The smut fungus Ustilago maydis needs the host plant maize for completion o
f its sexual life cycle. Recent experiments highlight the importance of cAM
P and mitogen-activated protein (MAP) kinase signalling for cell fusion as
well as for subsequent stages of plant colonisation and induction of diseas
e symptoms. During these distinct developmental stages the same signalling
cascades must be differentially regulated and accommodate multiple inputs a
nd outputs.