Two mitogen-activated protein kinase signalling cascades mediate basal resistance to antifungal plant defensins in Fusarium graminearum

Abstract

SummaryAntifungal defensins, MsDef1 and MtDef4, from Medicago spp., inhibit the growth of Fusarium graminearum, which causes head blight disease in cereals. In order to determine the signalling cascades that are modulated by these defensins, we have isolated several insertional mutants of F. graminearum that exhibit hypersensitivity to MsDef1, but not to MtDef4. The molecular characterization of two of these mutants, designated enhanced sensitivity to defensin (esd), has revealed that the Mgv1 and Gpmk1 MAP kinase signalling cascades play a major role in regulating sensitivity of F. graminearum to MsDef1, but not to MtDef4. The Hog1 MAP kinase signalling cascade, which is responsible for adaptation of this fungus to hyperosmotic stress, does not participate in the fungal response to these defensins. Significantly, the esd mutants also exhibit hypersensitivity to other tested defensins and are highly compromised in their pathogenesis on wheat heads and tomato fruits. The studies reported here for the first time implicate two MAP kinase signalling cascades in a plant defensin-mediated alteration of fungal growth. Based on our findings, we propose that specific MAP kinase signalling cascades are essential for protection of a fungal pathogen from the antimicrobial proteins of its host plant.

The Tig1 Histone Deacetylase Complex Regulates Infectious Growth in the Rice Blast Fungus Magnaporthe oryzae

“…Plant cells appeared to mount strong defense responses and displayed HR-like cell death. The tig1 mutant also had increased sensitivities to PR protein osmotin and plant defensin MsDef1 (Coca et al, 2000;Ramamoorthy et al, 2007). It is possible that attenuated invasive growth of the tig1 mutant triggered strong defense responses in host cells.…”

“…Hyphal growth was completely inhibited by 5 mM H 2 O 2 in the tig1 mutant KT-1 but not in Guy11 ( Figure 5A). We also assayed the effects of two plant proteins, osmotin and MsDef1, that are toxic to fungal pathogens (Coca et al, 2000;Ramamoorthy et al, 2007). In the presence of 50 mg/mL osmotin, appressorium formation was normal in Guy11 but blocked in KT-1 on hydrophobic surfaces ( Figure 5B).…”

“…For RNA isolation, infected leaves of cultivar Sariceltik were sampled at 1, 2, 3, and 4 DAI. Osmotin (Coca et al, 2000) and MsDef1 (Ramamoorthy et al, 2007) were added to conidium suspensions to final concentrations of 20 and 50 mM, respectively for assaying their effects on germination and appressorium formation.…”

Antimicrobial peptides

“…MAP kinase signaling cascades in F. graminearum regulate the fungus sensitivity to plant defensins. 83 It is giving proof that plant defensins can act as stimuli to launch MAP kinase signaling cascades involved in regulating the fungal genes important for overcoming the plant defense. It would seem, upon binding of defensins to their receptors, activation of MAPKKK(s) occurs owing to physical interaction and/ or phosphorylation by either the receptor itself or intermediary factors or an interlinking kinase.…”

“…It would seem, upon binding of defensins to their receptors, activation of MAPKKK(s) occurs owing to physical interaction and/ or phosphorylation by either the receptor itself or intermediary factors or an interlinking kinase. 83 Moreover, MsDef1 has been characterized to block the mammalian L-type Ca 2+ channel in a manner akin to structurally unrelated antifungal toxin KP4 from Ustilago maydis. 84 It is, as well, documented that bamboo defensins-like AMPs, PpAMP1 and PpAMP2, bind to chitin in microbial cell walls, 85 and Aspergillus giganteus defensin, AFP, inhibits the chitin biosynthesis in susceptible fungi.…”