  Signed graphs are studied since the middle of the last century. Recently, the
notion of homomorphism of signed graphs has been introduced since this notion
captures a number of well known conjectures which can be reformulated using the
definitions of signed homomorphism.
  In this paper, we introduce and study the properties of some target graphs
for signed homomorphism. Using these properties, we obtain upper bounds on the
signed chromatic numbers of graphs with bounded acyclic chromatic number and of
signed planar graphs with given girth.
