How can a plant resist glyphosate (Roundup) herbicide?
I found a paper investigating the mechanism of glyphosate resistance in Conyza canadensis. They used 31P NMR to investigate the fate of the glyphosate in vivo. What they found is that the resistant plants are able to transport glyphosate into the vacuole:


  The following view of horseweed resistance to glyphosate emerges from
  the data presented herein. Glyphosate enters the cytoplasm of both R
  and S plant variants at the same rate. Within hours, however,
  glyphosate begins to occupy the vacuole in the R but not the S
  biotype. The identical pH values of R and S vacuoles speak against the
  possibility of a pH-driven process. This, coupled with the
  preferential movement of glyphosate from the cytosol to the vacuole in
  R tissue but not in S, suggests the presence of a transporter for
  glyphosate either specific to R or at a substantially greater
  concentration in R than in S tissue.


Another paper titled "Two non-target mechanisms are involved in glyphosate-resistant horseweed (Conyza canadensis L. Cronq.) biotypes" found a different mechanism for glyphosate resistance in addition to the sequestration into the vacuole.


  Glyphosate was metabolized faster in the R (C004) biotype than in the
  S biotype. The herbicide disappeared completely from the R (C004)
  biotype by conversion into glyoxylate, sarcosine and
  aminomethylphosphonic acid within 96 HAT.


The resistant plants seem to be able to metabolize glyphosate more effectively.

It is of course not guarenteed that these are the only mechanisms for glyphosate resistance, there are likely more variations.