  We discuss the role of striction in the intertwined magnetic and structural
phase transitions in the underdoped iron-pnictides. The magneto-elastic
coupling to acoustic modes is then derived and estimated in framework of the
multiband spectrum for itinerant electrons with nesting features. We argue that
the 1-st order character of the magneto-elastic phase transition originates
from the lattice instabilities near the onset of spin-density wave order
introducing, thus, a shear acoustic mode as a new order parameter. Taking
non-harmonic termis in the lattice energy into account may explain the
splitting of the structural and magnetic transitions in some oxypnictides.
Fluctuations of the magnetic order parameter show up in the precursory
temperature dependence of the elastic moduli.
