Two-Component Elements Mediate Interactions between Cytokinin and Salicylic Acid in Plant Immunity

Recent studies have revealed an important role for hormones in plant immunity. We are now beginning to understand the contribution of crosstalk among different hormone signaling networks to the outcome of plant–pathogen interactions. Cytokinins are plant hormones that regulate development and responses to the environment. Cytokinin signaling involves a phosphorelay circuitry similar to two-component systems used by bacteria and fungi to perceive and react to various environmental stimuli. In this study, we asked whether cytokinin and components of cytokinin signaling contribute to plant immunity. We demonstrate that cytokinin levels in Arabidopsis are important in determining the amplitude of immune responses, ultimately influencing the outcome of plant–pathogen interactions. We show that high concentrations of cytokinin lead to increased defense responses to a virulent oomycete pathogen, through a process that is dependent on salicylic acid (SA) accumulation and activation of defense gene expression. Surprisingly, treatment with lower concentrations of cytokinin results in increased susceptibility. These functions for cytokinin in plant immunity require a host phosphorelay system and are mediated in part by type-A response regulators, which act as negative regulators of basal and pathogen-induced SA–dependent gene expression. Our results support a model in which cytokinin up-regulates plant immunity via an elevation of SA–dependent defense responses and in which SA in turn feedback-inhibits cytokinin signaling. The crosstalk between cytokinin and SA signaling networks may help plants fine-tune defense responses against pathogens

Loss of a Conserved tRNA Anticodon Modification Perturbs Plant Immunity

[EN] tRNA is the most highly modified class of RNA species, and modifications are found in tRNAs from all organisms that have been examined. Despite their vastly different chemical structures and their presence in different tRNAs, occurring in different locations in tRNA, the biosynthetic pathways of the majority of tRNA modifications include a methylation step(s). Recent discoveries have revealed unprecedented complexity in the modification patterns of tRNA, their regulation and function, suggesting that each modified nucleoside in tRNA may have its own specific function. However, in plants, our knowledge on the role of individual tRNA modifications and how they are regulated is very limited. In a genetic screen designed to identify factors regulating disease resistance and activation of defenses in Arabidopsis, we identified SUPPRESSOR OF CSB3 9 (SCS9). Our results reveal SCS9 encodes a tRNA methyltransferase that mediates the 2'-O-ribose methylation of selected tRNA species in the anticodon loop. These SCS9-mediated tRNA modifications enhance during the course of infection with the bacterial pathogen Pseudomonas syringae DC3000, and lack of such tRNA modification, as observed in scs9 mutants, severely compromise plant immunity against the same pathogen without affecting the salicylic acid (SA) signaling pathway which regulates plant immune responses. Our results support a model that gives importance to the control of certain tRNA modifications for mounting an effective immune response in Arabidopsis, and therefore expands the repertoire of molecular components essential for an efficient disease resistance response.This work was supported by the National Science Foundation of China (grant 31100268 to PC) and the Spanish MINECO (BFU2012 to PV) and Generalitat Valenciana (Prometeo2014/020 to PV). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Ramirez Garcia, V.; González-García, B.; López Sánchez, A.; Castelló Llopis, MJ.; Gil, M.; Zheng, B.; Cheng, P.... (2015). Loss of a Conserved tRNA Anticodon Modification Perturbs Plant Immunity. PLoS Genetics. 11(10):1-27. https://doi.org/10.1371/journal.pgen.1005586S127111

An allele of Arabidopsis COI1 with hypo- and hypermorphic phenotypes in plant growth, defence and fertility

Resistance to biotrophic pathogens is largely dependent on the hormone salicylic acid (SA) while jasmonic acid (JA) regulates resistance against necrotrophs. JA negatively regulates SA and is, in itself, negatively regulated by SA. A key component of the JA signal transduction pathway is its receptor, the COI1 gene. Mutations in this gene can affect all the JA phenotypes, whereas mutations in other genes, either in JA signal transduction or in JA biosynthesis, lack this general effect. To identify components of the part of the resistance against biotrophs independent of SA, a mutagenised population of NahG plants (severely depleted of SA) was screened for suppression of susceptibility. The screen resulted in the identification of intragenic and extragenic suppressors, and the results presented here correspond to the characterization of one extragenic suppressor, coi1-40. coi1-40 is quite different from previously described coi1 alleles, and it represents a strategy for enhancing resistance to biotrophs with low levels of SA, likely suppressing NahG by increasing the perception to the remaining SA. The phenotypes of coi1-40 lead us to speculate about a modular function for COI1, since we have recovered a mutation in COI1 which has a number of JA-related phenotypes reduced while others are equal to or above wild type levels.This work was supported by grant BIO201018896 from "Ministerio de Economia y Competitividad" (MINECO) of Spain and by grant ACOMP/2012/105 from "Generalitat Valenciana" to PT, a JAE-CSIC Fellowship to JVC, a FPI-MINECO to AD, and Fellowships from the European Molecular Biology Organization and the Human Frontier Science Program to BBHW. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Dobón Alonso, A.; Wulff, BBH.; Canet Perez, JV.; Fort Rausell, P.; Tornero Feliciano, P. (2013). An allele of Arabidopsis COI1 with hypo- and hypermorphic phenotypes in plant growth, defence and fertility. PLoS ONE. 1(8):55115-55115. https://doi.org/10.1371/journal.pone.0055115S551155511518Vlot, A. C., Dempsey, D. A., & Klessig, D. F. (2009). Salicylic Acid, a Multifaceted Hormone to Combat Disease. Annual Review of Phytopathology, 47(1), 177-206. doi:10.1146/annurev.phyto.050908.135202Mauch, F., Mauch-Mani, B., Gaille, C., Kull, B., Haas, D., & Reimmann, C. (2001). Manipulation of salicylate content in Arabidopsis thaliana by the expression of an engineered bacterial salicylate synthase. The Plant Journal, 25(1), 67-77. doi:10.1046/j.1365-313x.2001.00940.xGaffney, T., Friedrich, L., Vernooij, B., Negrotto, D., Nye, G., Uknes, S., … Ryals, J. (1993). 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beta-carbonic anhydrases play a role in salicylic acid perception in Arabidopsis

[EN] The plant hormone salicylic acid (SA) is required for defense responses. NON EXPRESSER OF PATHOGENESIS RELATED 1 (NPR1) and NON RECOGNITION OF BTH-4 (NRB4) are required for the response to SA in Arabidopsis (Arabidopsis thaliana). Here, we isolated several interactors of NRB4 using yeast two-hybrid assays. Two of these interactors, beta CA1 and beta CA2, are beta-carbonic anhydrase family proteins. Since double mutant beta ca1 beta ca2 plants did not show any obvious phenotype, we investigated other beta CAs and found that NRB4 also interacts with beta CA3 and beta CA4. Moreover, several beta CAs interacted with NPR1 in yeast, including one that interacted in a SA-dependent manner. This interaction was abolished in loss-of-function alleles of NPR1. Interactions between beta CAs and both NRB4 and NPR1 were also detected in planta, with evidence for a triple interaction, NRB4-beta CA1-NPR1. The quintuple mutant beta ca1 beta ca2 beta ca3 beta ca4 beta ca6 showed partial insensitivity to SA. These findings suggest that one of the functions of carbonic anhydrases is to modulate the perception of SA in plants.This study was supported by "Ministerio de Economia y Competitividad" of Spain (grant BIO2013-45444-P to PT, http://mmidi.mineco.gob.es/portal/site/MICINN/) and "Generalitat Valenciana" of Spain (grant ACOMP/2013/052 to PT, http://www.ceice.gva.es/web/ciencia/becas-ayudas-y-subvenciones-cientificas). MLC was awarded with a fellowship from CONICET (Argentina, http://convocatorias.conicet.gov.ar/becas/) and Erasmus Mundus Action 2, Arcoiris Project (EU, http://www.arcoiris.polito.it/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Medina-Puche, L.; Castelló Llopis, MJ.; Canet Perez, JV.; Lamilla, J.; Colombo, M.; Tornero Feliciano, P. (2017). beta-carbonic anhydrases play a role in salicylic acid perception in Arabidopsis. PLoS ONE. 12(7). https://doi.org/10.1371/journal.pone.0181820S12