We performed molecular dynamics simulations of silicon nanostructures submitted to various stresses and temperatures. For a given stress orientation, a transition in the onset of silicon plasticity is revealed depending on the temperature and stress magnitude. At high temperature and low stress, partial dislocation loops are nucleated in the {111} glide set planes. But at low temperature and very high stress, perfect dislocation loops are formed in the other set of {111} planes called shuffle. This result confirmed by three different classical potentials suggests that plasticity in silicon nanostructures could be controlled by dislocation nucleation.