Functional analysis of the Hikeshi-like protein and its interaction with HSP70 in Arabidopsis

Beyond heat waves: Unlocking epigenetic heat stress memory in Arabidopsis

Plants remember their exposure to environmental changes and respond more effectively the next time they encounter a similar change by flexibly altering gene expression. Epigenetic mechanisms play a crucial role in establishing such memory of environmental changes and fine-tuning gene expression. With the recent advancements in biochemistry and sequencing technologies, it has become possible to characterize the dynamics of epigenetic changes on scales ranging from short term (minutes) to long term (generations). Here, our main focus is on describing the current understanding of the temporal regulation of histone modifications and chromatin changes during exposure to short-term recurring high temperatures and reevaluating them in the context of natural environments. Investigations of the dynamics of histone modifications and chromatin structural changes in Arabidopsis after repeated exposure to heat at short intervals have revealed the detailed molecular mechanisms of short-term heat stress memory, which include histone modification enzymes, chromatin remodelers, and key transcription factors. In addition, we summarize the spatial regulation of heat responses. Based on the natural temperature patterns during summer, we discuss how plants cope with recurring heat stress occurring at various time intervals by utilizing 2 distinct types of heat stress memory mechanisms. We also explore future research directions to provide a more precise understanding of the epigenetic regulation of heat stress memory.

Evolutionary analysis and expression profiling of the HSP70 gene family in response to abiotic stresses in tomato (Solanum lycopersicum)

Heat shock protein 70 (HSP70) genes play essential roles in guarding plants against abiotic stresses, including heat, drought, and salt. In this study, the SlHSP70 gene family in tomatoes has been characterized using bioinformatic tools. 25 putative SlHSP70 genes in the tomato genome were found and classified into five subfamilies, with multi-subcellular localizations. Twelve pairs of gene duplications were identified, and segmental events were determined as the main factor for the gene family expansion. Based on public RNA-seq data, gene expression analysis identified the majority of genes expressed in the examined organelles. Further RNA-seq analysis and then quantitative RT-PCR validation showed that many SlHSP70 members are responsible for cellular feedback to heat, drought, and salt treatments, in which, at least five genes might be potential key players in the stress response. Our results provided a thorough overview of the SlHSP70 gene family in the tomato, which may be useful for the evolutionary and functional analysis of SlHSP70 under abiotic stress conditions.

Overexpression of Zostera japonica heat shock protein gene ZjHsp70 enhances the thermotolerance of transgenic Arabidopsis

BACKGROUND

Heat shock protein 70s (Hsp70s) are major members of the heat shock protein family and play a variety of roles to protect plants against stress. Plant Hsp70s are a conserved and widely expressed family of heat shock proteins. They have two main functional regions: N-terminal nucleic acid binding region and C-terminal substrate binding region.

METHODS AND RESULTS

In this study, we cloned the Hsp70 gene of Zostera japonica (ZjHsp70) based on the sequence obtained by transcriptome sequencing. The transcriptional levels of ZjHsp70 increased significantly at 1 h after heat treatment. ZjHsp70 was located in the cytoplasm and nucleus. The overexpression of ZjHsp70 in Arabidopsis resulted in increased heat tolerance, lower contents of malondialdehyde and higher antioxidant enzyme activity than in the wild type. ZjHsp70 may achieve this goal by maintaining highly active antioxidant enzymes.

CONCLUSIONS

We show that ZjHsp70 can improve plant heat tolerance by maintaining high antioxidant enzyme activity under high temperature stress. This study provided a basis to study the role of ZjHsp70 in thermotolerance in more detail.

Lack of Hikeshi activates HSF1 activity under normal conditions and disturbs the heat-shock response

Hikeshi mediates the nuclear import of the molecular chaperone HSP70 under heat-shock (acute heat stress) conditions, which is crucial for recovery from cellular damage. The cytoplasmic function of HSP70 is well studied, but its nuclear roles, particularly under nonstressed conditions, remain obscure. Here, we show that Hikeshi regulates the nucleocytoplasmic distribution of HSP70 not only under heat-shock conditions but also under nonstressed conditions. Nuclear HSP70 affects the transcriptional activity of HSF1 and nuclear proteostasis under nonstressed conditions. Depletion of Hikeshi induces a reduction in nuclear HSP70 and up-regulation of the mRNA expression of genes regulated by HSF1 under nonstressed conditions. In addition, the heat-shock response is impaired in Hikeshi-knockout cells. Our results suggest that HSF1 transcriptional activity is tightly regulated by nuclear HSP70 because nuclear-localized Hsp70 effectively suppresses transcriptional activity in a dose-dependent manner. Furthermore, the cytotoxicity of nuclear pathologic polyglutamine proteins was increased by Hikeshi depletion. Thus, proper nucleocytoplasmic distribution of HSP70, mediated by Hikeshi, is required for nuclear proteostasis and adaptive response to heat shock.

Thermo-Priming Mediated Cellular Networks for Abiotic Stress Management in Plants

Plants can adapt to different environmental conditions and can survive even under very harsh conditions. They have developed elaborate networks of receptors and signaling components, which modulate their biochemistry and physiology by regulating the genetic information. Plants also have the abilities to transmit information between their different parts to ensure a holistic response to any adverse environmental challenge. One such phenomenon that has received greater attention in recent years is called stress priming. Any milder exposure to stress is used by plants to prime themselves by modifying various cellular and molecular parameters. These changes seem to stay as memory and prepare the plants to better tolerate subsequent exposure to severe stress. In this review, we have discussed the various ways in which plants can be primed and illustrate the biochemical and molecular changes, including chromatin modification leading to stress memory, with major focus on thermo-priming. Alteration in various hormones and their subsequent role during and after priming under various stress conditions imposed by changing climate conditions are also discussed.

Exogenous Methyl Jasmonate Improves Heat Tolerance of Perennial Ryegrass Through Alteration of Osmotic Adjustment, Antioxidant Defense, and Expression of Jasmonic Acid-Responsive Genes

Perennial ryegrass (Lolium perenne L.) is an important cool-season grass species that is widely cultivated in temperate regions worldwide but usually sensitive to heat stress. Jasmonates (JAs) may have a positive effect on plant tolerance under heat stress. In this study, results showed that exogenous methyl jasmonic acid (MeJA) could significantly improve heat tolerance of perennial ryegrass through alteration of osmotic adjustment, antioxidant defense, and the expression of JA-responsive genes. MeJA-induced heat tolerance was involved in the maintenance of better relative water content (RWC), the decline of chlorophyll (Chl) loss for photosynthetic maintenance, as well as maintained lower electrolyte leakage (EL) and malondialdehyde (MDA) content under heat condition, so as to avoid further damage to plants. Besides, results also indicated that exogenous MeJA treatment could increase the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), thus enhancing the scavenging ability of reactive oxygen species, alleviating the oxidative damage caused by heat stress. Heat stress and exogenous MeJA upregulated transcript levels of related genes (LpLOX2, LpAOC, LpOPR3, and LpJMT) in JA biosynthetic pathway, which also could enhance the accumulation of JA and MeJA content. Furthermore, some NAC transcription factors and heat shock proteins may play a positive role in enhancing resistance of perennial ryegrass with heat stress.

AtHsc70-1 negatively regulates the basal heat tolerance in Arabidopsis thaliana through affecting the activity of HsfAs and Hsp101

Heat shock protein 70 (Hsp70) chaperones are highly conserved and essential proteins with diverse cellular functions, including plant abiotic stress tolerance. Hsp70 proteins have been linked with basal heat tolerance in plants. Hsp101 likewise is an important chaperone protein that plays a critical role in heat tolerance in plants. We observed that Arabidopsis hsc70-1 mutant seedlings show elevated basal heat tolerance compared with wild-type. Over-expression of Hsc70-1 resulted in increased heat sensitivity. Hsp101 transcript and protein levels were increased during non-heat stress (HS) and post-HS conditions in hsc70-1 mutant seedlings. In contrast, Hsp101 was repressed in Hsc70-1 over-expressing plants after post-HS conditions. Hsc70-1 showed physical interaction with HsfA1d and HsfA1e protein in the cytosol under non-HS conditions. In transient reporter gene analysis, HsfA1d, HsfA1e and HsfA2 showed transcriptional response on the Hsp101 promoter. HsfA1d and HsfA2 transcripts were at higher levels in hsc70-1 mutant compared with wild-type. We provide genetic evidence that Hsc70-1 is a negative regulator affecting HsfA1d/A1e/A2 activators, which in turn regulate Hsp101 expression and basal thermotolerance.

Heat stress-induced nuclear transport mediated by Hikeshi confers nuclear function of Hsp70s

The prime feature of eukaryotic cells is the separation of the intracellular space into two compartments, the nucleus and the cytoplasm. Active nuclear transport is crucial for the maintenance of this separation. In this report, we focus on a nuclear transport receptor named Hikeshi, which mediates the heat stress-induced nuclear import of 70-kDa heat shock proteins (Hsp70s), and discuss how the same protein can function differently depending on the cellular compartment in which it is localized. Hsp70 is a molecular chaperone that is predominantly localized in the cytoplasm under normal conditions but is known to accumulate in the nucleus under conditions of heat stress. Although the reported function of Hsp70 is mostly attributed to its molecular function in the cytoplasm, the functions of Hsp70 may extend beyond molecular chaperone activity in the nucleus.

Hikeshi modulates the proteotoxic stress response in human cells: Implication for the importance of the nuclear function of HSP70s

Hikeshi mediates the heat stress-induced nuclear import of heat-shock protein 70 (HSP70s: HSP70/HSC70). Dysfunction of Hikeshi causes some serious effects in humans; however, the cellular function of Hikeshi is largely unknown. Here, we investigated the effects of Hikeshi depletion on the survival of human cells after proteotoxic stress and found opposite effects in HeLa and hTERT-RPE1 (RPE) cells; depletion of Hikeshi reduced the survival of HeLa cells, but increased the survival of RPE cells in response to proteotoxic stress. Hikeshi depletion sustained heat-shock transcription factor 1 (HSF1) activation in HeLa cells after recovery from stress, but introduction of a nuclear localization signal-tagged HSC70 in Hikeshi-depleted HeLa cells down-regulated HSF1 activity. In RPE cells, the HSF1 was efficiently activated, but the activated HSF1 was not sustained after recovery from stress, as in HeLa cells. Additionally, we found that p53 and subsequent up-regulation of p21 were higher in the Hikeshi-depleted RPE cells than in the wild-type cells. Our results indicate that depletion of Hikeshi renders HeLa cells proteotoxic stress-sensitive through the abrogation of the nuclear function of HSP70s required for HSF1 regulation. Moreover, Hikeshi depletion up-regulates p21 in RPE cells, which could be a cause of its proteotoxic stress resistant.

Molecular analysis of Hsp70 mechanisms in plants and their function in response to stress

Studying the strategies of improving abiotic stress tolerance is quite imperative and research under this field will increase our understanding of response mechanisms to abiotic stress such as heat. The Hsp70 is an essential regulator of protein having the tendency to maintain internal cell stability like proper folding protein and breakdown of unfolded proteins. Hsp70 holds together protein substrates to help in movement, regulation, and prevent aggregation under physical and or chemical pressure. However, this review reports the molecular mechanism of heat shock protein 70 kDa (Hsp70) action and its structural and functional analysis, research progress on the interaction of Hsp70 with other proteins and their interaction mechanisms as well as the involvement of Hsp70 in abiotic stress responses as an adaptive defense mechanism.

Proteomic analysis shows that stress response proteins are significantly up-regulated in resistant diploid wheat (Triticum monococcum) in response to attack by the grain aphid (Sitobion avenae)

The grain aphid Sitobion avenae (F.) is a major pest of wheat, acting as a virus vector as well as causing direct plant damage. Commonly grown wheat varieties in the UK have only limited resistance to this pest. The present study was carried out to investigate the potential of a diploid wheat line (ACC20 PGR1755), reported as exhibiting resistance to S. avenae, to serve as a source of resistance genes. The diploid wheat line was confirmed as partially resistant, substantially reducing the fecundity, longevity and growth rate of the aphid. Proteomic analysis showed that approximately 200 protein spots were reproducibly detected in leaf extracts from both the resistant line and a comparable susceptible line (ACC5 PGR1735) using two-dimensional gel electrophoresis and image comparison software. Twenty-four spots were significantly up-regulated (>2-fold) in the resistant line after 24 h of aphid feeding (13 and 11 involved in local and systemic responses, respectively). Approximately 50 % of all differentially expressed protein spots were identified by a combination of database searching with MS and MS/MS data, revealing that the majority of proteins up-regulated by aphid infestation were involved in metabolic processes (including photosynthesis) and transcriptional regulation. However, in the resistant line only, several stress response proteins (including NBS-LRR-like proteins) and oxidative stress response proteins were identified as up-regulated in response to aphid feeding, as well as proteins involved in DNA synthesis/replication/repair. This study indicates that the resistant diploid line ACC20 PGR1755 may provide a valuable resource in breeding wheat for resistance to aphids.