Small heat shock proteins protect electron transport in chloroplasts and mitochondria during stress

Evidence indicates that small heat-shock proteins (Hsps) are involved in st ress tolerance, but the specific cell components or functions that small Hs ps protect or repair are mostly unidentified, We recently showed that the c hloroplast 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 interacting with the oxygen-evolving-comple x proteins of Photosystem II (PSII) within the thylakoid lumen. However, in vivo evidence of the importance of these Hsps to photosynthetic stress tol erance is lacking. Here we report positive relationships between chloroplas t small Hsp production and PSII thermotolerance in (1) a heat-tolerant geno type 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. s outhern U.S. (New Pork vs. Georgia); and (3) nine Lycopersicon (tomato) cul tivars/species differing in heat tolerance. These in vivo results are consi stent with our previous irt vitro observations and indicate that genetic va riation in production of the chloroplast small Hsp is an important determin ant 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 mitocho ndria of mammal (rat) cells which also protect Complex I during heat stress . These results suggest that the mitochondrial small Hsps, like the small c hloroplast Hsps, are general stress proteins that contribute significantly to cell and organismal stress tolerance.