Fungal derived cytokinins are necessary for normal Ustilago maydis infection of maize

Phytohormones derived from fungi play a key role in regulating plant–pathogen interactions; however, deciphering the separate contributions of pathogen and plant during infection has been difficult. Here, the Ustilago maydis–Zea mays pathosystem was used to investigate this chemical exchange. Ustilago maydis, the causative agent of maize smut, produces cytokinins (CK), which are a group of phytohormones responsible for directing plant development. The characteristic symptom of smut disease is the formation of tumours composed of plant and fungal tissue. Isopentenyltransferase (IPT) catalyses the rate‐limiting step in CK biosynthesis, and U. maydis strains in which the sole tRNA‐ipt gene was deleted no longer produced CKs. These deletion strains elicited fewer, smaller tumours than the pathogenic strain SG200. High performance liquid chromatography‐electrospray ionization tandem mass spectrometry (HPLC‐ESI MS/MS) was used to detect and quantify phytohormone levels in infected tissue. This revealed that key hormone changes in SG200 infections were not present in infections by deletion strains, suggesting that CK production by U. maydis is required for the altered phytohormone profile in infected tissue relative to uninfected tissue. Separate analyses indicated that U. maydis tRNA‐ipt mutants might be altered in their ability to metabolize CKs taken up from the environment. Mining the U. maydis genome identified genes encoding putative CK signalling and biosynthesis proteins.