Variation in chloroplast small heat-shock protein function is a major determinant of variation in thermotolerance of photosynthetic electron transport among ecotypes of Chenopodium album

Chloroplast small (low-molecular-weight) heat-shock proteins (csHsps) can protect photosynthetic electron transport (P-et), and quantitative variation in csHsps is correlated with thermotolerance of net photosynthesis and Photosystem II. However, the functional (i.e. protective) consequence of natural variation in csHsps is unknown. To investigate this, we used an in vitro assay to determine the contribution of csHsps to the tolerance of P-et to high temperatures in five ecotypes of Chenopodium album collected from habitats ranging from cool to warm, and we partitioned total P-et thermotolerance into basal and induced P-et components (without and with a pre-heat treatment, respectively, to induce csHsps). The ecotypes varied in total P-et thermotolerance and this was correlated with habitat temperature. Variation in total P-et thermotolerance was associated primarily with variation in induced P-et thermotolerance, and not with basal P-et thermotolerance. Variation in induced P-et was highly correlated with csHsp protection of P-et. Variation in csHsp function was associated with variation in csHsp content among ecotypes. These results are the first to demonstrate the direct functional consequences for natural variation in Hsps in plants, and show that functional variation is associated with evolutionary adaptation to specific habitats among ecotypes.