Chaperone protein required for seedling and chloroplast development under heat stress, probably by maintaining plastid-encoded RNA polymerase (PEP)-dependent transcription. Forms oligomeric structures: dodecameric arrangement of two trimer- of-dimer disks stabilized by the C-terminal tails, possibly through tail-to-tail interactions between the disks (PubMed:28325834). Interacts with PTAC5; the formed complex associates with the plastid-encoded RNA polymerase (PEP) complex not only during transcription initiation, but also during elongation and termination, and with a stronger efficiency in illuminated chloroplasts. Binds to promoter regions of PEP-dependent genes (PubMed:23922206). Confined to pollen grains of budding flowers. Highly induced in roots, stems, leaves, flowers and siliques after a heat shock at 30 degrees Celsius. Highly and quickly induced by heat stress during early seedling development. Ivory phenotype seedling under heat stress (30 degrees Celsius). Decrease in transcript levels of class I genes, but enhanced expression of class III genes (e.g. accD, rpoB and clpP) after heat shock. Belongs to the small heat shock protein (HSP20) family. Extended C-terminus.