Plants synthesize several families of low molecular weight (LMW) heat shock proteins (HSPs) in response to elevated temperatures. We have characterized two cDNAs, HSP18.1 and HSP17.9, that encode members of the class I family of LMW HSPs from pea (Pisum sativum). In addition, we investigated the expression of these HSPs at the mRNA and protein levels during heat stress and recovery. HSP18.1 and HSP17.9 are 82.1% identical at the amino acid level and are 80.8 to 92.9% identical to class I LMW HSPs of other angiosperms. Heat stress experiments were performed using intact seedlings subjected to a gradual temperature increase and held at a maximum temperature of 30 to 42 degrees Celsius for 4 hours. HSP18.1 and HSP17.9 mRNA levels peaked at the beginning of the maximum temperature period and declined rapidly after the stress period. Antiserum against a HSP18.1 fusion protein recognized both HSP18.1 and HSP17.9 but not members of other families of LMW HSPs. The accumulation of HSPI8.1-immunodetected protein was proportional to the severity of the heat stress, and the protein had a half-life of 37.7 ± 8 hours. The long half-life of these proteins supports the hypothesis that they are involved in establishing thermotolerance.Plants respond to elevated temperatures by expressing several families of evolutionarily conserved HSPs2 (12,17,23,31). Unlike yeast and most animals, which produce only one to four LMW HSPs (17), plants synthesize many LMW HSPs, between 12 and 27 different polypeptides, depending on the plant species (E. Vierling, unpublished data) (9,18,20). There are at least four families of nuclear encoded LMW HSPs in higher plants (22,26,31), all of which are members of the eukaryotic LMW HSP gene superfamily (17,23,31 (29), maize (24), petunia (4), and A. thaliana (4). The relationship between these families has not been closely examined at the amino acid level.It has long been known that plants can develop the ability to withstand otherwise lethal HS temperatures, a phenomenon referred to as acquired thermotolerance (12,17,23). Thermotolerance can be induced by several regimens: a previous moderate HS, a gradual temperature increase, a short, severe HS followed by a recovery period, and pretreatment with arsenite (1,12,16

Abstract. Diverse higher plant species synthesize low molecular weight (LMW) heat shock proteins (HSPs) which localize to chloroplasts. These proteins are homologous to LMW HSPs found in the cytoplasm of all eukaryotes, a class of HSPs whose molecular mode of action is not understood. To obtain basic information concerning the role of chloroplast HSPs, we examined the accumulation, stability, tissue specificity, and intra-chloroplast localization of HSP21, the major LMW chloroplast HSP in pea. Intact pea plants were subjected to heat stress conditions which would be encountered in the natural environment and HSP21 mRNA and protein levels were measured in leaves and roots. HSP21 was not detected in leaves or roots before stress, but the mature, 21-kD protein accumulated in direct proportion to temperature and HSP21 mRNA levels in both tissues. All of the HSP21 in leaves was localized to chloroplasts; there was no evidence for its transport into other organelles. In chloroplast fractionation experiments, >80% of HSP21 was recovered in the soluble chloroplast protein fraction. The half-life of HSP21 at control temperatures was 52 + 12 h, suggesting the protein's function is critical during recovery as well as during stress. We hypothesize that HSP21 functions in a catalytic fashion in both photosynthetic and nonphotosynthetic plastids.

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