Editors' ChoicePlant biology

Life and Death with Salicylic Acid

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Sci. Signal.  19 Jun 2012:
Vol. 5, Issue 229, pp. ec170
DOI: 10.1126/scisignal.2003315

When a plant is attacked by a pathogen, effector-triggered immunity is induced, which results in programmed cell death (PCD) at the site of infection and systemic acquired resistance (SAR) in other areas of the plant. SAR depends on the hormone salicylic acid (SA), which is produced at the site of infection and gradually decreases in concentration with increasing distance from the site of infection (see commentary by Gust and Nürnberger). SA controls the nuclear translocation of the transcriptional cofactor nonexpresser of PR genes 1 (NPR1), which is required for SAR. When NPR1 is degraded, cells undergo PCD, whereas when NPR1 accumulates in the nucleus, it induces the expression of genes required for SAR. Noting that degradation of NPR1 acts as a molecular switch to determine the response to infection, Fu et al. sought binding partners of NPR1 that might regulate its targeting by the CUL3 E3 ubiquitin ligase system in Arabidopsis thaliana. They found that loss of the NPR1 homologs NPR3 or NPR4, which contain domains typical of CUL3 adaptor proteins, prevented CUL3-dependent degradation of NPR1. Yeast two-hybrid studies and pull-down assays showed that NPR3 and NPR4 individually interacted with NPR1; however, NPR3 interacted with NPR1 only in the presence of SA and NPR4 interacted with NPR1 only in the absence of SA. Binding studies with [3H]-SA showed that NPR3 was a low-affinity receptor for SA, whereas NPR4 was a high-affinity receptor. Studies of wild-type or NPR3- or NPR4-mutant Arabidopsis infected with a pathogen showed that loss of either NPR3 or NPR4 had differential effects on plant responses. These data led the authors to propose a model in which SA, which is at high concentration at the site of infection, binds to NPR3, enabling it to recruit CUL3 to NPR1, which leads to NPR1 degradation and PCD. In contrast, at more distant sites, SA is lower in concentration and binds only to the high-affinity receptor NPR4, which blocks its interaction with NPR1, causing NPR1 to accumulate, translocate to the nucleus, and induce SAR.

Z. Q. Fu, S. Yan, A. Saleh, W. Wang, J. Ruble, N. Oka, R. Mohan, S. H. Spoel, Y. Tada, N. Zheng, X. Dong, NPR3 and NPR4 are receptors for the immune signal salicylic acid in plants. Nature 486, 228–232 (2012). [Online Journal]

A. D. Gust, T. Nürnberger, A life or death switch. Nature 486, 198–199 (2012). [Online Journal]

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