  The contributions to the cross section $\sigma_{t\bar{t}}$ and to the
forward-backward asymmetry $A_{\rm FB}^{t \bar t}$ of $t\bar{t}$ production at
the Tevatron from $Z'$-boson and scalar leptoquarks $S^{(\pm)}_a$ and scalar
gluons $F_a$ predicted by the minimal model with four color quark-lepton
symmetry are calculated. These contributions are shown to be small in tree
approximation and can be significant with account of the 1-loop $gt\bar{t}$
effective vertex induced by the scalar doublets. The lower mass limit for
scalar gluons $m_F \gtrsim 320\, GeV$ from the Tevatron data is obtained and it
is shown that for $m_{F_1}\lesssim 990\,GeV$ the scalar gluon $F_1$ can be
evident at LHC at the significance not less that $3\sigma$ (for $\sqrt{s}=14$
TeV, $L=10\,fb^{-1}$).
