p50 helicase protein to mediate an innate immune response, and that the TIR domain of N is indispensable for pathogen detection. But how does the N protein detect p50? Caplan et al. first used the immune protein N as bait in a yeast two-hybrid assay to search for host proteins that it might interact with. They pulled out a chloroplast protein that they named N receptor interacting protein 1 (NRIP1). In vitro (yeast two hybrid) and in vivo analyses (co-immunoprecipitation of tagged proteins) confirmed that N binds NRIP1. Silencing NRIP1 with RNA interference rendered plants susceptible to TMV, which showed that NRIP1 was necessary for the N-mediated immune response to this virus. Finally, NRIP1 was shown to interact with p50 in vitro and in vivo, which led the authors to propose that N guards the host protein NRIP1 to mediate an antiviral immune response.

How can a chloroplast protein interact with both a viral protein that is present in the cytoplasm and nucleus, and a cytoplasmic cellular immune protein? When the authors examined the localization of NRIP1 in transgenic plants upon transient expression of p50, they found NRIP1 in the chloroplast, cytoplasm and nucleus. Caplan et al. scrutinized protein localization in leaves, and found that in live plants NRIP1 only interacts with N if p50 is present. Targeting NRIP1 to the cytoplasm by removing the chloroplast-targeting signal failed to induce an N-mediated immune response, thereby ruling out the possibility that p50 merely alters the localization of NRIP1 to activate N.

Future work will examine how p50 alters the localization of NRIP1, as well as how the NRIP1–p50 complex triggers the plant innate immune response.

Susan Jones

References

Caplan, J. L., Mamillapalli, P., Burch-Smith, T. M., Czymmek, K., & Dinesh-Kumar, S. P. Chloroplastic protein NRIP1 mediates innate immune receptor recognition of a viral effector. Cell 132, 449–462 (2008) | Article | PubMed |