Editors' ChoicePlant Innate Immunity

Differential Treatment in the ER

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Science Signaling  17 Nov 2009:
Vol. 2, Issue 97, pp. ec368
DOI: 10.1126/scisignal.297ec368

The Arabidopsis thaliana pattern recognition receptors (PRRs) EFR and FLS2 detect the bacterial proteins EF-Tu and flagellin, respectively. Activation of these two structurally similar PRRs leads to stimulation of mitogen-activated protein kinases (MAPKs), production of reactive oxygen species (ROS), seedling growth inhibition (SGI), and powdery mildew resistant 4 (PMR4)–mediated deposition of callose (a polysaccharide produced during pathogen infection). Nekrasov et al. identified Arabidopsis strains with mutations in stromal cell–derived factor-2 (sdf2) that were insensitive to SGI caused by the EF-Tu–derived peptide elf18. As determined by MAPK activation and ROS production, sdf2-2 plants showed reduced activation of EFR by elf18 treatment compared with wild-type plants; activation of FLS2 by the flagellin-derived peptide flg22 was decreased to a lesser extent. sdf2-2 plants were more susceptible than wild-type plants to infection by a virulent bacterium and by two different necrotopic fungi. Fluorescently tagged SDF2 localized to the endoplasmic reticulum (ER), where it interacted with the co-chaperone protein ERdj3B and, through ERdj3B, with the chaperone BiP. Similar to sdf2-2 plants, erdj3b-1 plants were insensitive to elf18-induced SGI and showed reduced ROS production in response to elf18 and, to a lesser extent, to flg22. The abundance of fluorescently tagged EFR was reduced in sdf2-2 plants compared with efr-1 plants; furthermore, their relative resistance to endoglycosidase H treatment indicated that EFR was localized in the ER in sdf2-2 plants and at the plasma membrane in efr-1 plants. Plants with mutations in staurosporine and temperature sensitive-3A (SST3A), a component of the ER complex involved in N-glycosylation of proteins, were insensitive to SGI induced by elf18, but not by flg22, suggesting that N-glycosylation was more critical for EFR function than for FLS2 function. In a second paper, Saijo et al. independently explored the role of ER quality control in EFR function. They searched for mutations (which they called “priority in sweet life” or psl) that could relieve the inhibitory effect of elf18 and flg22 on the sucrose-mediated enhancement of accumulation of anthocyanins. In response to elf18 but not to flg22, psl1 and psl2 plants showed reduced ROS production, MAPK activation, and PMR4-mediated callose deposition. Furthermore, psl1 and psl2 plants, like efr plants, had higher colony numbers than wild-type plants when sprayed with bacteria. The abundance of endogenous EFR was lower in psl1 and psl2 plants than in wild-type plants. Positional cloning determined that PSL1 encoded the ER chaperone protein calreticulin 3 (CRT3), whereas PSL2 encoded UDP-glucose:glycoprotein glucosyltransferase (UGGT), an ER enzyme that serves as a sensor for unfolded proteins. Similar to psl1 and psl2 plants (and to the results obtained by Nekrasov et al.), sst3a plants were defective in the responses to elf18, but not to flg22, as assessed by anthocyanin accumulation, ROS production, and MAPK activation. Saijo et al. suggest that components of the ER quality control complex are involved in plant innate immunity to prevent premature activation of PRRs, because plants must compromise other processes, such as growth, when mounting immune responses.

V. Nekrasov, J. Li, M. Batoux, M. Roux, Z.-H. Chu, S. Lacombe, A. Rougon, P. Bittel, M. Kiss-Papp, D. Chinchilla, H. P. van Esse, L. Jorda, B. Schwessinger, V. Nicaise, B. P. H. J. Thomma, A. Molina, J. D. G. Jones, C. Zipfel, Control of the pattern-recognition receptor EFR by an ER protein complex in plant immunity. EMBO J. 28, 3428–3438 (2009). [PubMed]

Y. Saijo, N. Tintor, X. Lu, P. Rauf, K. Pajerowska-Mukhtar, H. Häweker, X. Dong, S. Robatzek, P. Schulze-Lefert, Receptor quality control in the endoplasmic reticulum for plant innate immunity. EMBO J. 28, 3439–3449 (2009). [PubMed]

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