  We investigate sustenance and dependence on magnetic Prandtl number (${\rm
Pm}$) for magnetorotational instability (MRI)-driven turbulence in
astrophysical Keplerian disks with zero net magnetic flux using standard
shearing box simulations. We focus on the turbulence dynamics in Fourier space,
capturing specific/noncanonical anisotropy of nonlinear processes due to disk
flow shear. This is a new type of nonlinear redistribution of modes over
wavevector orientations in Fourier space -- the nonlinear transverse cascade --
which is generic to shear flows and fundamentally different from usual
direct/inverse cascade. The zero flux MRI has no exponentially growing modes,
so its growth is transient, or nonmodal. Turbulence self-sustenance is governed
by constructive cooperation of the transient growth of MRI and the nonlinear
transverse cascade. This cooperation takes place at small wavenumbers (on the
flow size scales) referred to as the vital area in Fourier space. The direct
cascade transfers mode energy from the vital area to larger wavenumbers. At
large ${\rm Pm}$, the transverse cascade prevails over the direct one, keeping
most of modes' energy contained in small wavenumbers. With decreasing ${\rm
Pm}$, however, the action of the transverse cascade weakens and can no longer
oppose the action of direct cascade which more efficiently transfers energy to
higher wavenumbers, leading to increased resistive dissipation. This undermines
the sustenance scheme, resulting in the turbulence decay. Thus, the decay of
zero net flux MRI-turbulence with decreasing ${\rm Pm}$ is attributed to
topological rearrangement of the nonlinear processes when the direct cascade
begins to prevail over the transverse cascade.
