Iron as a determinant of virulence and resistance in the Colletotrichum graminicola – maize interaction
Anja Raschke*, Mario Lange, Emad Albarouki, Holger B. Deising
*
Key words: Colletotrichum graminicola, maize, plant-pathogen interaction, iron uptake, fungal iron homeostasis
In all kingdoms of life iron is an essential microelement. However, iron is hardly soluble in aerobic biogeosphere. Therefore, all living beings evolved strategies for an efficient uptake of iron. On the other hand, the redox mediation of iron can produce highly reactive oxygen species - via the Fenton- and Haber-Weiss-Reaction - damaging bio-molecules and ultimately organisms. Thus, a tight regulation of iron uptake and storage is essential. Pathogenic fungi employ several strategies for iron uptake from the host tissue: (i) reductive iron assimilation (RIA), (ii) siderophore-mediated Fe3+ acquisition (SIA), (iii) heme uptake, and (iv) low affinity iron uptake. As free heme is rare in the maize host plant of Colletotrichum graminicola, this hemibiotrophic fungus mainly applies RIA and SIA. Saprophytically hyphae growing under iron starvation conditions leads to an up-regulation of both pathways - RIA and SIA. However, during the biotrophic stage of the infection RIA is highly active, while SIA is specifically suppressed. At thelater necrotrophic stage it is vice versa. Maize leaves pretreated with the C. graminicola siderophore Coprogen respond with a strong defense including a respiratory burst when these leaves were infected later on. However, Coprogen alone did not induce a defense response. In conclusion, C. graminicola specifically represses the SIA to cower itself. During necrotrophy hiding is not necessary anymore. This strategy resembles the specific repression of the synthesis of β-1,3-Glucane, a pathogen associated molecular pattern (PAMP), during biotrophy. From other fungal species (e.g. Aspergillus spp.)the tight regulation of the SIA und RIA pathways occurs on transcriptional level mediated via two transcription factors SreA and HapX, respectively. However, the studied Aspergillus spp. are either necrotrophs or saprophytes. Homologues of these genes were identified in the hemibiotrophic fungusC. graminicola as CgSRE1 (GLRG_02856) and CgHAP1 (GLRG_10059).Functional characterization of these transcription factors during biotrophic and necrotrophic stages will gain the knowledge of iron acquisition in fungal virulence and provide valuable data to develop novel protection strategies.With this,the long lasting question will be elucidated how the switch from biotrophy to necrotrophy operates.
REFERENCES: Albarouki & Deising 2013 MPMI 26:695; Albarouki et al. 2014 Mol Microbiol 92:338
FINANCIAL SUPPORT: Interdisciplinary Center of Crop Plant Research (Federal State Saxony-Anhalt, Germany)
PREFERED PRESENTATION FORMAT:oral
SELECTED SESSIONS:Iron interaction with the environment