Chloroplast small heat-shock proteins protect photosynthesis during heavy metal stress

Plants can accumulate heavy metals when exposed to them at high levels. These metals can interfere with photosynthesis. Limited evidence suggests that increased synthesis of some heat-shock proteins (Hsps) may be a general plant response to metal stress, but the specific functions or structures protected by Hsps remain unidentified. Chloroplast small Hsps (smHsps) protect photosynthetic electron transport (Phet) during heat, oxidative, and photoinhibitory stress, but it is not known if chloroplast smHsps are synthesized during metal stress and protect photosynthesis. Zea mays (corn) plants were exposed to varying soil concentrations of Cu, Ni, Pb, and Zn to determine if chloroplast smHsps are induced by heavy metals, if smHsps protect Phet, and any effects on chloroplast smHsp and photosynthesis. Net photosynthesis (Phn) decreased with all metals-more so at higher levels and with longer exposures. Decreases in Phn resulted from damage to photosynthetic metabolism, including Phet. All metals increased chloroplast smHsp content, which increased with time of exposure. In vitro, Phet was protected from Pb (but not Ni) by purified chloroplast smHsp added to thylakoids. In vivo, Phn was protected from Ni and Pb by increases in smHsp in a heat-tolerant Agrostis stolonifera selection genotype expressing additional chloroplast smHsps compared to a near-isogenic heat-sensitive genotype. These results are evidence that Hsps protect photosynthesis from heavy metals and are among the first to demonstrate specific functions protected by Hsps during metal stress.