HSP21,a nuclear-encoded chloroplast-localized sHSP,has been described for many plant species.In addition to two conserved regions(Consensus Regions Ⅰ and Ⅱ) found in all sHSPs,it has a unique amphipathic,methionine-rich domain at its N terminal which is highly conserved among all identified chloroplast sHSPs but not found in other sHSPs.Many studies have suggested that HSP21 plays an important role in protecting the thermolabile photosystem II(PSII) against heat stress.Several studies have also demonstrated that HSP21 protects PSII against oxidative stress.