  We study the effect of the chiral symmetry restoration (CSR) on heavy-ion
collisions observables in the energy range $\sqrt{s_{NN}}$ = 3-20 GeV within
the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes
the deconfinement phase transition as well as essential aspects of CSR in the
dense and hot hadronic medium, which are incorporated in the Schwinger
mechanism for particle production. Our systematic studies show that chiral
symmetry restoration plays a crucial role in the description of heavy-ion
collisions at $\sqrt{s_{NN}}$ = 3-20 GeV, realizing an increase of the hadronic
particle production in the strangeness sector with respect to the non-strange
one. Our results provide a microscopic explanation for the "horn" structure in
the excitation function of the $K^+/\pi^+$ ratio: the CSR in the hadronic phase
produces the steep increase of this particle ratio up to $\sqrt{s_{NN}} \approx
$ 7 GeV, while the drop at higher energies is associated to the appearance of a
deconfined partonic medium. Furthermore, the appearance/disappearance of the
"horn" structure is investigated as a function of the system size. We
additionally present an analysis of strangeness production in the
($T,\mu_B$)-plane (as extracted from the PHSD for central Au+Au collisions) and
discuss the perspectives to identify a possible critical point in the phase
diagram.
