The Expression of Heat Shock Genes — A Model for Environmental Stress Response. In: Verma DPS, Goldberg RB, editors. Temporal and Spatial Regulation of Plant Genes. Vienna: Springer; 1988. pp. 253-73. [DOI: 10.1007/978-3-7091-6950-6_14]

Gene expression under temperature stress

In this review, changes in plant gene expression in response to environmental stresses are discussed using the examples of high and low temperature treatments. While some changes may contribute to acclimatory processes which improve plant survival or performance under stress, others may be 'shock' responses indicative of sensitivity. The heat-shock response, which is almost ubiquitous among eukaryotic organisms, is characterized by repression of normal cellular protein synthesis mediated at both the transcriptional and the translational level, and induction of heat-shock protein (HSP) synthesis. There is a correlation between HSP synthesis and induced thermotolerance in plants, but the evidence for a causal relationship is not conclusive. The possible biochemical functions of some of the HSPs are now becoming apparent; they are believed to play an important role in preventing accumulation of damaged proteins in the cell during heat shock. Although no other environmental stress elicits the full heat-shock response, certain treatments do induce synthesis of subsets of the HSPs, and the reasons for this are considered. Alterations in gene expression in response to low temperatures are more diverse and usually less dramatic than the heat-shock response, with which they share little, if any, homology. Biochemical adjustments during cold treatment are discussed, with particular reference to those which contribute to acclimation. Several genes whose expression is induced by cold have been cloned and characterized, and in some cases it is possible to attribute in vivo functions to them; they include enzymes of lipid, carbohydrate and protein metabolism, structural proteins and putative cryoprotectants. The use of transgenic plants is further facilitating an investigation of the biochemical factors which are important in cold acclimation. Drought, osmotic stress and abscisic acid induce expression of many of the same genes as does cold treatment; it seems likely that some of the products of these genes contribute to increased freezing tolerance by protecting against intracellular dehydration. Contents Summary 1 I. Introduction 1 II. High temperature stress 3 III. Low temperature stress 10 IV. Concluding remarks 20 Acknowledgements 21 References 21.

Synergistic effect of upstream sequences, CCAAT box elements, and HSE sequences for enhanced expression of chimaeric heat shock genes in transgenic tobacco

The thermoregulated expression of the soybean heat shock (hs) gene Gmhsp17.3-B is regulated via the heat shock promoter elements (HSEs), but full promoter activity requires additional sequences located upstream of the HSE-containing region. Structural features within this putative enhancer region include a run of simple sequences which are also present upstream of HSE-like sequences of other soybean hs genes, and three perfect CCAAT box sequences located immediately upstream from the most distal HSE of the promoter. A series of heterologous and homologous promoter fusions linked to the chloramphenicol acetyl transferase (CAT) gene was constructed and examined in transgenic tobacco plants. The region containing the AT-rich domain of the 5' flanking region was unable to direct transcription from the TATA box of a truncated delta CaMV35S promoter. Heat-inducible CAT activity was detectable when additional sequences from the native promoter containing three CCAAT boxes and a single HSE were present in the constructions. Complete reconstitution of the native hs promoter/enhancer region increased hs specific CAT activities only very little, but deletion of CCAAT box sequences reduced CAT expression five-fold. Our results suggest that AT-rich sequences have a moderate effect on thermoinducible expression levels of the soybean heat shock gene and that CCAAT box sequences act cooperatively with HSEs to increase the hs promoter activity.

Alfalfa heat shock genes are differentially expressed during somatic embryogenesis

We have isolated two cDNA clones (Mshsp18-1; Mshsp18-2) from alfalfa (Medicago sativa L.) which encode for small heat shock proteins (HSPs) belonging to the hsp17 subfamily. The predicted amino acid sequences of the two alfalfa proteins are 92% identical and a similar degree of homology (90%) can be detected between Mshsp18-2 and the pea hsp17. In comparison to various members of small HSPs from soybean amino acid sequence similarities of 80-86% were identified. The alfalfa HSPs share a homologous stretch of amino acids in the carboxy terminal region with hsp22, 23, 26 from Drosophila. This region contains the GVLTV motif which is characteristic of several members of small HSPs. At room temperature alfalfa hsp18 mRNAs were not detectable in root and leaf tissues but northern analysis showed a low level of expression in microcallus suspension (MCS). The transcription of Mshsp18 genes is induced by elevated temperature, CdCl2 treatment and osmotic shock in cultured cells. In alfalfa somatic embryos derived from MCS a considerable amount of hsp18 mRNA can be detected during the early embryogenic stages under normal culture conditions. The differential expression of these genes during embryo development suggests a specific functional role for HSPs in plant cells at the time of the developmental switch in vitro.

The function of plant heat shock promoter elements in the regulated expression of chimaeric genes in transgenic tobacco

A series of deletion mutants of a soybean heat shock (hs) gene promoter was generated and linked to the chloramphenicol acetyl transferase (CAT) coding sequence. These chimaeric promoter/reporter gene constructs were introduced into tobacco and thermoregulated expression of CAT activity was examined in leaf extracts. Three different types of gene fusions were tested using two different BIN19 vector constructions: (1) translational fusion between the N-terminus of the protein coding sequence of the heat shock gene Gmhsp17.3-B and CAT; (2) transcriptional fusions between the 5' nontranslated RNA regions of Gmhsp17.3-B and CAT; and (3) promoter fusions joining the hs promoter upstream sequences to the TATA box sequence of the delta CaMV 35S-CATter vector. Alternatively, multiple copies of a synthetic deoxyoligonucleotide with the soybean hs consensus element (HSE2) were used. Heat inducible CAT activities were detected except in plants containing a transcriptional fusion devoid of all but 18 nucleotides at the 5' terminus of the hs gene transcript. CAT activity was detectable in these plants only during the recovery at 25 degrees C after a hs (40 degrees C). Overlapping HSE-like promoter sequences seem to be necessary for the induction of heat inducible transcription of linked genes; synthetic HSE2 sequences have the capacity to reconstitute a hs promoter in combination with a TATA box sequence. Effective translation during hs seems to require sequences in the 5' non-translated leader of the hs protein mRNA; these sequences can be functionally replaced by the 5' leader sequence of the delta CaMV 35S promoter.