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Tumor formation is a characteristic symptom of corn smut disease caused by the fungus Ustilago maydis. During the development of tumors, cytokinin levels are elevated. Cytokinins are plant hormones that stimulate cell growth and cell division. Although the level of cytokinins is increased, the biosynthetic origins of these cytokinins have not been determined. Are they produced by the corn or the fungus? Both can produce cytokinins when grown independently. In addition, it is unknown as to whether the increased production of cytokinins contribute towards the formation of tumors. To investigate these phenomena, several genes and proteins potentially involved in the biosynthesis and signalling of cytokinins were identified in both organisms. The plant proteins responsible for the biosynthesis and signalling of cytokinins are different from the fungal proteins and this allowed us to determine the changes in protein transcript levels over the course of pathogenesis by using reverse transcription PCR. The results indicate that the increase in cytokinins originate from both the corn and the fungus proteins. In addition, increased cytokinin signalling was also observed during the formation of tumors. This led to a model of the interaction in which the fungus secretes active cytokinins into the corn host. The increased production of active cytokinins leads to increased cytokinin signalling which promotes excessive cell division and the formation of tumors. The enhanced insight into how U. maydis induces the formation of tumors will aid in developing molecular approaches to inhibit tumour formation and mitigate the negative impacts of fungal diseases.
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