Planetary nebulae (PNe) are excellent tracers to study the chemistry, kinematics, and stellar populations of galaxies.  They can be used to
constrain the properties of galactic substructures and peer into the past tidal interactions.  In this talk, I present our successful GTC observations of PNe in the Northern Spur and the Giant Stream, two
most prominent substructures of M31.  The deep spectroscopy enabled detection of the weak [O III] 4363 temperature-diagnostic line in all target PNe and as a consequence, reliable determination of elemental abundances. Our PN sample have homogeneous oxygen abundances, although
slight difference between the two substructures are marginally noticed. The study of abundances and the spatial and kinematical properties of our sample leads to the tempting conclusions: 1) their progenitors might
belong to the same stellar population, and 2) the Northern Spur and the Giant Stream may have the same origin and may be explained by the stellar orbit proposed by Merrett et al.
The dwarf satellite M32 might be responsible for the two substructures. Deep spectroscopy of PNe in M32 will help to assess this hypothesis.