The Maternal-to-Zygotic Transition in Flowering Plants: Evidence, Mechanisms, and Plasticity

The maternal-to-zygotic transition (MZT) defines a developmental phase during which the embryo progressively emancipates itself from a developmental control relying largely on maternal information. The MZT is a functional readout of two processes: the clearance of maternally derived information and the de novo expression of the inherited, parental alleles enabled by zygotic genome activation (ZGA). In plants, for many years the debate about whether the MZT exists at all focused on the ZGA alone. However, several recent studies provide evidence for a progressive alleviation of the maternal control over embryogenesis that is correlated with a gradual ZGA, a process that is itself maternally controlled. Yet, several examples of zygotic genes that are expressed and/or functionally required early in embryogenesis demonstrate a certain flexibility in the dynamics and kinetics of the MZT among plant species and also intraspecific hybrids.