Behind the Cover: New Phytologist 217:4, March 2018

How to make a tumour

Ustilago maydis is a biotrophic fungus that causes smut disease on maize, by reprogramming plant cells to produce massive tumours. Just how this happens at the cellular level in maize leaves has remained unknown, until now.

In the paper behind the cover of New Phytologist 217:4, Alexandra Matei and Gunther Doehlemann, of the University of Cologne, together with colleagues from Germany and the USA, describe the cellular basis of tumour formation.

When a maize leaf is infected by U. maydis, bundle sheath cells, a key component of leaves, are completely disrupted and replaced by a strongly dividing cell type. Gunther and colleagues set out to identify which cell types underwent this strong division.

Image: Cross section of a maize leaf infected by Ustilago maydis. Incorporation of EdU (green) shows DNA synthesis in the nuclei (propidium iodine, red) being induced in bundle sheath cells. Courtesy of Alexandra Matei.
Cross section of a maize leaf infected by Ustilago maydis. Incorporation of EdU (green) shows DNA synthesis in the nuclei (propidium iodine, red) being induced in bundle sheath cells. Courtesy of Alexandra Matei.

The image shows a microsection of an U. maydis infected leaf, at the beginning of the tumour induction stage, three days after infection. By monitoring DNA synthesis in vivo using staining methods, the researchers were able to identify how cells were rearranged during tumour formation.

Leaf cells were rearranged in two different ways during the formation of tumours: bundle sheath cells proliferated (hyperplasia), and mesophyll cells enlarged (hypertrophy). Using the image above, the researchers found that the initiation of cell division began inside the bundle sheath cells, three days after infection. This confirms the bundle sheath cells as the origin of hyperplastic tumour cells.

Additional notes about the method, by Alexandra Matei:

DNA synthesis was visualised in vivo via the EdU (5-ethynyl-20-deoxyuridin) staining method (green) with subsequent costaining of nuclei via propidium iodide (PI). Cell walls (blue) were visualised with a filter for cell wall autofluorescence. The picture of this stained leaf section was captured using a Nikon Eclipse Ti inverted light microscope with a blue filter for cell wall autofluorescence (425–250 nm) overlaid with pictures taken with a green filter for visualisation of EdU/AF488 (455–490 nm) and a red filter for visualisation of costaining with PI (540–580 nm).

Read the paper: Matei, A., Ernst, C., Günl, M., Thiele, B., Altmüller, J., Walbot, V., Usadel, B. and Doehlemann, G. (2018) How to make a tumour: cell type specific dissection of Ustilago maydis-induced tumour development in maize leaves. New Phytologist. doi: 10.1111/nph.14960


 

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Mike Whitfield (@mgwhitfield)
Development Coordinator
New Phytologist