Expression of heat shock factor and heat shock protein 70 genes during maize pollen development

Heat stress at the bicellular stage inhibits sperm cell development and transport into pollen tubes

For successful double fertilization in flowering plants (angiosperms), pollen tubes deliver 2 nonmotile sperm cells toward female gametes (egg and central cell, respectively). Heatwaves, especially during the reproduction period, threaten male gametophyte (pollen) development, resulting in severe yield losses. Using maize (Zea mays) as a crop and grass model system, we found strong seed set reduction when moderate heat stress was applied for 2 d during the uni- and bicellular stages of pollen development. We show that heat stress accelerates pollen development and impairs pollen germination capabilities when applied at the unicellular stage. Heat stress at the bicellular stage impairs sperm cell development and transport into pollen tubes. To understand the course of the latter defects, we used marker lines and analyzed the transcriptomes of isolated sperm cells. Heat stress affected the expression of genes associated with transcription, RNA processing and translation, DNA replication, and the cell cycle. This included the genes encoding centromeric histone 3 (CENH3) and α-tubulin. Most genes that were misregulated encode proteins involved in the transition from metaphase to anaphase during pollen mitosis II. Heat stress also activated spindle assembly check point and meta- to anaphase transition genes in sperm cells. In summary, misregulation of the identified genes during heat stress at the bicellular stage results in sperm cell development and transport defects ultimately leading to sterility.

Mitigation of UV-B Radiation Stress in Tobacco Pollen by Expression of the Tardigrade Damage Suppressor Protein (Dsup)

Pollen, the male gametophyte of seed plants, is extremely sensitive to UV light, which may prevent fertilization. As a result, strategies to improve plant resistance to solar ultraviolet (UV) radiation are required. The tardigrade damage suppressor protein (Dsup) is a putative DNA-binding protein that enables tardigrades to tolerate harsh environmental conditions, including UV radiation, and was therefore considered as a candidate for reducing the effects of UV exposure on pollen. Tobacco pollen was genetically engineered to express Dsup and then exposed to UV-B radiation to determine the effectiveness of the protein in increasing pollen resistance. To establish the preventive role of Dsup against UV-B stress, we carried out extensive investigations into pollen viability, germination rate, pollen tube length, male germ unit position, callose plug development, marker protein content, and antioxidant capacity. The results indicated that UV-B stress has a significant negative impact on both pollen grain and pollen tube growth. However, Dsup expression increased the antioxidant levels and reversed some of the UV-B-induced changes to pollen, restoring the proper distance between the tip and the last callose plug formed, as well as pollen tube length, tubulin, and HSP70 levels. Therefore, the expression of heterologous Dsup in pollen may provide the plant male gametophyte with enhanced responses to UV-B stress and protection against harmful environmental radiation.

An overview of heat stress in tomato (Solanum lycopersicum L.)

Heat stress has been defined as the rise of temperature for a period of time higher than a threshold level, thereby permanently affecting the plant growth and development. Day or night temperature is considered as the major limiting factor for plant growth. Earlier studies reported that night temperature is an important factor in the heat reaction of the plants. Tomato cultivars capable of setting viable fruits under night temperatures above 21 °C are considered as heat-tolerant cultivars. The development of breeding objectives is generally summarized in four points: (a) cultivars with higher yield, (b) disease resistant varieties in the 1970s, (c) long shelf-life in 1980s, and (d) nutritive and taste quality during 1990s. Some unique varieties like the dwarf "Micro-Tom", and the first transgenic tomato (FlavrSavr) were developed through breeding; they were distributed late in the 1980s. High temperature significantly affects seed, pollen viability and root expansion. Researchers have employed different parameters to evaluate the tolerance to heat stress, including membrane thermo stability, floral characteristics (Stigma exertion and antheridia cone splitting), flower number, and fruit yield per plant. Reports on pollen viability and fruit set/plant under heat stress by comparing the pollen growth and tube development in heat-treated and non-heat-stressed conditions are available in literature. The electrical conductivity (EC) have been used to evaluate the tolerance of some tomato cultivars in vitro under heat stress conditions as an indication of cell damage due to electrolyte leakage; they classified the cultivars into three groups: (a) heat tolerant, (b) moderately heat tolerant, and (c) heat sensitive. It is important to determine the range in genetic diversity for heat tolerance in tomatoes. Heat stress experiments under field conditions offer breeders information to identify the potentially heat tolerant germplasm.

Isolation of Staged and Viable Maize Microspores for DH Production

Isolated microspore culture systems have been designed in maize by several groups, mainly from the late 1980s to early 2000s. However, even with optimized protocols, microspore embryogenesis induction has remained very dependent on the genotype in maize, with elite germplasm generally displaying no response or very low response. Yet, these last few years, significant progress has been accomplished in understanding and controlling microspore embryogenesis induction in model dicot and monocot species. This knowledge may be transferred to maize, and isolated microspore culture may gain new interest in this crop, at least for embryogenesis research. The methods we hereby present in detail permit the purification of 3-12 × 10⁵ viable microspores per maize tassel, at the favorable stage for microspore embryogenesis. When cultured in appropriate liquid media, microspores from responsive genotypes give rise to androgenic embryos, which can then be regenerated into fertile doubled haploid plants.

ZmSTK1 and ZmSTK2, encoding receptor-like cytoplasmic kinase, are involved in maize pollen development with additive effect

Pollen germination and pollen tube growth are important physiological processes of sexual reproduction of plants and also are involved in signal transduction. Our previous study reveals that ZmSTK1 and ZmSTK2 are two receptor-like cytoplasmic kinases (RLCK) homologs in Zea mays as members of receptor-like protein kinase (RLK) subfamily, sharing 86% identity at the amino acid level. Here, we report that ZmSTK1 and ZmSTK2, expressed at late stages of pollen development, regulate maize pollen development with additive effect. ZmSTK1 or ZmSTK2 mutation exhibited severe pollen transmission deficiency, which thus influenced pollen fertility. Moreover, the kinase domains of ZmSTKs were cross-interacted with C-terminus of enolases detected by co-immunoprecipitation (Co-IP) and yeast two-hybrid system (Y2H), respectively. Further, the detective ZmSTK1 or ZmSTK2 was associated with decreased activity of enolases and also reduced downstream metabolite contents, which enolases are involved in glycolytic pathway, such as phosphoenolpyruvate (PEP), pyruvate, ADP/ATP, starch, glucose, sucrose and fructose. This study reveals that ZmSTK1 and ZmSTK2 regulate maize pollen development and indirectly participate in glycolytic pathway.

Melatonin Alleviates High Temperature-Induced Pollen Abortion in Solanum lycopersicum

Melatonin is a pleiotropic signal molecule that plays critical roles in regulating plant growth and development, as well as providing physiological protections against various environmental stresses. Nonetheless, the mechanisms for melatonin-mediated pollen thermotolerance remain largely unknown. In this study, we report that irrigation treatment with melatonin (20 µM) effectively ameliorated high temperature-induced inactivation of pollen and inhibition of pollen germination in tomato (Solanum lycopersicum) plants. Melatonin alleviated reactive oxygen species production in tomato anthers under high temperature by the up-regulation of the transcription and activities of several antioxidant enzymes. Transmission electron micrograph results showed that high temperature-induced pollen abortion is associated with a premature degeneration of the tapetum cells and the formation of defective pollen grains with degenerated nuclei at the early uninuclear microspore stage, whilst melatonin protected degradation of organelles by enhancing the expression of heat shock protein genes to refold unfolded proteins and the expression of autophagy-related genes and formation of autophagosomes to degrade denatured proteins. These findings suggest a novel function of melatonin to protect pollen activity under high temperature and support the potential effects of melatonin on reproductive development of plants.

Acclimation to high temperature during pollen development

KEY MESSAGE

Pollen heat acclimation. As a consequence of global warming, plants have to face more severe and more frequently occurring periods of high temperature stress. While this affects the whole plant, development of the male gametophyte, the pollen, seems to be the most sensitive process. Given the great importance of functioning pollen for the plant life cycle and for agricultural production, it is necessary to understand this sensitivity. While changes in temperature affect different components of all cells and require a cellular response and acclimation, high temperature effects and responses in developing pollen are distinct from vegetative tissues at several points. This could be related to specific physiological characteristics of developing pollen and supporting tissues which make them vulnerable to high temperature, or its derived effects such as ROS accumulation and carbohydrate starvation. But also expression of heat stress-responsive genes shows unique patterns in developing pollen when compared to vegetative tissues that might explain the failure to withstand high temperatures. As an alternative to viewing pollen failure under high temperature as a result of inherent sensitivity of a specific developmental process, we end by discussing whether it might actually be an adaptation.

Expression of maize heat shock transcription factor gene ZmHsf06 enhances the thermotolerance and drought-stress tolerance of transgenic Arabidopsis

Based on the information of 25 heat shock transcription factor (Hsf) homologues in maize according to a genome-wide analysis, ZmHsf06 was cloned from maize leaves and transformed into Arabidopsis thaliana (L. Heynh.) (ecotype, Col-0). Three transgenic positive lines were selected to assess the basic and acquired thermotolerance and drought-stress tolerance under stresses and for some physiological assays. The sequence analysis indicates that ZmHsf06 contained the characteristic domains of class A type plant Hsfs. The results of qRT-PCR showed that the expression levels of ZmHsf06 were elevated by heat shock and drought stress to different extents in three transgenic lines. Phenotypic observation shows that compared with the Wt (wild-type) controls, the overexpressing ZmHsf06 of Arabidopsis plants have enhanced basal and acquired thermotolerance, stronger drought-stress tolerance and growth advantages under mild heat stress conditions. These results are further confirmed by physiological and biochemical evidence that transgenic Arabidopsis plants exhibit higher seed germination rate, longer axial-root length, higher activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), higher leaf chlorophyll content, but lower relative electrical conductivity (REC), malondialdehyde (MDA) and osmotic potential (OP) than the Wt controls after heat shock and drought treatments. ZmHsf06 may be a central representative of maize Hsfs and could be useful in molecular breeding of maize or other crops for enhanced tolerances, particularly during terminal heat and drought stresses.

Enhancement of reproductive heat tolerance in plants

Comparison of average crop yields with reported record yields has shown that major crops exhibit annual average yields three- to seven-fold lower than record yields because of unfavorable environments. The current study investigated the enhancement of pollen heat tolerance through expressing an Arabidopsis thaliana heat shock protein 101 (AtHSP101) that is not normally expressed in pollen but reported to play a crucial role in vegetative thermotolerance. The AtHSP101 construct under the control of the constitutive ocs/mas 'superpromoter' was transformed into cotton Coker 312 and tobacco SRI lines via Agrobacterium mediated transformation. Thermotolerance of pollen was evaluated by in vitro pollen germination studies. Comparing with those of wild type and transgenic null lines, pollen from AtHSP101 transgenic tobacco and cotton lines exhibited significantly higher germination rate and much greater pollen tube elongation under elevated temperatures or after a heat exposure. In addition, significant increases in boll set and seed numbers were also observed in transgenic cotton lines exposed to elevated day and night temperatures in both greenhouse and field studies. The results of this study suggest that enhancing heat tolerance of reproductive tissues in plant holds promise in the development of crops with improved yield production and yield sustainability in unfavorable environments.

Heat shock factor HSFB2a involved in gametophyte development of Arabidopsis thaliana and its expression is controlled by a heat-inducible long non-coding antisense RNA

Heat stress transcription factors (HSFs) are central regulators of the heat stress response. Plant HSFs of subgroup B lack a conserved sequence motif present in the transcriptional activation domain of class A-HSFs. Arabidopsis members were found to be involved in non-heat shock functions. In the present analysis we investigated the expression, regulation and function of HSFB2a. HSFB2a expression was counteracted by a natural long non-coding antisense RNA, asHSFB2a. In leaves, the antisense RNA gene is only expressed after heat stress and dependent on the activity of HSFA1a/HSFA1b. HSFB2a and asHSFB2a RNAs were also present in the absence of heat stress in the female gametophyte. Transgenic overexpression of HSFB2a resulted in a complete knock down of the asHSFB2a expression. Conversely, asHSFB2a overexpression leads to the absence of HSFB2a RNA. The knockdown of HSFB2a by asHSFB2a correlated with an improved, knockdown of asHSFB2a by HSFB2a overexpression with an impaired biomass production early in vegetative development. In both cases the development of female gametophytes was impaired. A T-DNA knock-out line did not segregate homozygous mutant plants, only heterozygots hsfB2a-tt1/+ were viable. Approximately 50% of the female gametophytes were arrested in early development, before mitosis 3, resulting in 45% of sterile ovules. Our analysis indicates that the "Yin-Yang" regulation of gene expression at the HSFB2a locus influences vegetative and gametophytic development in Arabidopsis.

Proteomic analysis of early seed development in Pinus massoniana L

Understanding seed development is important for large-scale propagation and germplasm conservation for the Masson pine. We undertook a proteomic analysis of Masson pine seeds during the early stages of embryogenesis. Two-dimensional difference gel electrophoresis (2D DIGE) was used to quantify the differences in protein expression during early seed development. Using electrospray ionization mass spectrometry/mass spectrometry, we identified proteins from 43 gel spots that had been excised from preparative "pick" gels. Proteins involved in carbon metabolism were identified and were predominantly expressed at higher levels during the cleavage polyembryony and columnar embryo stages. Functional annotation of one seed protein revealed it involvement in programmed cell death and translation of selective mRNAs, which may play an important role in subordinate embryo elimination and suspensor degeneration in polyembryonic seed gymnosperms. Other identified proteins were associated with protein folding, nitrogen metabolism, disease/defense response, and protein storage, synthesis and stabilization. The comprehensive protein expression profiles generated by this study will provide new insights into the complex developmental process of seed development in Masson pine.

Genome-wide identification, classification and analysis of heat shock transcription factor family in maize

Background

Heat shock response in eukaryotes is transcriptionally regulated by conserved heat shock transcription factors (Hsfs). Hsf genes are represented by a large multigene family in plants and investigation of the Hsf gene family will serve to elucidate the mechanisms by which plants respond to stress. In recent years, reports of genome-wide structural and evolutionary analysis of the entire Hsf gene family have been generated in two model plant systems, Arabidopsis and rice. Maize, an important cereal crop, has represented a model plant for genetics and evolutionary research. Although some Hsf genes have been characterized in maize, analysis of the entire Hsf gene family were not completed following Maize (B73) Genome Sequencing Project.

Results

A genome-wide analysis was carried out in the present study to identify all Hsfs maize genes. Due to the availability of complete maize genome sequences, 25 nonredundant Hsf genes, named ZmHsfs were identified. Chromosomal location, protein domain and motif organization of ZmHsfs were analyzed in maize genome. The phylogenetic relationships, gene duplications and expression profiles of ZmHsf genes were also presented in this study. Twenty-five ZmHsfs were classified into three major classes (class A, B, and C) according to their structural characteristics and phylogenetic comparisons, and class A was further subdivided into 10 subclasses. Moreover, phylogenetic analysis indicated that the orthologs from the three species (maize, Arabidopsis and rice) were distributed in all three classes, it also revealed diverse Hsf gene family expression patterns in classes and subclasses. Chromosomal/segmental duplications played a key role in Hsf gene family expansion in maize by investigation of gene duplication events. Furthermore, the transcripts of 25 ZmHsf genes were detected in the leaves by heat shock using quantitative real-time PCR. The result demonstrated that ZmHsf genes exhibit different expression levels in heat stress treatment.

Conclusions

Overall, data obtained from our investigation contributes to a better understanding of the complexity of the maize Hsf gene family and provides the first step towards directing future experimentation designed to perform systematic analysis of the functions of the Hsf gene family.

Developmental and heat stress-regulated expression of HsfA2 and small heat shock proteins in tomato anthers

The high sensitivity of male reproductive cells to high temperatures may be due to an inadequate heat stress response. The results of a comprehensive expression analysis of HsfA2 and Hsp17-CII, two important members of the heat stress system, in the developing anthers of a heat-tolerant tomato genotype are reported here. A transcriptional analysis at different developmental anther/pollen stages was performed using semi-quantitative and real-time PCR. The messengers were localized using in situ RNA hybridization, and protein accumulation was monitored using immunoblot analysis. Based on the analysis of the gene and protein expression profiles, HsfA2 and Hsp17-CII are finely regulated during anther development and are further induced under both short and prolonged heat stress conditions. These data suggest that HsfA2 may be directly involved in the activation of protection mechanisms in the tomato anther during heat stress and, thereby, may contribute to tomato fruit set under adverse temperatures.

The maize heat shock factor-binding protein paralogs EMP2 and HSBP2 interact non-redundantly with specific heat shock factors

The heat shock response (HSR) is a conserved mechanism by which transcripts of heat shock protein (hsp) genes accumulate following mobilization of heat shock transcription factors (HSFs) in response to thermal stress. Studies in animals identified the heat shock factor-binding protein1 (HSBP1) that interacts with heat shock transcription factor1 (HSF1) during heat shock attenuation; overexpression analyses revealed that the coiled-coil protein HSBP1 functions as a negative regulator of the HSR. Zea mays contains two HSBP paralogs, EMP2 and HSBP2, which exhibit differential accumulation during the HSR and plant development. Embryo-lethal recessive emp2 mutations revealed that EMP2 is required for the down-regulation of hsp transcription during embryogenesis, whereas accumulation of HSBP2 is induced in seedlings following heat shock. Notwithstanding, no interaction has yet been demonstrated between a plant HSBP and a plant HSF. In this report 22 maize HSF isoforms are identified comprising three structural classes: HSF-A, HSF-B and HSF-C. Phylogenetic analysis of Arabidopsis, maize and rice HSFs reveals that at least nine ancestral HSF isoforms were present prior to the separation of monocot and eudicots, followed by differential amplification of HSF members in these lineages. Yeast two-hybrid analyses show that EMP2 and HSBP2 interact non-redundantly with specific HSF-A isoforms. Site-specific mutagenesis of HSBP2 reveals that interactions between hydrophobic residues within the coiled coil are required for HSF::HSBP2 binding; domain swapping demonstrate that the isoform specificity of HSF::HSBP interaction is conferred by residues outside of the coiled coil. These data suggest that the non-redundant functions of the maize HSBPs may be explained, at least in part, by the specificity of HSBP::HSF interactions during plant development.

Engrailed-ZmOCL1 fusions cause a transient reduction of kernel size in maize

ZmOCL1 is the founding member of the ZmOCL (Outer Cell Layer) family encoding putative transcription factors of the HD-ZIP IV class. It is expressed in the L1 cell layer of the embryo and several other tissues of maize. After determination of the intron/exon structure a mutator insertion was isolated in the upstream region. No notable phenotypes and wildtype levels of ZmOCL1 transcript were observed in homozygous mutant plants. In contrast transgenic plants carrying a fusion of the repressor domain of the Drosophila Engrailed gene with the DNA binding and dimerisation domains of ZmOCL1 showed a transient reduction of embryo, endosperm and kernel size that was most obvious around 15 DAP. An inverse relationship was observed between the degree of size reduction and the expression level of the transcript. In reciprocal crosses the size reduction was only observed when the transgenic plants were used as females and no expression of male transmitted transgenes was detected. Smaller kernels resembled younger kernels of wild-type siblings indicating that interference with ZmOCL1 function leads to an overall slow-down of early kernel development. Based on marker gene analysis ZmOCL1 may act via a modification of gibberellin levels. Phylogenetic analyses based on the intron/exon structure and sequence similarities of ZmOCL1 and other HD-ZIP IV proteins from maize, rice and Arabidopsis helped to identify orthologues and suggested an evolution in the function of individual genes after the divergence of monocots and dicots.

Identification of candidate genes for in vitro androgenesis induction in maize

Extensive studies have been conducted to understand the genetic control of in vitro androgenesis, but little is know about the genes and the mechanisms involved in the switch that allows an immature pollen grain to develop as an embryo. We have developed two maize isogenic lines with high androgenetic aptitude, named AH5-44 and AH5-49, through backcross and selection from a high-responsive DH229 line on the non-responding A188 line genetic background. The genomic structure of these two lines was precisely described with microsatellite markers. Five regions retained from the parent DH229 highly responsive to androgenesis were localised in both AH5-44 and AH5-49. Sequences expressed on microspores extracted from the four lines were amplified using a cDNA-AFLP protocol. For each line, eight culture conditions were compared: microspores extracted after tassel recovery, after 7 or 14 days in cold room and after 1-4 days of in vitro culture. This genetic and developmental screening allowed us to identify four sequences, including a new HSP70-like candidate gene. Possible implication of the identified sequences in androgenesis response is discussed.

Plant class B HSFs inhibit transcription and exhibit affinity for TFIIB and TBP

Plant heat shock transcription factors (HSFs) are capable of transcriptional activation (class A HSFs) or both, activation and repression (class B HSFs). However, the details of mechanism still remain unclear. It is likely, that the regulation occurs through interactions of HSFs with general transcription factors (GTFs), as has been described for numerous other transcription factors. Here, we show that class A HSFs may activate transcription through direct contacts with TATA-binding protein (TBP). Class A HSFs can also interact weakly with TFIIB. Conversely, class B HSFs inhibit promoter activity through an active mechanism of repression that involves the C-terminal regulatory region (CTR) of class B HSFs. Deletion analysis revealed two sites in the CTR of soybean GmHSFB1 potentially involved in protein-protein interactions with GTFs: one is the repressor domain (RD) located in the N-terminal half of the CTR, and the other is a TFIIB binding domain (BD) that shows affinity for TFIIB and is located C-terminally from the RD. A Gal4 DNA binding domain-RD fusion repressed activity of LexA-activators, while Gal4-BD proteins synergistically activated strong and weak transcriptional activators. In vitro binding studies were consistent with this pattern of activity since the BD region alone interacted strongly with TFIIB, and the presence of RD had an inhibitory effect on TFIIB binding and transcriptional activation.

Mitochondrial Respiratory Deficiencies Signal Up-regulation of Genes for Heat Shock Proteins

The consequences of mitochondrial dysfunction are not limited to the development of oxidative stress or initiation of apoptosis but can result in the establishment of stress tolerance. Using maize mitochondrial mutants, we show that permanent mitochondrial deficiencies trigger novel Ca(2+)-independent signaling pathways, leading to constitutive expression of genes for molecular chaperones, heat shock proteins (HSPs) of different classes. The signaling to activate hsp genes appears to originate from a reduced mitochondrial transmembrane potential. Upon depolarization of mitochondrial membranes in transient assays, gene induction for mitochondrial HSPs occurred more rapidly than that for cytosolic HSPs. We also demonstrate that in the nematode Caenorhabditis elegans transcription of hsp genes can be induced by RNA interference of nuclear respiratory genes. In both organisms, activation of hsp genes in response to mitochondrial impairment is distinct from their responses to heat shock and is not associated with oxidative stress. Thus, mitochondria-to-nucleus signaling to express a hsp gene network is apparently a widespread retrograde mechanism to facilitate cell defense and survival.

High temperature stress of Brassica napus during flowering reduces micro- and megagametophyte fertility, induces fruit abortion, and disrupts seed production

High temperature stress (HTS), during flowering, decreases seed production in many plants. To determine the effect of a moderate HTS on flowering, fruit and seed set in Brassica napus, plants were exposed to a HTS (8/16 h dark/light, 18 degrees C night, ramped at 2 degrees C h-1, over 6 h, to 35 degrees C for 4 h, ramped at 2 degrees C h-1 back to 23 degrees C for 6 h) for 1 or 2 weeks after the initiation of flowering. Although flowering on the HTS-treated plants, during both the 1 week and 2 week HTS treatments, was equal to that of control-grown plants, fruit and seed development, as well as seed weight, were significantly reduced. Under HTS, flowers either developed into seedless, parthenocarpic fruit or aborted on the stem. At the cessation of the HTS, plants compensated for the lack of fruit and seed production by increasing the number of lateral inflorescences produced. During the HTS, pollen viability and germinability were slightly reduced. In vitro pollen tube growth at 35 degrees C, from both control pollen and pollen developed under a HTS, appeared abnormal, however, in vivo tube growth to the micropyle appeared normal. Reciprocal pollination of HTS or control pistils with HTS or control pollen indicated that the combined effects of HTS on both micro- and megagametophytes was required to knock out fruit and seed development. Expression profiles for a subset of HEAT SHOCK PROTEINs (HSP101, HSP70, HSP17.6) showed that both micro- and megagametophytes were thermosensitive despite HTS-induced expression from these genes.

Identification of higher plant GlsA, a putative morphogenesis factor of gametic cells

GlsA has been identified in an asexual-reproductive-cell (gonidia)-deficient mutant of Volvox as a chaperone-like protein essential for gonidia production. In this study, we isolated an angiosperm glsA (LlglsA) gene expressed during Lilium longiflorum pollen development. Immunoblot analyses showed that the strong LlGlsA expression occurred in the generative cell and its pattern during pollen development corresponded to that of alpha-tubulin. Morphological analyses succeeded in visualizing the dispersion of the strong LlGlsA signal in developing generative cells. In addition, multiple-immunofluorescence staining of LlGlsA and alpha-tubulin revealed that some of the dot-like LlGlsA signals were co-localized with microtubule filaments. From those results, we suggest that angiosperm GlsA functions as a chaperone modifying various structures during male gametic cell formation.

A rice spotted leaf gene, Spl7, encodes a heat stress transcription factor protein

A rice spotted leaf (lesion-mimic) gene, Spl7, was identified by map-based cloning. High-resolution mapping with cleaved amplified polymorphic sequence markers enabled us to define a genomic region of 3 kb as a candidate for Spl7. We found one ORF that showed high similarity to a heat stress transcription factor (HSF). Transgenic analysis verified the function of the candidate gene for Spl7: leaf spot development was suppressed in spl7 mutants with a wild-type Spl7 transgene. Thus, we conclude that Spl7 encodes the HSF protein. The transcript of spl7 was observed in mutant plants. The levels of mRNAs (Spl7 in wild type and spl7 in mutant) increased under heat stress. Sequence analysis revealed only one base substitution in the HSF DNA-binding domain of the mutant allele, causing a change from tryptophan to cysteine.

Stress-specific activation and repression of heat shock factors 1 and 2

Vertebrate cells express a family of heat shock transcription factors (HSF1 to HSF4) that coordinate the inducible regulation of heat shock genes in response to diverse signals. HSF1 is potent and activated rapidly though transiently by heat shock, whereas HSF2 is a less active transcriptional regulator but can retain its DNA binding properties for extended periods. Consequently, the differential activation of HSF1 and HSF2 by various stresses may be critical for cells to survive repeated and diverse stress challenges and to provide a mechanism for more precise regulation of heat shock gene expression. Here we show, using a novel DNA binding and detection assay, that HSF1 and HSF2 are coactivated to different levels in response to a range of conditions that cause cell stress. Above a low basal activity of both HSFs, heat shock preferentially activates HSF1, whereas the amino acid analogue azetidine or the proteasome inhibitor MG132 coactivates both HSFs to different levels and hemin preferentially induces HSF2. Unexpectedly, we also found that heat shock has dramatic adverse effects on HSF2 that lead to its reversible inactivation coincident with relocalization from the nucleus. The reversible inactivation of HSF2 is specific to heat shock and does not occur with other stressors or in cells expressing high levels of heat shock proteins. These results reveal that HSF2 activity is negatively regulated by heat and suggest a role for heat shock proteins in the positive regulation of HSF2.

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.

Sites of expression of DnaJ homologs and Hsp70 in male and female flowers of the Japanese willow Salix gilgiana Seemen

We did in situ hybridization to identify the sites at which DnaJ homologs (SGJ1 and SGJ3) and Hsp70 were expressed in male and female flowers of the Japanese willow, Salix gilgiana Seemen. Transcripts of SGJ1, SGJ3, and Hsp70 were detected in microspores and in the tapetal layers of immature anthers, in the inner integument of ovules, and in the funicular epidermal layers of ovaries. Transcripts of SGJ3 and Hsp70 were also detected in the nectaries of male and female flowers, in which transcripts of SGJ1 were undetectable.

Multiple S gene family members including natural antisense transcripts are differentially expressed during development of maize flowers

Within the large Brassica S gene family, SLG (S locus glycoprotein) and SRK (S locus receptor kinase) participate to the control of pollen-stigma self-incompatibility. In the self-compatible species maize, S gene family members are predominantly expressed in vegetative organs but are also expressed to a lesser extent in the stigma (silk). To determine if the expression of any S gene family members correlates with female receptivity, we analyzed their expression in developing maize silks. We show that a large family of maize S transcripts is expressed in developing silks. Surprisingly, we isolated a cDNA complementary to a large portion of the antisense strand of the maize receptor kinase S domain. Rapid amplification of cDNA ends (RACE)-polymerase chain reaction, RNase protection, and Northern hybridization with single-stranded riboprobes confirmed that natural antisense S transcripts exist in leaves and seedling shoots and in all sexual tissues tested except mature pollen. These natural antisense S transcripts co-exist with several less abundant sense S transcripts. The accumulation of sense and antisense S transcripts is differentially regulated during pollen and silk development. Thus, these results support a role for S gene family members in sexual tissue development and/or compatible pollination and reveal a new level of complexity in the regulation and function of the S gene family in maize.

Isolation and characterization of HsfA3, a new heat stress transcription factor of Lycopersicon peruvianum

Stress-induced transcription of heat shock proteins (Hsps) in eukaryotes is mediated by a conserved class of transcription factors called heat stress transcription factors (Hsfs). Here we report the isolation and functional characterization of HsfA3, a new member of the Hsf family. HsfA3 was cloned from a tomato heat stress cDNA library by yeast two-hybrid screening, using HsfA1 as a bait. HsfA3 is a single-copy gene with all the conserved sequence elements characteristic of a heat stress transcription factor. The constitutively expressed HsfA3 is mainly found in the cytoplasm under control conditions and in the nucleus under heat stress conditions. Functionally, HsfA3 behaves similarly to the already known members of tomato Hsf family. It is able to substitute yeast Hsf for viability functions and is a strong activator of Hsf-dependent reporter constructs both in tobacco protoplasts and yeast. Finally, similar to the AHA motifs in HsfA1 and HsfA2, the activator function depends on four short peptide motifs with a central tryptophan residue found in the C-terminal domain of HsfA3.

Heat-shock proteins, molecular chaperones, and the stress response: evolutionary and ecological physiology

Molecular chaperones, including the heat-shock proteins (Hsps), are a ubiquitous feature of cells in which these proteins cope with stress-induced denaturation of other proteins. Hsps have received the most attention in model organisms undergoing experimental stress in the laboratory, and the function of Hsps at the molecular and cellular level is becoming well understood in this context. A complementary focus is now emerging on the Hsps of both model and nonmodel organisms undergoing stress in nature, on the roles of Hsps in the stress physiology of whole multicellular eukaryotes and the tissues and organs they comprise, and on the ecological and evolutionary correlates of variation in Hsps and the genes that encode them. This focus discloses that (a) expression of Hsps can occur in nature, (b) all species have hsp genes but they vary in the patterns of their expression, (c) Hsp expression can be correlated with resistance to stress, and (d) species' thresholds for Hsp expression are correlated with levels of stress that they naturally undergo. These conclusions are now well established and may require little additional confirmation; many significant questions remain unanswered concerning both the mechanisms of Hsp-mediated stress tolerance at the organismal level and the evolutionary mechanisms that have diversified the hsp genes.

The tomato Hsf system: HsfA2 needs interaction with HsfA1 for efficient nuclear import and may be localized in cytoplasmic heat stress granules

In heat-stressed (HS) tomato (Lycopersicon peruvianum) cell cultures, the constitutively expressed HS transcription factor HsfA1 is complemented by two HS-inducible forms, HsfA2 and HsfB1. Because of its stability, HsfA2 accumulates to fairly high levels in the course of a prolonged HS and recovery regimen. Using immunofluorescence and cell fractionation experiments, we identified three states of HsfA2: (i) a soluble, cytoplasmic form in preinduced cultures maintained at 25 degrees C, (ii) a salt-resistant, nuclear form found in HS cells, and (iii) a stored form of HsfA2 in cytoplasmic HS granules. The efficient nuclear transport of HsfA2 evidently requires interaction with HsfA1. When expressed in tobacco protoplasts by use of a transient-expression system, HsfA2 is mainly retained in the cytoplasm unless it is coexpressed with HsfA1. The essential parts for the interaction and nuclear cotransport of the two Hsfs are the homologous oligomerization domain (HR-A/B region of the A-type Hsfs) and functional nuclear localization signal motifs of both partners. Direct physical interaction of the two Hsfs with formation of relatively stabile hetero-oligomers was shown by a two-hybrid test in Saccharomyces cerevisiae as well as by coimmunoprecipitation using tomato and tobacco whole-cell lysates.