Title Co-option of EDM 2 to distinct regulatory modules in Arabidopsis thaliana development

Background: Strong immunity of plants to pathogenic microorganisms is often mediated by highly specific mechanisms of non-self recognition that are dependent on disease resistance (R) genes. The Arabidopsis thaliana protein EDM2 is required for immunity mediated by the R gene RPP7. EDM2 is nuclear localized and contains typical features of transcriptional and epigenetic regulators. In addition, to its role in immunity, EDM2 plays also a role in promoting floral transition. This developmental function of EDM2, but not its role in RPP7-mediated disease resistance, seems to involve the protein kinase WNK8, which physically interacts with EDM2 in nuclei. Results: Here we report that EDM2 affects additional developmental processes which include the formation of leaf pavement cells and leaf expansion as well as the development of morphological features related to vegetative phase change. EDM2 has a promoting effect of each of these processes. While WNK8 seems not to exhibit any vegetative phase change-related function, it has a promoting effect on the development of leaf pavement cells and leaf expansion. Microarray data further support regulatory interactions between WNK8 and EDM2. The fact that the effects of EDM2 and WNK8 on leaf pavement cell formation and leaf expansion are co-directional, while WNK8 counteracts the promoting effect of EDM2 on floral transition, is surprising and suggests that WNK8 can modulate the activity of EDM2. Conclusion: We propose that EDM2 has been co-opted to distinct regulatory modules controlling a set of different processes in plant immunity and development. WNK8 appears to modulate some functions of EDM2. Background The defense regulator EDM (Enhanced Downy Mildew) 2 was previously shown in Col (Columbia) accessions of Arabidopsis thaliana (Arabidopsis) to be specifically required for immunity mediated by the disease resistance (R)-gene RPP (Resistance to Peronospora parasitica) 7 against the Hiks1 isolate of the pathogenic oomycete Hyaloperonospora arabidopsidis (formerly Peronospora parasitica; Hpa) [1]. Unlike many other plant defense mechanisms RPP7-mediated immunity is independent from the defense hormone salicylic acid. Furthermore, RPP7-mediated immunity appears thus far to be the only defense mechanism that EDM2 is involved in, as no other EDM2-dependent R-gene functions have been reported yet. EDM2 also does not contribute to basal defense, a weaker non-specific plant immune response [2]. Thus, the role of EDM2 in plant defense seems to be restricted to a single or a limited number of defense pathways. The EDM2 protein is nuclear-localized and bears typical features of transcription factors and epigenetic regulators, but does not belong to any established class of plant proteins controlling transcription [1,2]. Consistent with a possible role in transcriptional regulation, EDM2 seems to contribute to disease resistance by promoting transcription of RPP7 [1]. We recently reported that a second function of EDM2 is the regulation of flowering time, as edm2 mutants exhibit a delay in floral transition [2]. Consistent with this phenotype, transcript levels of the negative flowering regulator gene FLC (Flowering Locus C) were substantially elevated in edm2 mutants. As theses edm2 phenotypes are independent from the photoperiod, EDM2 may formally be considered as a member of the autonomous floral promotion pathway, which controls the floral transition by suppressing FLC expression independent from day-length [3]. * Correspondence: thomas.eulgem@ucr.edu Center for Plant Cell Biology, Institute for Integrative Genome Biology, Department of Botany and Plant Sciences, University of California at Riverside, Riverside, CA 92521, USA Tsuchiya and Eulgem BMC Plant Biology 2010, 10:203 http://www.biomedcentral. com/1471-2229/10/203 © 2010 Tsuchiya and Eulgem; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. EDM2 interacts in nuclei with the WNK (With No Lysine) 8 protein kinase that can phosphorylate EDM2 [2]. Mutants of this kinase or WNK8 overexpressor lines, however, do not exhibit reduced immunity to HpaHiks1, but wnk8 mutants flower early [2,4]. Double mutant analyses showed that EDM2 is epistatic to WNK8. Similarly, FLC proved to be epistatic to EDM2. Thus, WNK8 may act upstream of EDM2 in a regulatory module affecting the floral transition by modulating FLC transcript levels [2]. In this model, WNK8 counteracts the promoting effect of EDM2 on flowering. Here we report that besides its roles in immunity and the floral transition, EDM2 has several additional functions in Arabidopsis development, such as promoting vegetative leaf growth and leaf pavement cell morphology. WNK8 also has a promoting effect on these processes, suggesting that it participates in these EDM2 functions. However, in contrast to its role in the floral transition, WNK8 seems not to counteract the function of EDM2, but to positively contribute to EDM2 activity. Overlapping transcript profiles of edm2 and wnk8 mutants support common biological roles of both regulators. In addition, EDM2 seems to have a WNK8-independent role in a developmental process related to vegetative phase change. We propose that EDM2 has multiple distinct regulatory functions related to developmental processes and defense. These EDM2 functions differ with respect to their dependency on WNK8. A possible function of WNK8 may be to fine-tune EDM2 activity in some biological processes.