The biotrophic smut fungus Ustilago maydis infects all aerial organs of maize (Zea mays) and induces
tumors in the plant tissues. U. maydis deploys many effector
proteins to manipulate its host. Previously, deletion analysis demonstrated that several effectors have important functions in inducing
tumor expansion specifically in maize leaves. Here, we present the functional characterization of the effector See1 (Seedling efficient effector1). See1 is required for the reactivation of
plant DNA synthesis, which is crucial for
tumor progression in leaf cells. By contrast, See1 does not affect
tumor formation in immature tassel floral tissues, where maize cell proliferation occurs independent of
fungal infection. See1 interacts with a maize homolog of SGT1 (Suppressor of G2 allele of skp1),
a factor acting in cell cycle progression in yeast (Saccharomyces cerevisiae) and an important component of plant and human innate immunity. See1 interferes with the MAPK-triggered phosphorylation of maize SGT1 at a monocot-specific phosphorylation site. We propose that See1 interferes with SGT1 activity, resulting in both modulation of immune responses and reactivation of
DNA synthesis in leaf cells. This identifies See1 as a fungal effector that directly and specifically contributes to the formation of leaf
tumors in maize.