Molecular chaperone essential for chloroplast development and seedling viability. Mediates internal thylakoid membrane formation and confers thermotolerance to chloroplasts during heat stress. By heat stress. Seedling lethal when grown on soil. On agar plates supplied with sucrose, seedlings grow slowly with a chlorotic phenotype. They display irregular and small chloroplasts without starch grains in the stroma and with disorganized grana stacks and undeveloped thylakoid membranes. Plants overexpressing CLPB3 show inhibition of chloroplast development and a mild pale-green phenotype. Belongs to the ClpA/ClpB family. Truncated N-terminus.