Figure 1. Arabidopsis thaliana infected with G. orontii, at 15 days post infection (dpi). The visible white powdery mildew is comprised of masses of asexual reproductive structures, conidiophores.
Image Credit: Jyoti Taneja.

Figure 2. A light microscopic image of a colony of G. orontii at 5 dpi on A. thaliana leaf stained with Trypan Blue. The image shows the germinated spore (blue arrow), haustoria in epidermal cells (red arrow), mycelium spread over the leaf surface, and conidiophores containing new spores (black arrow). Magnification 200X.
Image Credit: Divya Chandran

Figure 3. Masses of G. orontii conidiophores on the surface of an Arabidopsis leaf. Magnification 80X.
Image Credit: Jyoti Taneja.

Golovinomyces orontii isolate MGH1 is in the Golovinomyceae tribe of the Erysiphaceaea family of Ascomycota fungi. G. orontii causes powdery mildew disease on Arabidopsis thaliana (Figure 1) and may also colonize other members of the Brassicaceae as well as Solanaceae and Cucurbitaceae (Micali et al. 2008). Like all powdery mildews, it is an obligate biotroph that requires a living plant host for its growth and reproduction. The asexual infection cycle includes germination of the conidium (spore) on a leaf surface and differentiation of the germ tube to form an appressorium that penetrates the plant cell wall to gain entry. Successful colonization then requires formation of a specialized structure, the haustorium (Figure 2), which is encased in a plant-derived membrane within the host epidermis. The powdery mildew draws nutrients and water from the host via this structure, which also appears to export fungal effectors. Once a functioning haustorium is established, epiphytic hyphal growth continues resulting in expansion of the colony on the leaf surface as well as secondary penetration and formation of haustoria. Conidiophores, asexual reproductive structures that contain chains of conidia, are observed at 5 days post infection (Figures 2 and 3). Masses of conidiophores look like a white powder (Figure 1), thus the name powdery mildew. The most mature conidia at the tips of conidiophores are released into the wind, propagating the infection. Sexual reproduction has not been observed or studied for G. orontii; it is likely it would occur infrequently in the field, similar to findings with other powdery mildews.

The genome of G. orontii strain MGH1 was sequenced by JGI as part of a Community Science Program Project (CSP) #1657, Comparative Genomics of Powdery Mildews and Associated Host Plants, led by Mary Wildermuth and Shauna Somerville at the University of California at Berkeley and involving a large group of international collaborators. The powdery mildew fungi are one of the most ubiquitous plant pathogen groups worldwide, and are well-established models of biotrophic interactions with over 100 years of research history. The genome and transcriptome data provided through this CSP will allow comparative analyses of phylogenetically diverse powdery mildew species infecting a broad range of host plants, as well as comparisons with other parasitic biotrophic microbes (e.g., rust and smut fungi, Oomycetes) and mutualistic biotrophs, such as arbuscular mycorrhizal fungi. It will facilitate our understanding of conserved powdery mildew genes/processes required for successful infection and divergent powdery mildew genes/processes that dictate host specialization. It will also accelerate studies by the powdery mildew community to enhance the resistance of economically important crops to powdery mildew infection, limit fungicide use, and facilitate more sustainable agricultural practices.
The G. orontii MGH1 isolate DNA and RNA was provided by the Wildermuth laboratory at UC Berkeley.

Reference:
Micali, C., Gollner, K., Humphry, M., Consonni, C., and R. Panstruga. (2008) The Powdery Mildew Disease of Arabidopsis: A Paradigm for the Interaction between Plants and Biotrophic Fungi. The Arabidopsis Book, doi: 10.1199/tab.0115