Small Heat Shock Proteins Protect Electron Transport in Chloroplasts and Mitochondria During Stress

Abstract

SYNOPSIS. Evidence indicates that small heat-shock proteins (Hsps) are involved in stress tolerance, but the specific cell components or functions that small Hsps protect or repair are mostly unidentified. We recently showed that the chloroplast small Hsps of higher plants (1) are produced in response to many environmental stresses (e.g., heat, oxidative, and high-light stress); and (2) protect (but do not repair) photosynthetic electron transport in vitro during stress, specifically by in- teracting with the oxygen-evolving-complex proteins of Photosystem II (PSII) with- in the thylakoid lumen. However, in vivo evidence of the importance of these Hsps to photosynthetic stress tolerance is lacking. Here we report positive relationships between chloroplast small Hsp production and PSII thermotolerance in (1) a heat- tolerant genotype of Agrostis palustris (bentgrass) and a heat sensitive genotype which lacks one or more chloroplast small Hsps produced by the tolerant genotype; (2) ecotypes of Chenopodium album (lambs quarters) from the northern vs. south- ern U.S. (New York vs. Georgia); and (3) nine Lycopersicon (tomato) cultivars/ species differing in heat tolerance. These in vivo results are consistent with our previous in vitro observations and indicate that genetic variation in production of the chloroplast small Hsp is an important determinant of photosynthetic and, thereby, whole-plant thermotolerance. Recently, we showed that the mitochondrial small Hsp of plants protects respiratory (specifically Complex I) electron transport in vitro during heat stress, and here we present evidence for previously unidentified small Hsps in mitochondria of mammal (rat) cells which also protect Complex I during heat stress. These results suggest that the mitochondrial small Hsps, like the small chloroplast Hsps, are general stress proteins that contribute significantly to cell and organismal stress tolerance.