Protein encoded by or localized in a plastid, a semi-autonomous, self-reproducing organelle. Plastids are remnants of a photosynthetic organism that was engulfed by the host, although not all are now photosynthetic. Plastid genomes encode genes for rRNAs, tRNAs and between about 28 and 150 proteins. Plastids can be categorized in 4 main groups: chloroplasts, cyanelles, apicoplasts and non-photosynthetic. The later is found is some land plants (Epifagus virginiana), chlorophyte algae (Prototheca wickerhamii) and euglenoids (Astasis longa), which do not encode the genes necessary for photosynthesis and so are not photosynthetic but still contain a plastid. They probably do not contain thylakoids.