It looks like some kind of microbe contamination. See some contamination (cloudy, 'smear'-like stuff in the medium) on the right-handed side of the picture.

Please see the attached paper.

The authors have surveyed the common bacterial contamination on in vitro plant tissue culture. Their survey of contaminated in vitro plant cultures obtained from different laboratories yielded 104 isolates of bacteria (see yellow highlights of the paper).

Some bacteria turned pink (due to pink pigment), such as M. extorquens strain (p.44).

Thank you very much for the nice information Dr. Yuan and Dr. Stefano.

bacterial contamination.

Tissue browning (common in Tissue culture tech.)