  One of the prime manifestations of an anisotropic electronic state in
underdoped cuprates is the in-plane resistivity anisotropy
$\Delta\rho\equiv(\rho_{a}-\rho_{b})/\rho_{b}$. Here we use a
Boltzmann-equation approach to compute the contribution to $\Delta\rho$ arising
from scattering by anisotropic charge and spin fluctuations, which have been
recently observed experimentally. While the anisotropy in the charge
fluctuations is manifested in the correlation length, the anisotropy in the
spin fluctuations emerges only in the structure factor. As a result, we find
that spin fluctuations favor $\Delta\rho>0$, whereas charge fluctuations
promote $\Delta\rho<0$, which are both consistent with the doping dependence of
$\Delta\rho$ observed in YBa$_{2}$Cu$_{3}$O$_{7}$. We also discuss the role
played by CuO chains in these materials, and propose transport experiments in
strained HgBa$_{2}$CuO$_{4}$ and Nd$_{2}$CuO$_{4}$ to probe directly the
different resistivity anisotropy regimes.
