AM fungal exudates activate MAP kinases in plant cells in dependence from cytosolic Ca2+ increase

Received 3 January 2011 Accepted 18 April 2011 Available online 23 April 2011 Keywords: Arbuscular-mycorrhizal fungi Signaling Diffusible factor MAPK Calcium Sym pathway 1. Introduction Protein phosphorylation plays a central role in propagating signals from the cell surface to the nucleus initiated by biotic or abiotic stress [1e3]. The recognition of the presence of a potential pathogen on a leaf surface has been repeatedly shown to be signaled via mitogen-activated protein kinase (MAPK) cascades. In the model plant Arabidopsis thaliana, the three proteins MPK3, MPK4, and MPK6 appear to be activated irrespective of the identity of the pathogen [4]; other MAPKs may also be involved in this process, but these have been only rarely identi!ed as yet, perhaps because of their low abundance and/or faint activation [1]. The same 3 kinases and their homologues in other species have been shown to be activated also by a range of abiotic stresses [5e10]. As examples, in Nicotiana sp., the activity of both SIPK and WIPK (orthologues of AtMPK6 and AtMPK3, respectively) is inducible not only by various pathogens, but also by wounding and various abiotic stresses [11]. MAPKs are not * Corresponding author. Tel.: !39 0116708875; fax: !39 0112368875. E-mail address: francesca.cardinale@unito.it (F. Cardinale). 0981-9428/$ e see front matter ! 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.plaphy.2011.04.008 abstract The molecular dialogue occurring prior to direct contact between the fungal and plant partners of arbuscular-mycorrhizal (AM) symbioses begins with the release of fungal elicitors, so far only partially identi!ed chemically, which can activate speci!c signaling pathways in the host plant. We show here that the activation of MAPK is also induced by exudates of germinating spores of Gigaspora margarita in cultured cells of the non-leguminous species tobacco (Nicotiana tabacum), as well as in those of the model legume Lotus japonicus. MAPK activity peaked about 15 min after the exposure of the host cells to the fungal exudates (FE). FE were also responsible for a rapid and transient increase in free cytosolic Ca2! in Nicotiana plumbaginifolia and tobacco cells, and pre-treatment with a Ca2!-channel blocker (La3!) showed that in these cells, MAPK activation was dependent on the cytosolic Ca2! increase. A partial dependence of MAPK activity on the common Sym pathway could be demonstrated for a cell line of L. japonicus defective for LjSym4 and hence unable to establish an AM symbiosis. Our results show that MAPK activation is triggered by an FE-induced cytosolic Ca2! transient, and that a Sym genetic deter- minant acts to modulate the intensity and duration of this activity.