The early infection phase of Ustilago maydis: adaption to the plant environment

• Applicant: Professor Dr. Gunther Döhlemann
• Subject Area: Organismic Interactions, Chemical Ecology and Microbiomes of Plant Systems ; Plant Breeding and Plant Pathology
• Term: from 2009 to 2012
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 15847441

• Final Report Year: 2013

Final Report Abstract

In this project, different aspects of the early phase of the biotrophic interaction of maize and U. maydis have been studied. A preliminary transcriptome analysis revealed that the initial defense response of maize that is induced upon inoculation of U. maydis is suppressed when biotrophy is established immediately upon epidermal penetration. Based on this observation, we hypothesized that compatibility in this biotrophic interaction depends on an active suppression of plant immunity by the pathogen. In this context, the main goals of this project were i) to identify and functionally characterize novel compatibility factors in the maize – U. maydis interaction, and ii) to study the programmed cell death responses during incompatible Ustilago-interactions. To identify maize genes that act as susceptibility factors in the U. maydis interaction, a virus induced gene silencing (VIGS) protocol was established that allows systemic gene silencing in U. maydis infected maize plants and a correlation of gene-silencing efficiency and fungal growth. This approach identified the conserved suppressor of apoptotic cell death, Bax-Inhibitor-1 (BI-1) as a compatibility factor in the maize-U. maydis interaction. Complementary, the maize terpene-synthase TPS6/11 was found to restrict fungal virulence, i.e. its silencing caused an increased susceptibility of maize to U. maydis. A novel maize compatibility factor being identified in this study is corn cystatin-9 (CC9). This apoplastic protein is an inhibitor of papain-like cysteine proteases. Silencing of cc9 causes resistance of maize to U. maydis infection, which coincides with an HR-like epidermal defense response and the transcriptional activation of SA-marker genes. This drastic phenotype could be explained by functional analysis of the CC9-targeted proteases, which revealed a central role of these apoplastic enzymes in the activation of SA-associated defense signaling in maize. Regulation of epidermal cell death was studied in barley, where infection with the maize pathogen U. maydis causes an epidermal cell death response. This defense response was partially suppressed in barley plant overexpressing BI-1. By contrast, epidermal cell death caused by Ustilago mutants deleted for the secreted effector Pep1 were not influence by BI-1, which suggests distinct programs of cell death being activated in the two types of incompatible interactions. This assumption could be substantiated by enzyme essays, transcriptional analyses and life-cell as well as transmission electron microscopy. Together, these analyses revealed hallmarks for apoptotic cell death being activated in the non-host interaction, while ∆pep1 mutants cause an apoplastic oxidative burst that induces autophagy rather than apoptosis-like cell death.

Publications

• (2010) A model of Ustilago maydis leaf tumor metabolism. Plant Signaling & Behavior 5: 11, 1-4(2010) A model of Ustilago maydis leaf tumor metabolism. Plant Signaling & Behavior 5: 11, 1-4
  Horst RJ, Doehlemann G, Wahl R, Hofmann J, Schmiedl A, Kahmann R, Kämper J, Voll LM
  
• (2010) Maize tumors caused by Ustilago maydis require organ-specific genes in host and pathogen. Science 328:89-92
  Skibbe D, Doehlemann G, Fernandes J, Walbot V
  
• (2010) Ustilago maydis infection strongly alters organic nitrogen allocation in maize and stimulates productivity of systemic source leaves. Plant Physiology 152: 293-30
  Horst R, Doehlemann G, Wahl R, Hofmann J, Schmiedl A, Kahmann R, Kämper J, Sonnewald U, Voll L
  
• (2011) Common motifs in the response of cereal primary metabolism to fungal pathogens are not based on similar transcriptional reprogramming. Frontiers in Plant Physiology 2:81-6
  Voll LM, Horst RJ, Voitsik AM, Zajic D, Samans B, Pons-Kühnemann J, Doehlemann G, Münch S, Wahl R, Molitor A, Hofmann J, Schmiedl A, Waller F, Deising HB, Kahmann R, Kämper J, Kogel HK, Sonnewald U
  
• (2011) Systemic virus induced gene silencing allows functional characterization of maize genes during the biotrophic interaction with Ustilago maydis. New Phytologist 189: 471-483
  van der Linke K, Kastner C, Kumlehn J, Kahmann R, Doehlemann G
  
• (2012) A maize cystatin suppresses host immunity by inhibition of apoplastic cysteine proteases. Plant Cell 24: 1285-1300
  van der Linde K, Hemetsberger C, Kastner C, Kaschani F, van der Hoorn RAL, Kumlehn J, Doehlemann G
  
• (2012) The maize cystatin CC9 interacts with apoplastic cysteine proteases. Plant Signaling & Behavior 7: 1397 - 1401
  van der Linde K, Mueller A, Hemetsberger C, Kaschani F, van der Hoorn RAL, Doehlemann G
  
• (2013) Utilizing virus induced gene silencing for the functional characterization of maize genes during infection with the fungal pathogen Ustilago maydis. Methods in Molecular Biology, Springer Science 975:47-60
  van der Linde K and Doehlemann G