Regulation of the heat-shock response

Shared and independent roles for a Galpha(i) protein and adenylyl cyclase in regulating development and stress responses in Neurospora crassa

Growth and development are regulated using cyclic AMP (cAMP)-dependent and -independent pathways in Neurospora crassa. The cr-1 adenylyl cyclase mutant lacks detectable cAMP and exhibits numerous defects, including colonial growth habit, short aerial hyphae, premature conidiation on plates, inappropriate conidiation in submerged culture, and increased thermotolerance. Evidence suggests that the heterotrimeric Galpha protein GNA-1 is a direct positive regulator of adenylyl cyclase. deltagna-1 strains are female-sterile, and deltagna-1 strains have, reduced apical extension rates on normal and hyperosmotic medium, greater resistance to oxidative and heat stress, and stunted aerial hyphae compared to the wild-type strain. In this study, a deltagna-1 cr-1 double mutant was analyzed to differentiate cAMP-dependent and -independent signaling pathways regulated by GNA-1. deltagna-1 cr-1 mutants have severely restricted colonial growth and do not produce aerial hyphae on plates or in standing liquid cultures. Addition of cAMP to plates or standing liquid cultures rescues cr-1, but not deltagna-1 cr-1, defects, which is consistent with previous results demonstrating that deltagna-1 mutants do not respond to exogenous cAMP. The females of all strains carrying the deltagna-1 mutation are sterile; however, unlike cr-1 and deltagna-1 strains, the deltagna-1 cr-1 mutant does not produce protoperithecia. The deltagna-1 and cr-1 mutations were synergistic with respect to inappropriate conidiation during growth in submerged culture. Thermotolerance followed the order wild type < deltaga-1 < cr-1 = deltagna-1 cr-1, consistent with a cAMP-dependent process. Taken together, the results suggest that in general, GNA-1 and CR-1 regulate N. crassa growth and development using parallel pathways, while thermotolerance is largely dependent on cAMP.

Combinatorial interaction of cis elements specifies the expression of the Arabidopsis AtHsp90-1 gene

The promoter region of the Arabidopsis AtHsp90-1 gene is congested with heat shock elements and stress response elements, as well as with other potential transcriptional binding sites (activating protein 1, CCAAT/enhancer-binding protein element, and metal regulatory element). To determine how the expression of this bona fide AtHsp90-1 gene is regulated, a comprehensive quantitative and qualitative promoter deletion analysis was conducted under various environmental conditions and during development. The promoter induces gene expression at high levels after heat shock and arsenite treatment. However, our results show that the two stress responses may involve common but not necessarily the same regulatory elements. Whereas for heat induction, heat shock elements and stress response elements act cooperatively to promote high levels of gene expression, arsenite induction seems to require the involvement of activating protein 1 regulatory sequences. In stressed transgenic plants harboring the full-length promoter, beta-glucuronidase activity was prominent in all tissues. Nevertheless, progressive deletion of the promoter decreases the level of expression under heat shock and restricts it predominantly in the two meristems of the plant. In contrast, under arsenite induction, proximal sequences induce AtHsp90-1 gene expression only in the shoot meristem. Distally located elements negatively regulate AtHsp90-1 gene expression under unstressed conditions, whereas flower-specific regulated expression in mature pollen grains suggests the prominent role of the AtHsp90-1 in pollen development. The results show that the regulation of developmental expression, suppression, or stress induction is mainly due to combinatorial contribution of the cis elements in the promoter region of the AtHsp90-1 gene.

Heat stress- and heat shock transcription factor-dependent expression and activity of ascorbate peroxidase in Arabidopsis

To find evidence for a connection between heat stress response, oxidative stress, and common stress tolerance, we studied the effects of elevated growth temperatures and heat stress on the activity and expression of ascorbate peroxidase (APX). We compared wild-type Arabidopsis with transgenic plants overexpressing heat shock transcription factor 3 (HSF3), which synthesize heat shock proteins and are improved in basal thermotolerance. Following heat stress, APX activity was positively affected in transgenic plants and correlated with a new thermostable isoform, APX(S). This enzyme was present in addition to thermolabile cytosolic APX1, the prevalent isoform in unstressed cells. In HSF3-transgenic plants, APX(S) activity was detectable at normal temperature and persisted after severe heat stress at 44 degrees C. In nontransgenic plants, APX(S) was undetectable at normal temperature, but could be induced by moderate heat stress. The mRNA expression profiles of known and three new Apx genes were determined using real-time PCR. Apx1 and Apx2 genes encoding cytosolic APX were heat stress and HSF dependently expressed, but only the representations of Apx2 mRNA met the criteria that suggest identity between APX(S) and APX2: not expressed at normal temperature in wild type, strong induction by heat stress, and HSF3-dependent expression in transgenic plants. Our data suggest that Apx2 is a novel heat shock gene and that the enzymatic activity of APX2/APX(S) is required to compensate heat stress-dependent decline of APX1 activity in the cytosol. The functional roles of modulations of APX expression and the interdependence of heat stress and oxidative stress response and signaling mechanisms are discussed.

Limitation to photosynthesis in water-stressed leaves: stomata vs. metabolism and the role of ATP

Decreasing relative water content (RWC) of leaves progressively decreases stomatal conductance (gs), slowing CO2 assimilation (A) which eventually stops, after which CO2 is evolved. In some studies, photosynthetic potential (Apot), measured under saturating CO2, is unaffected by a small loss of RWC but becomes progressively more inhibited, and less stimulated by elevated CO2, below a threshold RWC (Type 1 response). In other studies, Apot and the stimulation of A by elevated CO2 decreases progressively as RWC falls (Type 2 response). Decreased Apot is caused by impaired metabolism. Consequently, as RWC declines, the relative limitation of A by g(s) decreases, and metabolic limitation increases. Causes of decreased Apot are considered. Limitation of ribulose bisphosphate (RuBP) synthesis is the likely cause of decreased Apot at low RWC, not inhibition or loss of photosynthetic carbon reduction cycle enzymes, including RuBP carboxylase/oxygenase (Rubisco). Limitation of RuBP synthesis is probably caused by inhibition of ATP synthesis, due to progressive inactivation or loss of Coupling Factor resulting from increasing ionic (Mg2+) concentration, not to reduced capacity for electron or proton transport, or inadequate trans-thylakoid proton gradient (ApH). Inhibition of Apot by accumulation of assimilates or inadequate inorganic phosphate is not considered significant. Decreased ATP content and imbalance with reductant status affect cell metabolism substantially: possible consequences are discussed with reference to accumulation of amino acids and alterations in protein complement under water stress.

Photosynthetic carbon assimilation and associated metabolism in relation to water deficits in higher plants

Experimental studies on CO2 assimilation of mesophytic C3 plants in relation to relative water content (RWC) are discussed. Decreasing RWC slows the actual rate of photosynthetic CO2 assimilation (A) and decreases the potential rate (Apot). Generally, as RWC falls from c. 100 to c. 75%, the stomatal conductance (gs) decreases, and with it A. However, there are two general types of relation of Apot to RWC, which are called Type 1 and Type 2. Type 1 has two main phases. As RWC decreases from 100 to c. 75%, Apot is unaffected, but decreasing stomatal conductance (gs) results in smaller A, and lower CO2 concentration inside the leaf (Ci) and in the chloroplast (Cc), the latter falling possibly to the compensation point. Down-regulation of electron transport occurs by energy quenching mechanisms, and changes in carbohydrate and nitrogen metabolism are considered acclimatory, caused by low Ci and reversible by elevated CO2. Below 75% RWC, there is metabolic inhibition of Apot, inhibition of A then being partly (but progressively less) reversible by elevated CO2; gs regulates A progressively less, and Ci and CO2 compensation point, Gamma rise. It is suggested that this is the true stress phase, where the decrease in Apot is caused by decreased ATP synthesis and a consequent decreased synthesis of RuBP. In the Type 2 response, Apot decreases progressively at RWC 100 to 75%, with A being progressively less restored to the unstressed value by elevated CO2. Decreased gs leads to a lower Ci and Cc but they probably do not reach compensation point: gs becomes progressively less important and metabolic limitations more important as RWC falls. The primary effect of low RWC on Apot is most probably caused by limited RuBP synthesis, as a result of decreased ATP synthesis, either through inhibition of Coupling Factor activity or amount due to increased ion concentration. Carbohydrate synthesis and accumulation decrease. Type 2 response is considered equivalent to Type 1 at RWC below c. 75%, with Apot inhibited by limited ATP and RuBP synthesis, respiratory metabolism dominates and Ci and Gamma rise. The importance of inhibited ATP synthesis as a primary cause of decreasing Apot is discussed. Factors determining the Type 1 and Type 2 responses are unknown. Electron transport is maintained (but down-regulated) in Types 1 and 2 over a wide range of RWC, and a large reduced/oxidized adenylate ratio results. Metabolic imbalance results in amino acid accumulation and decreased and altered protein synthesis. These conditions profoundly affect cell functions and ultimately cause cell death. Type 1 and 2 responses may reflect differences in gs and in sensitivity of metabolism to decreasing RWC.

Interaction between Arabidopsis heat shock transcription factor 1 and 70 kDa heat shock proteins

The activity of the Arabidopsis heat shock transcription factor (HSF) is repressed at normal conditions but activated by cellular stresses. Circumstantial evidence suggests that HSP70 may function as a negative feedback regulator of HSF activity. Here the interaction between HSF and HSP70 is reported using electrophoretic mobility shift and yeast two-hybrid assays. Subdomain mapping indicates an interaction of the activation domain and DNA-binding domain of HSF1 with HSP70.

At-HSP17.6A, encoding a small heat-shock protein in Arabidopsis, can enhance osmotolerance upon overexpression

Owing to their sessile lifestyle, it is crucial for plants to acquire stress tolerance. The function of heat-shock proteins, including small heat-shock proteins (smHSPs), in stress tolerance is not fully explored. To gain further knowledge about the smHSPs, the gene that encoded the cytosolic class II smHSP in Arabidopsis thaliana (At-HSP17.6A) was characterized. The At-HSP17.6A expression was induced by heat and osmotic stress, as well as during seed development. Accumulation of At-HSP17.6A proteins could be detected with heat and at a late stage of seed development, but not with osmotic stress, suggesting stress-induced post-transcriptional regulation of At-HSP17.6A expression. Overproduction of At-HSP17.6A could increase salt and drought tolerance in Arabidopsis. The chaperone activity of At-HSP17.6A was demonstrated in vitro.

Heat-shock responses in two leguminous plants: a comparative study

Relative growth rates, basal and acclimated thermotolerance, membrane damage, fluorescence emission, and relative levels of free and conjugated ubiquitin and HSP70 were compared after 2 h of treatment at different temperatures between Prosopis chilensis and Glycine max (soybean), cv. McCall, to evaluate if the thermotolerance of these two plants was related to levels of accumulation of heat shock proteins. Seedlings of P. chilensis germinated at 25 degrees C and at 35 degrees C and grown at temperatures above germination temperature showed higher relative growth than soybean seedlings treated under the same conditions. The lethal temperature of both species was 50 degrees C after germination at 25 degrees C. However, they were able to grow at 50 degrees C after germination at 35 degrees C. Membrane damage determinations in leaves showed that P. chilensis has an LT(50) 6 degrees C higher than that of soybean. There were no differences in the quantum yield of photosynthesis (F(v)/F(m)), between both plants when the temperatures were raised. P. chilensis showed higher relative levels of free ubiquitin, conjugated ubiquitin and HSP70 than soybean seedlings when the temperatures were raised. Time-course studies of accumulation of these proteins performed at 40 degrees C showed that the relative accumulation rates of ubiquitin, conjugated ubiquitin and HSP70 were higher in P. chilensis than in soybean. In both plants, free ubiquitin decreased during the first 5 min and increased after 30 min of heat shock, conjugated ubiquitin increased after 30 min and HSP70 began to increase dramatically after 20 min of heat shock. From these data it is concluded that P. chilensis is more tolerant to acute heat stress than soybean.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

Genomic analysis of the Hsp70 superfamily in Arabidopsis thaliana

The Arabidopsis genome contains at least 18 genes encoding members of the 70-kilodalton heat shock protein (Hsp70) family, 14 in the DnaK subfamily and 4 in the Hsp110/SSE subfamily. While the Hsp70s are highly conserved, a phylogenetic analysis including all members of this family in Arabidopsis and in yeast indicates the homology of Hsp70s in the subgroups, such as those predicted to localize in the same subcellular compartment and those similar to the mammalian Hsp110 and Grp170. Gene structure and genome organization suggest duplication in the origin of some genes. The Arabidopsis hsp70s exhibit distinct expression profiles; representative genes of the subgroups are expressed at relatively high levels during specific developmental stages and under thermal stress.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

Evidence for non-circadian light/dark-regulated expression of Hsp70s in spinach leaves

Expression of six Hsp70s in spinach (Spinacia oleracea cv Longstanding Bloomsdale) leaves grown under isothermal conditions is regulated by a light/dark (L/D) mechanism distinctly different from the light-regulated mechanism for the chlorophyll a/b-binding protein (cab) or small subunit of ribulose-1,5-bisphosphate carboxylase oxygenase (rbcS). Subjecting entrained plants to two or three L/D cycles within a 24-h period resulted in an equal number of oscillations in expression for five out of six 70-kD heat shock proteins (Hsp70s). Three cycles appear to be the maximum, as shorter L/D treatments do not consistently increase the number of cycles in a 24-h period. The expression response of Hsp70s to L/D is overridden by heat shock. Protein disulfide isomerase, a second molecular chaperone of the endoplasmic reticulum, has an expression pattern in entrained plants that is similar to hsc70-2, the endoplasmic reticulum luminal Hsp70 binding protein. The parallel expression patterns for the various Hsp70s and protein disulfide isomerase indicate a likely general coordinate L/D regulation for molecular chaperones in plants. Multiple inductions in response to successive L/D treatments within a 24-h period in entrained plants for five of six Hsp70s support the conclusion that expression is not a consequence of circadian control, but instead is independently cued by non-circadian-mediated L/D signals where peak Hsp70 expression precedes the daily thermoperiod maximum.

Arabidopsis transcription factors: genome-wide comparative analysis among eukaryotes

The completion of the Arabidopsis thaliana genome sequence allows a comparative analysis of transcriptional regulators across the three eukaryotic kingdoms. Arabidopsis dedicates over 5% of its genome to code for more than 1500 transcription factors, about 45% of which are from families specific to plants. Arabidopsis transcription factors that belong to families common to all eukaryotes do not share significant similarity with those of the other kingdoms beyond the conserved DNA binding domains, many of which have been arranged in combinations specific to each lineage. The genome-wide comparison reveals the evolutionary generation of diversity in the regulation of transcription.

A ditelosomic line of 'Chinese Spring' wheat with augmented acquired thermotolerance

A study of the ditelosomic series of 'Chinese Spring' wheat has yielded a number of lines displaying either an increased or decreased ability to acquire thermotolerance. One such ditelosomic (DT) is termed DT1BS which refers to the missing short arm of chromosome 1 in the B genome. The DT1BS line has the ability to acquire thermotolerance at lower induction temperatures and provide greater protection to the plant against otherwise lethal elevated temperatures. Using a chlorophyll accumulation assay to measure plant health, we show that DT1BS accumulates chlorophyll optimally at the same temperature, and to similar levels as 'Chinese Spring'. We also show that maximum acquired thermotolerance against a 48 degrees C challenge is induced at 40 degrees C, but significant levels of protection can be obtained at temperatures as low as 34 degrees C in DT1BS or 36 degrees C in 'Chinese Spring'. Heat-shock protein accumulation is observed in DT1BS at temperatures 4 degrees C lower than the 'Chinese Spring' and is correlated with the induction of acquired thermotolerance.

Dephosphorylation of photosystem II reaction center proteins in plant photosynthetic membranes as an immediate response to abrupt elevation of temperature

Kinetic studies of protein dephosphorylation in photosynthetic thylakoid membranes revealed specifically accelerated dephosphorylation of photosystem II (PSII) core proteins at elevated temperatures. Raising the temperature from 22 degrees C to 42 degrees C resulted in a more than 10-fold increase in the dephosphorylation rates of the PSII reaction center proteins D1 and D2 and of the chlorophyll a binding protein CP43 in isolated spinach (Spinacia oleracea) thylakoids. In contrast the dephosphorylation rates of the light harvesting protein complex and the 9-kD protein of the PSII (PsbH) were accelerated only 2- to 3-fold. The use of a phospho-threonine antibody to measure in vivo phosphorylation levels in spinach leaves revealed a more than 20-fold acceleration in D1, D2, and CP43 dephosphorylation induced by abrupt elevation of temperature, but no increase in light harvesting protein complex dephosphorylation. This rapid dephosphorylation is catalyzed by a PSII-specific, intrinsic membrane protein phosphatase. Phosphatase assays, using intact thylakoids, solubilized membranes, and the isolated enzyme, revealed that the temperature-induced lateral migration of PSII to the stroma-exposed thylakoids only partially contributed to the rapid increase in the dephosphorylation rate. Significant activation of the phosphatase coincided with the temperature-induced release of TLP40 from the membrane into thylakoid lumen. TLP40 is a peptidyl-prolyl cis-trans isomerase, which acts as a regulatory subunit of the membrane phosphatase. Thus dissociation of TLP40 caused by an abrupt elevation in temperature and activation of the membrane protein phosphatase are suggested to trigger accelerated repair of photodamaged PSII and to operate as possible early signals initiating other heat shock responses in chloroplasts.

Isolation of Arabidopsis mutants lacking components of acquired thermotolerance

Acquired thermotolerance is a complex physiological phenomenon that enables plants to survive normally lethal temperatures. This study characterizes the temperature sensitivity of Arabidopsis using a chlorophyll accumulation bioassay, describes a procedure for selection of acquired thermotolerance mutants, and provides the physiological characterization of one mutant (AtTS02) isolated by this procedure. Exposure of etiolated Arabidopsis seedlings to 48 degrees C or 50 degrees C for 30 min blocks subsequent chlorophyll accumulation and is eventually lethal. Arabidopsis seedlings can be protected against the effects of a 50 degrees C, 30-min challenge by a 4-h pre-incubation at 38 degrees C. By the use of the milder challenge, 44 degrees C for 30 min, and protective pretreatment, mutants lacking components of the acquired thermotolerance system were isolated. Putative mutants isolated by this procedure exhibited chlorophyll accumulation levels (our measure of acquired thermotolerance) ranging from 10% to 98% of control seedling levels following pre-incubation at 38 degrees C and challenge at 50 degrees C. The induction temperatures for maximum acquired thermotolerance prior to a high temperature challenge were the same in AtTS02 and RLD seedlings, although the absolute level of chlorophyll accumulation was reduced in the mutant. Genetic analysis showed that the loss of acquired thermotolerance in AtTS02 was a recessive trait. The pattern of proteins synthesized at 25 degrees C and 38 degrees C in the RLD and AtTS02 revealed the reduction in the level of a 27-kD heat shock protein in AtTS02. Genetic analysis showed that the reduction of this protein level was correlated with the acquired thermotolerance phenotype.

Arabidopsis RTM2 gene is necessary for specific restriction of tobacco etch virus and encodes an unusual small heat shock-like protein

Arabidopsis plants have a system to specifically restrict the long-distance movement of tobacco etch potyvirus (TEV) without involving either hypersensitive cell death or systemic acquired resistance. At least two dominant genes, RTM1 and RTM2, are necessary for this restriction. Through a series of coinfection experiments with heterologous viruses, the RTM1/RTM2-mediated restriction was shown to be highly specific for TEV. The RTM2 gene was isolated by a map-based cloning strategy. Isolation of RTM2 was confirmed by transgenic complementation and sequence analysis of wild-type and mutant alleles. The RTM2 gene product is a multidomain protein containing an N-terminal region with high similarity to plant small heat shock proteins (HSPs). Phylogenetic analysis revealed that the RTM2 small HSP-like domain is evolutionarily distinct from each of the five known classes of plant small HSPs. Unlike most other plant genes encoding small HSPs, expression of the RTM2 gene was not induced by high temperature and did not contribute to thermotolerance of seedlings. The RTM2 gene product was also shown to contain a large C-terminal region with multiple repeating sequences.

Functional analysis of oxidative stress-activated mitogen-activated protein kinase cascade in plants

Despite the recognition of H(2)O(2) as a central signaling molecule in stress and wounding responses, pathogen defense, and regulation of cell cycle and cell death, little is known about how the H(2)O(2) signal is perceived and transduced in plant cells. We report here that H(2)O(2) is a potent activator of mitogen-activated protein kinases (MAPKs) in Arabidopsis leaf cells. Using epitope tagging and a protoplast transient expression assay, we show that H(2)O(2) can activate a specific Arabidopsis mitogen-activated protein kinase kinase kinase, ANP1, which initiates a phosphorylation cascade involving two stress MAPKs, AtMPK3 and AtMPK6. Constitutively active ANP1 mimics the H(2)O(2) effect and initiates the MAPK cascade that induces specific stress-responsive genes, but it blocks the action of auxin, a plant mitogen and growth hormone. The latter observation provides a molecular link between oxidative stress and auxin signal transduction. Finally, we show that transgenic tobacco plants that express a constitutively active tobacco ANP1 orthologue, NPK1, display enhanced tolerance to multiple environmental stress conditions without activating previously described drought, cold, and abscisic acid signaling pathways. Thus, manipulation of key regulators of an oxidative stress signaling pathway, such as ANP1/NPK1, provides a strategy for engineering multiple stress tolerance that may greatly benefit agriculture.

An imperfect heat shock element and different upstream sequences are required for the seed-specific expression of a small heat shock protein gene

Chimeric constructs containing the promoter and upstream sequences of Ha hsp17.6 G1, a small heat shock protein gene, reproduced in transgenic tobacco (Nicotiana tabacum) its unique seed-specific expression patterns previously reported in sunflower. These constructs did not respond to heat shock, but were expressed without exogenous stress during late zygotic embryogenesis coincident with seed desiccation. Site-directed mutagenesis of its distal and imperfect heat shock element strongly impaired in vitro heat shock transcription factor binding and transgene expression in seeds. Deletion analyses of upstream sequences indicated the contribution of additional cis-acting elements with either positive or negative effects on transgene expression. These results show differences in the transcriptional activation through the heat shock element of small heat shock protein gene promoters in seeds compared with the heat shock response. In addition, they suggest that heat shock transcription factors and other distinct trans-acting factors cooperate in the regulation of Ha hsp17.6 G1 during seed desiccation.

Recovery of the yeast cell cycle from heat shock-induced G(1) arrest involves a positive regulation of G(1) cyclin expression by the S phase cyclin Clb5

In the yeast Saccharomyces cerevisiae, heat shock stress induces a variety of cellular responses including a transient cell cycle arrest before G(1)/S transition. Previous studies have suggested that this G(1) delay is probably attributable to a reduced level of the G(1) cyclin gene (CLN1 and CLN2) transcripts. Here we report our finding that the G(1) cyclin Cln3 and the S cyclin Clb5 are the key factors required for recovery from heat shock-induced G(1) arrest. Heat shock treatment of G(1) cells lacking either CLN3 or CLB5/CLB6 functions leads to prolonged cell cycle arrest before the initiation of DNA synthesis, concomitant with a severe deficiency in bud formation. The inability of the clb5 clb6 mutant to resume normal budding after heat shock treatment is unanticipated, since the S phase cyclins are generally thought to be required mainly for initiation of DNA synthesis and have no significant roles in bud formation in the presence of functional G(1) cyclins. Further studies reveal that the accumulation of G(1) cyclin transcripts is markedly delayed in the clb5 clb6 mutant following heat shock treatment, indicating that the CLN gene expression may require Clb5/Clb6 to attain a threshold level for driving the cell cycle through G(1)/S transition. Consistent with this assumption, overproduction of Clb5 greatly enhances the transcription of at least two G(1) cyclin genes (CLN1 and CLN2) in heat-shocked G(1) cells. These results suggest that Clb5 may positively regulate the expression of G(1) cyclins during cellular recovery from heat shock-induced G(1) arrest. Additional evidence is presented to support a role for Clb5 in maintaining the synchrony between budding and DNA synthesis during normal cell division as well.

Accumulation of small heat-shock protein homologs in the endoplasmic reticulum of cortical parenchyma cells in mulberry in association with seasonal cold acclimation

Cortical parenchyma cells of mulberry (Morus bombycis Koidz.) trees acquire extremely high freezing tolerance in winter as a result of seasonal cold acclimation. The amount of total proteins in endoplasmic reticulum (ER)-enriched fractions isolated from these cells increased in parallel with the process of cold acclimation. Protein compositions in the ER-enriched fraction also changed seasonally, with a prominent accumulation of 20-kD (WAP20) and 27-kD (WAP27) proteins in winter. The N-terminal amino acid sequence of WAP20 exhibited homology to ER-localized small heat-shock proteins (smHSPs), whereas that of WAP27 did not exhibit homology to any known proteins. Like other smHSPs, WAP20 formed a complex of high molecular mass in native-polyacrylamide gel electrophoresis. Furthermore, not only WAP20 but also 21-kD proteins reacted with antibodies against WAP20. Fractionation of the crude microsomes by isopycnic sucrose-gradient centrifugation revealed that both WAP27 and WAP20 were distributed on a density corresponding to the fractions with higher activity of ER marker enzyme, suggesting localization of these proteins in the ER. When ER-enriched fractions were treated with trypsin in the absence of detergent, WAP20 and WAP27 were undigested, suggesting localization of these proteins inside the ER vesicle. The accumulation of a large quantity of smHSPs in the ER in winter as a result of seasonal cold acclimation indicates that these proteins may play a significant role in the acquisition of freezing tolerance in cortical parenchyma cells of mulberry trees.

Characterization of a maize heat-shock protein 101 gene, HSP101, encoding a ClpB/Hsp100 protein homologue

Heat shock protein 101 (HSP101) cDNA and genomic clones from maize have been isolated. The structure of maize HSP101 reveals the presence of six exons interrupted by five introns. Maize HSP101 contains a predicted open reading frame that translates into a 912-aa sequence with a mass of 101kDa. Initiation of transcription was mapped 146 bases upstream of the AUG codon. Five heat shock element (HSE) boxes were found within the proximal 289 bases of the promoter region. Southern blot analysis of genomic DNA indicates that the maize genome contains only one copy of HSP101. A protein sequence comparison showed that maize Hsp101 belongs to the heat shock 100kDa and caseino-lytic protease B protein family (Hsp100/ClpB) that plays important roles in bacteria and yeast in the survival to extremely high temperatures and the control of proteolysis. Accumulation of HSP101 mRNA was strong under heat shock conditions, but not detectable after cold or osmotic stress treatments or by exogenous application of ABA. The analysis of the predicted supersecondary structure of maize Hsp101 showed that a coiled-coil located in the middle region of the protein is evolutionarily conserved in all members of the Clp A, B and C subfamilies. It is proposed that these supersecondary structures may have important roles in Clp function.