  Identifying generic physical mechanisms responsible for the generation of
magnetic fields and turbulence in differentially rotating flows is fundamental
to understand the dynamics of astrophysical objects such as accretion disks and
stars. In this paper, we discuss the concept of subcritical dynamo action and
its hydrodynamic analogue exemplified by the process of nonlinear transition to
turbulence in non-rotating wall-bounded shear flows. To illustrate this idea,
we describe some recent results on nonlinear hydrodynamic transition to
turbulence and nonlinear dynamo action in rotating shear flows pertaining to
the problem of turbulent angular momentum transport in accretion disks. We
argue that this concept is very generic and should be applicable to many
astrophysical problems involving a shear flow and non-axisymmetric
instabilities of shear-induced axisymmetric toroidal velocity or magnetic
fields, such as Kelvin-Helmholtz, magnetorotational, Tayler or global
magnetoshear instabilities. In the light of several recent numerical results,
we finally suggest that, similarly to a standard linear instability,
subcritical MHD dynamo processes in high-Reynolds number shear flows could act
as a large-scale driving mechanism of turbulent flows that would in turn
generate an independent small-scale dynamo.
