Sumoylation of Arabidopsis heat shock factor A2 (HsfA2) modifies its activity during acquired thermotholerance

N-terminal acetylation orchestrates glycolate-mediated ROS homeostasis to promote rice thermoresponsive growth

Climate warming poses a significant threat to global crop production and food security. However, our understanding of the molecular mechanisms governing thermoresponsive development in crops remains limited. Here we report that the auxiliary subunit of N-terminal acetyltransferase A (NatA) in rice OsNAA15 is a prerequisite for rice thermoresponsive growth. OsNAA15 produces two isoforms OsNAA15.1 and OsNAA15.2, via temperature-dependent alternative splicing. Among the two, OsNAA15.1 is more likely to form a stable and functional NatA complex with the potential catalytic subunit OsNAA10, leading to a thermoresponsive N-terminal acetylome. Intriguingly, while OsNAA15.1 promotes plant growth under elevated temperatures, OsNAA15.2 exhibits an inhibitory effect. We identified two glycolate oxidases (GLO1/5) as major substrates from the thermoresponsive acetylome. These enzymes are involved in hydrogen peroxide (H2O2) biosynthesis via glycolate oxidation. N-terminally acetylated GLO1/5 undergo their degradation through the ubiquitin-proteasome system. This leads to reduced reactive oxygen species (ROS) production, thereby promoting plant growth, particularly under high ambient temperatures. Conclusively, our findings highlight the pivotal role of N-terminal acetylation in orchestrating the glycolate-mediated ROS homeostasis to facilitate thermoresponsive growth in rice.

Proteome Analysis of the ROF-FKBP Mutants Reveals Functional Relations among Heat Stress Responses, Plant Development, and Protein Quality Control during Heat Acclimation in Arabidopsis thaliana

In the present study, a differential screening following heat stress acclimation was performed in Arabidopsis thaliana WT and ROF-FKBP mutated plants using mass spectrometry, and the results were used to understand and analyze the effect of the ROF PPIases during thermotolerance acquisition in plants. Our data highlight the central role of these two PPIases in heat stress and point to their direct or indirect effect on other proteins participating in cellular functions such as protein folding and quality control, cell division, photosynthesis, and other metabolic and signaling processes. Specifically, the heat stress response, protein folding, and protein ER processing pathways are enhanced following a 37 °C acclimation period independent of the mutation state. However, at 37 °C, and in the double-mutated rof1-/2- plants, a higher accumulation of proteins belonging to the above pathways is observed compared with all other conditions (WT, single mutants, control, and heat-acclimated plants). Furthermore, the proteasomal pathway, involving the common member of both the protasomal and the lysosomal degradation pathway, CDC48, is over-represented in the extracts of both the untreated and heat-stressed rof1-/2- mutants compared with the other extracts. In contrast, in the single rof1- mutation, the heat acclimation pathway is suppressed at 37 °C when compared to the WT. Protein accumulation related to the heat stress and the protein quality control pathways points to a differential but also synergistic role of the two proteins. Protein complexes of other biochemical and developmental mechanisms, such as the light-harvesting complex of the photosynthetic pathway and the phosphoinositide binding proteins involved in membrane-trafficking events during cell plate formation and cytokinesis (patellin 1, 2, and 4), are negatively regulated in the rof1-/2- mutant. Our results suggest that ROF1 and ROF2 FKBPs regulate stress response, and developmental and metabolic pathways via a complex feedback mechanism involving partners that ensure protein quality control and plant survival during heat stress.

Thermosensitive SUMOylation of TaHsfA1 defines a dynamic ON/OFF molecular switch for the heat stress response in wheat

Dissecting genetic components in crop plants associated with heat stress (HS) sensing and adaptation will facilitate the design of modern crop varieties with improved thermotolerance. However, the molecular mechanisms underlying the ON/OFF switch controlling HS responses (HSRs) in wheat (Triticum aestivum) remain largely unknown. In this study, we focused on the molecular action of TaHsfA1, a class A heat shock transcription factor, in sensing dynamically changing HS signals and regulating HSRs. We show that the TaHsfA1 protein is modified by small ubiquitin-related modifier (SUMO) and that this modification is essential for the full transcriptional activation activity of TaHsfA1 in triggering downstream gene expression. During sustained heat exposure, the SUMOylation of TaHsfA1 is suppressed, which partially reduces TaHsfA1 protein activity, thereby reducing the intensity of downstream HSRs. In addition, we demonstrate that TaHsfA1 interacts with the histone acetyltransferase TaHAG1 in a thermosensitive manner. Together, our findings emphasize the importance of TaHsfA1 in thermotolerance in wheat. In addition, they define a highly dynamic SUMOylation-dependent "ON/OFF" molecular switch that senses temperature signals and contributes to thermotolerance in crops.

Interfering small ubiquitin modifiers (SUMO) improves the thermotolerance of apple by facilitating the activity of MdDREB2A

Heat stress, which is caused by global warming, threatens crops yield and quality across the world. As a kind of post-translation modification, SUMOylation involves in plants heat stress response with a rapid and wide pattern. Here, we identified small ubiquitin modifiers (SUMO), which affect drought tolerance in apple, also participated in thermotolerance. Six isoforms of SUMOs located on six chromosomes in apple genome, and all the SUMOs were up-regulated in response to heat stress condition. The MdSUMO2 RNAi transgenic apple plants exhibited higher survival rate, lower ion leakage, higher catalase (CAT) activity, and Malondialdehyde (MDA) content under heat stress. MdDREB2A, the substrate of MdSUMO2 in apple, was accumulated in MdSUMO2 RNAi transgenic plants than the wild type GL-3 at the protein level in response to heat stress treatment. Further, the inhibited SUMOylation level of MdDREB2A in MdSUMO2 RNAi plants might repress its ubiquitination, too. The accumulated MdDREB2A in MdSUMO2 RNAi plants further induced heat-responsive genes expression to strengthen plants thermotolerance, including MdHSFA3, MdHSP26.5, MdHSP18.2, MdHSP70, MdCYP18-1 and MdTLP1. In summary, these findings illustrate that interfering small ubiquitin modifiers (SUMO) in apple improves plants thermotolerance, partly by facilitating the stability and activity of MdDREB2A.

SUMOylation facilitates the assembly of a Nuclear Factor-Y complex to enhance thermotolerance in Arabidopsis

Heat stress (HS) has serious negative effects on plant development and has become a major threat to agriculture. A rapid transcriptional regulatory cascade has evolved in plants in response to HS. Nuclear Factor-Y (NF-Y) complexes are critical for this mechanism, but how NF-Y complexes are regulated remains unclear. In this study, we identified NF-YC10 (NF-Y subunit C10), a central regulator of the HS response in Arabidopsis thaliana, as a substrate of SUMOylation, an important post-translational modification. Biochemical analysis showed that the SUMO ligase SIZ1 (SAP AND MIZ1 DOMAIN-CONTAINING LIGASE1) interacts with NF-YC10 and enhances its SUMOylation during HS. The SUMOylation of NF-YC10 facilitates its interaction with and the nuclear translocation of NF-YB3, in which the SUMO interaction motif (SIM) is essential for its efficient association with NF-YC10. Further functional analysis indicated that the SUMOylation of NF-YC10 and the SIM of NF-YB3 are critical for HS-responsive gene expression and plant thermotolerance. These findings uncover a role for the SIZ1-mediated SUMOylation of NF-YC10 in NF-Y complex assembly under HS, providing new insights into the role of a post-translational modification in regulating transcription during abiotic stress responses in plants.

Current perspectives of ubiquitination and SUMOylation in abiotic stress tolerance in plants

Post-translational modification (PTM) is a critical and rapid mechanism to regulate all the major cellular processes through the modification of diverse protein substrates. Substrate-specific covalent attachment of ubiquitin and Small Ubiquitin-Like Modifier (SUMO) with the target proteins, known as ubiquitination and SUMOylation, respectively, are crucial PTMs that regulate almost every process in the cell by modulating the stability and fidelity of the proteins. Ubiquitination and SUMOylation play a very significant role to provide tolerance to the plants in adverse environmental conditions by activating/deactivating the pre-existing proteins to a great extent. We reviewed the importance of ubiquitination and SUMOylation in plants, implicating its prospects in various abiotic stress regulations. An exhaustive study of molecular mechanisms of ubiquitination and SUMOylation of plant proteins and their role will contribute to the understanding of physiology underlying mitigation of the abiotic stresses and survival in plants. It will be helpful to strategize the improvement of crops for abiotic stress tolerance.

Reproductive-Stage Heat Stress in Cereals: Impact, Plant Responses and Strategies for Tolerance Improvement

Reproductive-stage heat stress (RSHS) poses a major constraint to cereal crop production by damaging main plant reproductive structures and hampering reproductive processes, including pollen and stigma viability, pollination, fertilization, grain setting and grain filling. Despite this well-recognized fact, research on crop heat stress (HS) is relatively recent compared to other abiotic stresses, such as drought and salinity, and in particular, RSHS studies in cereals are considerably few in comparison with seedling-stage and vegetative-stage-centered studies. Meanwhile, climate change-exacerbated HS, independently or synergistically with drought, will have huge implications on crop performance and future global food security. Fortunately, due to their sedentary nature, crop plants have evolved complex and diverse transient and long-term mechanisms to perceive, transduce, respond and adapt to HS at the molecular, cell, physiological and whole plant levels. Therefore, uncovering the molecular and physiological mechanisms governing plant response and tolerance to RSHS facilitates the designing of effective strategies to improve HS tolerance in cereal crops. In this review, we update our understanding of several aspects of RSHS in cereals, particularly impacts on physiological processes and yield; HS signal perception and transduction; and transcriptional regulation by heat shock factors and heat stress-responsive genes. We also discuss the epigenetic, post-translational modification and HS memory mechanisms modulating plant HS tolerance. Moreover, we offer a critical set of strategies (encompassing genomics and plant breeding, transgenesis, omics and agronomy) that could accelerate the development of RSHS-resilient cereal crop cultivars. We underline that a judicious combination of all of these strategies offers the best foot forward in RSHS tolerance improvement in cereals. Further, we highlight critical shortcomings to RSHS tolerance investigations in cereals and propositions for their circumvention, as well as some knowledge gaps, which should guide future research priorities. Overall, our review furthers our understanding of HS tolerance in plants and supports the rational designing of RSHS-tolerant cereal crop cultivars for the warming climate.

Overexpression of a novel heat-inducible ethylene-responsive factor gene LlERF110 from Lilium longiflorum decreases thermotolerance

AP2/ERF (APETALA2/ethylene-responsive factor) family transcription factors are involved in various plant-specific processes, especially in plant development and response to abiotic stress. However, their roles in thermotolerance are still largely unknown. In the current study, we identified a heat-inducible ERF member LlERF110 from Lilium longiflorum that was rapidly induced by high temperature. Its protein was localized in the nucleus, and transcriptional activation activity was observed in yeast and plant cells. In addition, LlERF110 was able to bind to GCC- and CGG-elements, but not to DRE-elements. Overexpression of LlERF110 conferred delayed bolting and bushy phenotype, with decreased thermotolerance accompanied by a disrupted ROS (reactive oxygen species) homeostasis in transgenic plants. The accumulation of LlERF110 may activate certain repressors related to heat stress response (HSR) and indirectly damage the normal expression of heat stress (HS)-protective genes such as AtHSFA2, which consequently leads to reduced thermotolerance. Our results implied that LlERF110 might function as a heat-inducible gene but may hinder the establishment of thermotolerance.

A MITE variation-associated heat-inducible isoform of a heat-shock factor confers heat tolerance through regulation of JASMONATE ZIM-DOMAIN genes in rice

High temperatures cause huge yield losses in rice. Heat-shock factors (Hsfs) are key transcription factors which regulate the expression of heat stress-responsive genes, but natural variation in and functional characterization of Hsfs have seldom been reported. A significant heat response locus was detected via a genome-wide association study (GWAS) using green leaf area as an indicative trait. A miniature inverted-repeat transposable element (MITE) in the promoter of a candidate gene, HTG3 (heat-tolerance gene on chromosome 3), was found to be significantly associated with heat-induced expression of HTG3 and heat tolerance (HT). The MITE-absent variant has been selected in heat-prone rice-growing regions. HTG3a is an alternatively spliced isoform encoding a functional Hsf, and experiments using overexpression and knockout rice lines showed that HTG3a positively regulates HT at both vegetative and reproductive stages. The HTG3-regulated genes were enriched for heat shock proteins and jasmonic acid signaling. Two heat-responsive JASMONATE ZIM-DOMAIN (JAZ) genes were confirmed to be directly upregulated by HTG3a, and one of them, OsJAZ9, positively regulates HT. We conclude that HTG3 plays an important role in HT through the regulation of JAZs and other heat-responsive genes. The MITE-absent allele may be valuable for HT breeding in rice.

Post-translational modification: a strategic response to high temperature in plants

With the increasing global warming, high-temperature stress is affecting plant growth and development with greater frequency. Therefore, an increasing number of studies examining the mechanism of temperature response contribute to a more optimal understanding of plant growth under environmental pressure. Post-translational modification (PTM) provides the rapid reconnection of transcriptional programs including transcription factors and signaling proteins. It is vital that plants quickly respond to changes in the environment in order to survive under stressful situations. Herein, we discuss several types of PTMs that occur in response to warm-temperature and high-temperature stress, including ubiquitination, SUMOylation, phosphorylation, histone methylation, and acetylation. This review provides a valuable resolution to this issue to enable increased crop productivity at high temperatures.

Elongation factor TFIIS is essential for heat stress adaptation in plants

Elongation factor TFIIS (transcription factor IIS) is structurally and biochemically probably the best characterized elongation cofactor of RNA polymerase II. However, little is known about TFIIS regulation or its roles during stress responses. Here, we show that, although TFIIS seems unnecessary under optimal conditions in Arabidopsis, its absence renders plants supersensitive to heat; tfIIs mutants die even when exposed to sublethal high temperature. TFIIS activity is required for thermal adaptation throughout the whole life cycle of plants, ensuring both survival and reproductive success. By employing a transcriptome analysis, we unravel that the absence of TFIIS makes transcriptional reprogramming sluggish, and affects expression and alternative splicing pattern of hundreds of heat-regulated transcripts. Transcriptome changes indirectly cause proteotoxic stress and deterioration of cellular pathways, including photosynthesis, which finally leads to lethality. Contrary to expectations of being constantly present to support transcription, we show that TFIIS is dynamically regulated. TFIIS accumulation during heat occurs in evolutionary distant species, including the unicellular alga Chlamydomonas reinhardtii, dicot Brassica napus and monocot Hordeum vulgare, suggesting that the vital role of TFIIS in stress adaptation of plants is conserved.

Hsp70, sHsps and ubiquitin proteins modulate HsfA6a-mediated Hsp101 transcript expression in rice (Oryza sativa L.)

Hsp100 chaperones disaggregate the aggregated proteins and are vital for maintenance of protein homeostasis. The level of Hsp100 synthesised in the cells has a bearing on the survival of plants under heat stress. The Hsp100 transcription machinery is activated within minutes of the onset of heat stress. The heat shock factor HsfA6a plays a major role in the transcriptional regulation of the Hsp101 gene in rice plants. Through yeast-2-hybrid library screening, we identified small heat shock proteins (sHSPs), Hsp70 and ubiquitin as HsfA6a interacting proteins (HIPs). The bimolecular fluorescence complementation assays showed the physical interaction of HsfA6a with Hsp16.9A-CI and Hsp18.0-CII in the cytosolic region and with cHsp70-1 in the nucleus. With the Hsp101 promoter: reporter gene assays, using yeast cells and rice protoplasts, we show that CI-sHsps and CII-sHsps are negative regulators and Hsp70 positive regulator of the HsfA6a activity in modulation of Hsp101 transcription. We also noted that the HsfA6a interactors, Hsp70 and CI-sHsps and CII-sHsps, physically interact with each other. We noted that HsfA6a binds with the CI-sHsp and Hsp70 promoters, implying that HsfA6a has a role in transcriptional regulation of its interacting proteins. Furthermore, we noted that the mutation of the ubiquitin/sumoylation acceptor site lysine 10 to alanine (K10A) of HsfA6a enhanced its DNA binding potential on the Hsp101 promoter, implying that these modifiers are possibly involved in modulation of HsfA6a activity. Our work shows that Hsp70, CI-sHsps and CII-sHsp, and ubiquitin proteins coordinate with HsfA6a in mediating the Hsp101 transcription process in rice.

SUMO conjugating enzyme: a vital player of SUMO pathway in plants

Plants face numerous challenges such as biotic and abiotic stresses during their whole lifecycle. As they are sessile in nature, they ought to develop multiple ways to act during stressed conditions to maintain cellular homeostasis. Among various defense mechanisms, the small ubiquitin-like modifiers (SUMO) pathway is considered as the most important because several nuclear proteins regulated by this pathway are involved in several cellular functions such as response to stress, transcription, translation, metabolism of RNA, energy metabolism, repairing damaged DNA, ensuring genome stability and nuclear trafficking. In general, the SUMO pathway has its own particular set of enzymes E1, E2, and E3. The SUMO conjugating enzyme [SCE (E2)] is a very crucial member of the pathway which can transfer SUMO to its target protein even without the involvement of E3. More than just a middle player, it has shown its involvement in effective triggered immunity in crops like tomato and various abiotic stresses like drought and salinity in maize, rice, and Arabidopsis. This review tries to explore the importance of the SUMOylation process, focusing on the E2 enzyme and its regulatory role in the abiotic stress response, plant immunity, and DNA damage repair.

SUMOylation: A critical transcription modulator in plant cells

Gene transcription is critical for various cellular processes and is precisely controlled at multiple levels, and posttranslational modification (PTM) is a fast and powerful way to regulate transcription factors (TFs). SUMOylation, which conjugates small ubiquitin-related modifier (SUMO) molecules to protein substrates, is a crucial PTM that modulates the activity, stability, subcellular localization, and partner interactions of TFs in plant cells. Here, we summarize the mechanisms of SUMOylation in the regulation of transcription in plant development and stress responses. We also discuss the crosstalk between SUMOylation and other PTMs, as well as the potential functions of SUMOylation in the regulation of transcription-associated complexes on plant chromatin. This summary and perspective will improve understanding of the molecular mechanism of PTMs in plant transcription regulation.

The protein modifier SUMO is critical for integrity of the Arabidopsis shoot apex at warm ambient temperatures

SUMO is a protein modification whose conjugate levels peak during acute heat stress. We find that SUMO is also critical for plant longevity when Arabidopsis experiences a prolonged non-damaging period of only 28 degrees Celsius. Remarkably, this thermo-lethality at 28 degrees was not seen with any other mutant of the SUMO pathway tested. Autoimmunity due to low SUMO1/2 expression levels was not causal for this thermo-lethality. The role of SUMO for thermo-resilience was also distinct from its requirement for thermomorphogenesis - a growth response triggered by the same warm temperature, as only the latter response was dependent on the SUMO ligase SIZ1 as well. Thermo-resilience at 28 degrees Celsius and (acquired) thermotolerance (a response that allows plants to recover and acclimate to brief extreme temperatures) both depend on the HEAT SHOCK TRANSCRIPTION FACTOR A1 (HSFA1). Acquired thermotolerance was, however, normal in the sumo1/2 knockdown mutant. Thus, SUMO-dependent thermo-resilience is potentially controlled in a different way than the protein damage pathway that underpins thermotolerance. Close inspection of shoot apices revealed that the cell patterning and tissue integrity of the shoot apex of the SUMO1/2 knockdown mutant was lost at 28, but not 22 degrees Celsius. We thus describe a novel SUMO-dependent phenotype.

Diversity of plant heat shock factors: regulation, interactions, and functions

Plants heat shock factors (HSFs) are encoded by large gene families with variable structure, expression, and function. HSFs are components of complex signaling systems that control responses not only to high temperatures but also to a number of abiotic stresses such as cold, drought, hypoxic conditions, soil salinity, toxic minerals, strong irradiation, and to pathogen threats. Here we provide an overview of the diverse world of plant HSFs through compilation and analysis of their functional versatility, diverse regulation, and interactions. Bioinformatic data on gene expression profiles of Arabidopsis HSF genes were re-analyzed to reveal their characteristic transcript patterns. While HSFs are regulated primarily at the transcript level, alternative splicing and post-translational modifications such as phosphorylation and sumoylation provides further variability. Plant HSFs are involved in an intricate web of protein-protein interactions which adds considerable complexity to their biological function. A list of such interactions was compiled from public databases and published data, and discussed to pinpoint their relevance in transcription control. Although most fundamental studies of plant HSFs have been conducted in the model plant, Arabidopsis, information on HSFs is accumulating in other plants such as tomato, rice, wheat, and sunflower. Understanding the function, interactions, and regulation of HSFs will facilitate the design of novel strategies to use engineered proteins to improve tolerance and adaptation of crops to adverse environmental conditions.

Apple SUMO E3 ligase MdSIZ1 facilitates SUMOylation of MdARF8 to regulate lateral root formation

Post-translational modification of proteins mediated by SIZ1, a small ubiquitin-like modifier (SUMO) E3 ligase, regulates multiple biological processes in plants. However, its role in the regulation of lateral root formation remains unclear. Here, we demonstrate that the apple SUMO E3 ligase MdSIZ1 promotes lateral root formation. Using a yeast-two-hybrid (Y2H) system, the auxin response factor MdARF8 was screened out as a protein-protein interaction partner of the SUMO-conjugating E2 enzyme MdSCE1, indicating that MdARF8 may be a substrate for MdSIZ1. The interaction between MdARF8 and MdSCE1 was confirmed by pull-down, Y2H and Co-immunoprecipitation assays. MdSIZ1 enhanced the conjugating enzyme activity of MdSCE1 to form a MdSCE1-MdSIZ1-MdARF8 complex, thereby facilitating SUMO modification. We identified two arginine substitution mutations at K342 and K380 in MdARF8 that blocked MdSIZ1-mediated SUMOylation, indicating that K342 and K380 are the principal SUMOylation sites of the MdARF8 protein. Moreover, MdARF8 promoted lateral root formation in transgenic apple plants, and the phenotype of reduced lateral roots in the Arabidopsis siz1-2 mutant was restored in siz1-2/MdARF8 complementary plants. Our findings reveal an important role for sumoylation in the regulation of lateral root formation in plants.

Chromatin-associated SUMOylation controls the transcriptional switch between plant development and heat stress responses

The post-translational protein modification known as SUMOylation has conserved roles in the heat stress responses of various species. The functional connection between the global regulation of gene expression and chromatin-associated SUMOylation in plant cells is unknown. Here, we uncovered a genome-wide relationship between chromatin-associated SUMOylation and transcriptional switches in Arabidopsis thaliana grown at room temperature, exposed to heat stress, and exposed to heat stress followed by recovery. The small ubiquitin-like modifier (SUMO)-associated chromatin sites, characterized by whole-genome ChIP-seq, were generally associated with active chromatin markers. In response to heat stress, chromatin-associated SUMO signals increased at promoter-transcriptional start site regions and decreased in gene bodies. RNA-seq analysis supported the role of chromatin-associated SUMOylation in transcriptional activation during rapid responses to high temperature. Changes in SUMO signals on chromatin were associated with the upregulation of heat-responsive genes and the downregulation of growth-related genes. Disruption of the SUMO ligase gene SIZ1 abolished SUMO signals on chromatin and attenuated rapid transcriptional responses to heat stress. The SUMO signal peaks were enriched in DNA elements recognized by distinct groups of transcription factors under different temperature conditions. These observations provide evidence that chromatin-associated SUMOylation regulates the transcriptional switch between development and heat stress response in plant cells.

Plant SUMO E3 Ligases: Function, Structural Organization, and Connection With DNA

Protein modification by the small ubiquitin-like modifier (SUMO) plays an important role in multiple plant processes, including growth, development, and the response to abiotic stresses. Mechanistically, SUMOylation is a sequential multi-enzymatic process where SUMO E3 ligases accelerate SUMO conjugation while also influencing target identity and interactions. This review explores the biological functions of plant SUMO E3 ligases [SAP AND MIZ1 DOMAIN-CONTAINING LIGASE (SIZs), METHYL METHANESULFONATE-SENSITIVITY PROTEIN 21 (MMS21s), and PROTEIN INHIBITOR OF ACTIVATED STAT-LIKE (PIALs)] in relation to their molecular activities and domains. We also explore the sub-cellular localization of SUMO E3 ligases and review evidence suggesting a connection between certain SUMO E3 ligases and DNA that contributes to gene expression regulation.

Molecular and genetic bases of heat stress responses in crop plants and breeding for increased resilience and productivity

To ensure the food security of future generations and to address the challenge of the 'no hunger zone' proposed by the FAO (Food and Agriculture Organization), crop production must be doubled by 2050, but environmental stresses are counteracting this goal. Heat stress in particular is affecting agricultural crops more frequently and more severely. Since the discovery of the physiological, molecular, and genetic bases of heat stress responses, cultivated plants have become the subject of intense research on how they may avoid or tolerate heat stress by either using natural genetic variation or creating new variation with DNA technologies, mutational breeding, or genome editing. This review reports current understanding of the genetic and molecular bases of heat stress in crops together with recent approaches to creating heat-tolerant varieties. Research is close to a breakthrough of global relevance, breeding plants fitter to face the biggest challenge of our time.

Molecular Regulation of Plant Responses to Environmental Temperatures

Temperature is a key factor governing the growth and development, distribution, and seasonal behavior of plants. The entire plant life cycle is affected by environmental temperatures. Plants grow rapidly and exhibit specific changes in morphology under mild average temperature conditions, a response termed thermomorphogenesis. When exposed to chilling or moist chilling low temperatures, flowering or seed germination is accelerated in some plant species; these processes are known as vernalization and cold stratification, respectively. Interestingly, once many temperate plants are exposed to chilling temperatures for some time, they can acquire the ability to resist freezing stress, a process termed cold acclimation. In the face of global climate change, heat stress has emerged as a frequent challenge, which adversely affects plant growth and development. In this review, we summarize and discuss recent progress in dissecting the molecular mechanisms regulating plant thermomorphogenesis, vernalization, and responses to extreme temperatures. We also discuss the remaining issues that are crucial for understanding the interactions between plants and temperature.

Plant abiotic stress response and nutrient use efficiency

Abiotic stresses and soil nutrient limitations are major environmental conditions that reduce plant growth, productivity and quality. Plants have evolved mechanisms to perceive these environmental challenges, transmit the stress signals within cells as well as between cells and tissues, and make appropriate adjustments in their growth and development in order to survive and reproduce. In recent years, significant progress has been made on many fronts of the stress signaling research, particularly in understanding the downstream signaling events that culminate at the activation of stress- and nutrient limitation-responsive genes, cellular ion homeostasis, and growth adjustment. However, the revelation of the early events of stress signaling, particularly the identification of primary stress sensors, still lags behind. In this review, we summarize recent work on the genetic and molecular mechanisms of plant abiotic stress and nutrient limitation sensing and signaling and discuss new directions for future studies.

Pyrophosphate modulates plant stress responses via SUMOylation

Pyrophosphate (PPi), a byproduct of macromolecule biosynthesis is maintained at low levels by soluble inorganic pyrophosphatases (sPPase) found in all eukaryotes. In plants, H⁺-pumping pyrophosphatases (H⁺-PPase) convert the substantial energy present in PPi into an electrochemical gradient. We show here, that both cold- and heat stress sensitivity of fugu5 mutants lacking the major H⁺-PPase isoform AVP1 is correlated with reduced SUMOylation. In addition, we show that increased PPi concentrations interfere with SUMOylation in yeast and we provide evidence that SUMO activating E1-enzymes are inhibited by micromolar concentrations of PPi in a non-competitive manner. Taken together, our results do not only provide a mechanistic explanation for the beneficial effects of AVP1 overexpression in plants but they also highlight PPi as an important integrator of metabolism and stress tolerance.

SUMO modification of LBD30 by SIZ1 regulates secondary cell wall formation in Arabidopsis thaliana

A wide range of biological processes are regulated by sumoylation, a post-translational modification involving the conjugation of SUMO (Small Ubiquitin-Like Modifier) to protein. In Arabidopsis thaliana, AtSIZ1 encodes a SUMO E3 ligase for SUMO modification. siz1 mutants displayed defective secondary cell walls (SCWs) in inflorescence fiber cells. Such defects were caused by repression of SND1/NST1-mediated transcriptional networks. Yeast two-hybrid assay indicated that SIZ1 interacts with the LBD30 C-terminal domain, which was further confirmed using bimolecular fluorescence complementation and immunoprecipitation. Mass spectrometry and co-immunoprecipitation indicated that SIZ1 mediates SUMO conjugation to LBD30 at the K226 residue. Genes controlling SCW formation were activated by the overexpression of LBD30, but not in the LBD30(K226R) mutant. LBD30 enhancement of SCW formation resulted from upregulation of SND1/NST1-mediated transcriptional networks. This study presents a mechanism by which sumoylation of LBD30, mediated by SIZ1, regulates SCW formation in A. thaliana.

Arabidopsis thaliana SPF1 and SPF2 are nuclear-located ULP2-like SUMO proteases that act downstream of SIZ1 in plant development

Post-translational modifiers such as the small ubiquitin-like modifier (SUMO) peptide act as fast and reversible protein regulators. Functional characterization of the sumoylation machinery has determined the key regulatory role that SUMO plays in plant development. Unlike components of the SUMO conjugation pathway, SUMO proteases (ULPs) are encoded by a relatively large gene family and are potential sources of specificity within the pathway. This study reports a thorough comparative genomics and phylogenetic characterization of plant ULPs, revealing the presence of one ULP1-like and three ULP2-like SUMO protease subgroups within plant genomes. As representatives of an under-studied subgroup, Arabidopsis SPF1 and SPF2 were subjected to functional characterization. Loss-of-function mutants implicated both proteins with vegetative growth, flowering time, and seed size and yield. Mutants constitutively accumulated SUMO conjugates, and yeast complementation assays associated these proteins with the function of ScUlp2 but not ScUlp1. Fluorescence imaging placed both proteins in the plant cell nucleoplasm. Transcriptomics analysis indicated strong regulatory involvement in secondary metabolism, cell wall remodelling, and nitrate assimilation. Furthermore, developmental defects of the spf1-1 spf2-2 (spf1/2) double-mutant opposed those of the major E3 ligase siz1 mutant and, most significantly, developmental and transcriptomic characterization of the siz1 spf1/2 triple-mutant placed SIZ1 as epistatic to SPF1 and SPF2.

Unraveling Field Crops Sensitivity to Heat Stress：Mechanisms, Approaches, and Future Prospects

The astonishing increase in temperature presents an alarming threat to crop production worldwide. As evident by huge yield decline in various crops, the escalating drastic impacts of heat stress (HS) are putting global food production as well as nutritional security at high risk. HS is a major abiotic stress that influences plant morphology, physiology, reproduction, and productivity worldwide. The physiological and molecular responses to HS are dynamic research areas, and molecular techniques are being adopted for producing heat tolerant crop plants. In this article, we reviewed recent findings, impacts, adoption, and tolerance at the cellular, organellar, and whole plant level and reported several approaches that are used to improve HS tolerance in crop plants. Omics approaches unravel various mechanisms underlying thermotolerance, which is imperative to understand the processes of molecular responses toward HS. Our review about physiological and molecular mechanisms may enlighten ways to develop thermo-tolerant cultivars and to produce crop plants that are agriculturally important in adverse climatic conditions.

Analysis of transactivation potential of rice (Oryza sativa L.) heat shock factors

Based on yeast one-hybrid assays, we show that the presence of C-terminal AHA motifs is not a prerequisite for transactivation potential in rice heat shock factors. Transcriptional activation or transactivation (TA) of heat stress responsive genes takes place by binding of heat shock factors (Hsfs) to heat shock elements. Analysis of TA potential of thirteen rice (Oryza sativa L.) Hsfs (OsHsfs) carried out in this study by yeast one-hybrid assay showed that OsHsfsA3 possesses strong TA potential while OsHsfs A1a, A2a, A2b, A4a, A4d, A5, A7b, B1, B2a, B2b, B2c and B4d lack TA potential. From a near complete picture of TA potential of the OsHsf family (comprising of 25 members) emerging from this study and an earlier report from our group (Mittal et al. in FEBS J 278(17):3076-3085, 2011), it is concluded that (1) overall, six OsHsfs, namely A3, A6a, A6b, A8, C1a and C1b possess TA potential; (2) four class A OsHsfs, namely A3, A6a, A6b and A8 have TA potential out of which A6a and A6b contain AHA motifs while A3 and A8 lack AHA motifs; (3) nine class A OsHsfs, namely A1a, A2a, A2b, A2e, A4a, A4d, A5, A7a and A7b containing AHA motif(s) lack TA function in the yeast assay system; (4) all class B OsHsfs lack AHA motifs and TA potential (B4a not analyzed) and (5) though all class C OsHsf members lack AHA motifs, two members C1a and C1b possess TA function, while one member C2a lacks TA potential (C2b not analyzed). Thus, the presence or absence of AHA motif is possibly not the only factor determining TA potential of OsHsfs. Our findings will help to identify the transcriptional activators of rice heat shock response.

SUMO E3 Ligase SlSIZ1 Facilitates Heat Tolerance in Tomato

High temperature has become a major abiotic stress that limits crop productivity. Heat shock transcription factors (HSFs) and heat shock proteins (HSPs) play important roles in enhancing thermotolerance of plants. SUMOylation is an important post-translational modification in regulating cellular functions in eukaryotes. SIZ1, a well-characterized SUMO E3 ligase, mediates the process of SUMOylation. In this study, SUMO conjugations were clearly induced by high temperature. Overexpression of SIZ1 SUMO E3 ligase (SlSIZ1) in tomato could enhance the tolerance to heat stress in tomato. The RNA interference (RNAi) plants were more wilted than the wild type with heat treatment. Under heat stress, SlSIZ1 could decrease the accumulation of reactive oxygen species (ROS) and induce some genes of HSF and HSP transcription. Furthermore, overexpression of SlSIZ1 could increase the level of Hsp70 under high temperature. Yeast two-hybrid and bimolecular fluorescence complementation (BiFC) assays showed that SlSIZ1 could interact with SlHsfA1 to mediate the SUMOylation of SlHsfA1 and consequently enhance thermotolerance of tomato. In conclusion, overexpression of SlSIZ1 enhanced heat tolerance by regulating the activities of HsfA1 and increasing the content Hsp70.

SIZ1-mediated SUMOylation during phosphate homeostasis in plants: Looking beyond the tip of the iceberg

Availability of phosphate (Pi) is often limited in rhizospheres in different agroclimatic zones and adversely affects growth and development of plants. To circumvent this impasse, there is an urgent need and global consensus to develop Pi use efficient crops. To achieve this goal, it is essential to identify the molecular entities that exert regulatory influences on the sensing and signaling cascade governing Pi homeostasis. SIZ1 encodes a small ubiquitin-like modifier (SUMO E3) ligase, and plays a pivotal role in the post-translational SUMOylation of proteins. In this review, we discuss the reverse genetics approach conventionally used for providing circumstantial evidence towards the regulatory influences of SIZ1 on several morphophysiological and molecular traits that govern Pi homeostasis in taxonomically diverse Arabidopsis thaliana (Arabidopsis) and Oryza sativa (rice) model species. However, the efforts have been rather modest in identifying SUMO protein targets that play key roles in the maintenance of Pi homeostasis in these model plants contrary to the plethora of them now known in lower organisms and animals. Therefore, to predict the SIZ1-mediated SUMOylome involved in Pi homeostasis, the state-of-the-art high-throughput technologies often used for animals thus provide an attractive paradigm towards achieving the long-term goal of developing Pi use efficient crops.

SUMO-Dependent Synergism Involving Heat Shock Transcription Factors with Functions Linked to Seed Longevity and Desiccation Tolerance

A transcriptional synergism between HaHSFA9 (A9) and HaHSFA4a (A4a) contributes to determining longevity and desiccation tolerance of sunflower (Helianthus annuus, L.) seeds. Potential lysine SUMOylation sites were identified in A9 and A4a and mutated to arginine. We show that A9 is SUMOylated in planta at K38. Although we did not directly detect SUMOylated A4a in planta, we provide indirect evidence from transient expression experiments indicating that A4a is SUMOylated at K172. Different combinations of wild type and SUMOylation site mutants of A9 and A4a were analyzed by transient expression in sunflower embryos and leaves. Although most of the precedents in literature link SUMOylation with repression, the A9 and A4a synergism was fully abolished when the mutant forms for both factors were combined. However, the combination of mutant forms of A9 and A4a did not affect the nuclear retention of A4a by A9; therefore, the analyzed mutations would affect the synergism after the mutual interaction and nuclear co-localization of A9 and A4a. Our results suggest a role for HSF SUMOylation during late, zygotic, embryogenesis. The SUMOylation of A9 (or A4a) would allow a crucial, synergic, transcriptional effect that occurs in maturing sunflower seeds.

The heat-shock protein/chaperone network and multiple stress resistance

Crop yield has been greatly enhanced during the last century. However, most elite cultivars are adapted to temperate climates and are not well suited to more stressful conditions. In the context of climate change, stress resistance is a major concern. To overcome these difficulties, scientists may help breeders by providing genetic markers associated with stress resistance. However, multistress resistance cannot be obtained from the simple addition of single stress resistance traits. In the field, stresses are unpredictable and several may occur at once. Consequently, the use of single stress resistance traits is often inadequate. Although it has been historically linked with the heat stress response, the heat-shock protein (HSP)/chaperone network is a major component of multiple stress responses. Among the HSP/chaperone 'client proteins', many are primary metabolism enzymes and signal transduction components with essential roles for the proper functioning of a cell. HSPs/chaperones are controlled by the action of diverse heat-shock factors, which are recruited under stress conditions. In this review, we give an overview of the regulation of the HSP/chaperone network with a focus on Arabidopsis thaliana. We illustrate the role of HSPs/chaperones in regulating diverse signalling pathways and discuss several basic principles that should be considered for engineering multiple stress resistance in crops through the HSP/chaperone network.

Dissecting the Biochemical and Transcriptomic Effects of a Locally Applied Heat Treatment on Developing Cabernet Sauvignon Grape Berries

Reproductive development of grapevine and berry composition are both strongly influenced by temperature. To date, the molecular mechanisms involved in grapevine berries response to high temperatures are poorly understood. Unlike recent data that addressed the effects on berry development of elevated temperatures applied at the whole plant level, the present work particularly focuses on the fruit responses triggered by direct exposure to heat treatment (HT). In the context of climate change, this work focusing on temperature effect at the microclimate level is of particular interest as it can help to better understand the consequences of leaf removal (a common viticultural practice) on berry development. HT (+ 8°C) was locally applied to clusters from Cabernet Sauvignon fruiting cuttings at three different developmental stages (middle green, veraison and middle ripening). Samples were collected 1, 7, and 14 days after treatment and used for metabolic and transcriptomic analyses. The results showed dramatic and specific biochemical and transcriptomic changes in heat exposed berries, depending on the developmental stage and the stress duration. When applied at the herbaceous stage, HT delayed the onset of veraison. Heating also strongly altered the berry concentration of amino acids and organic acids (e.g., phenylalanine, γ-aminobutyric acid and malate) and decreased the anthocyanin content at maturity. These physiological alterations could be partly explained by the deep remodeling of transcriptome in heated berries. More than 7000 genes were deregulated in at least one of the nine experimental conditions. The most affected processes belong to the categories "stress responses," "protein metabolism" and "secondary metabolism," highlighting the intrinsic capacity of grape berries to perceive HT and to build adaptive responses. Additionally, important changes in processes related to "transport," "hormone" and "cell wall" might contribute to the postponing of veraison. Finally, opposite effects depending on heating duration were observed for genes encoding enzymes of the general phenylpropanoid pathway, suggesting that the HT-induced decrease in anthocyanin content may result from a combination of transcript abundance and product degradation.

Transcriptional Regulatory Network of Plant Heat Stress Response

Heat stress (HS) is becoming an increasingly significant problem for food security as global warming progresses. Recent studies have elucidated the complex transcriptional regulatory networks involved in HS. Here, we provide an overview of current knowledge regarding the transcriptional regulatory network and post-translational regulation of the transcription factors involved in the HS response. Increasing evidence suggests that epigenetic regulation and small RNAs are important in heat-induced transcriptional responses and stress memory. It remains to be elucidated how plants sense and respond to HS. Several recent reports have discussed the heat sensing and signaling that activate transcriptional cascades; thus, we also highlight future directions of promoting crop tolerance to HS using these factors or other strategies for agricultural applications.

Functions of heat shock transcription factors involved in response to photooxidative stresses in Arabidopsis

Because plants are continually exposed to various environmental stresses, they possess numerous transcription factors that regulate metabolism to adapt and acclimate to those conditions. To clarify the gene regulation systems activated in response to photooxidative stress, we isolated 76 high light and heat shock stress-inducible genes, including heat shock transcription factor (Hsf) A2 from Arabidopsis. Unlike yeast or animals, more than 20 genes encoding putative Hsfs are present in the genomes of higher plants, and they are categorized into three classes based on their structural characterization. However, the multiplicity of Hsfs in plants remains unknown. Furthermore, the individual functions of Hsfs are also largely unknown because of their genetic redundancy. Recently, the developments of T-DNA insertion knockout mutant lines and chimeric repressor gene-silencing technology have provided effective tools for exploring the individual functions of Hsfs. This review describes the current knowledge on the individual functions and activation mechanisms of Hsfs.

Functional identification of MdSIZ1 as a SUMO E3 ligase in apple

SUMOylation, the conjugation of target proteins with SUMO (small ubiquitin-related modifier), is a type of post-translational modification in eukaryotes and involves the sequential action of activation (E1), conjugation (E2) and ligation (E3) enzymes. In Arabidopsis, the AtSIZ1 protein is a SUMO E3 ligase that promotes the conjugation of SUMO proteins to target substrates. Here, we isolated and identified a SUMO E3 ligase, MdSIZ1, in apple, which was similar to AtSIZ1. SUMOylation analysis showed that MdSIZ1 had SUMO E3 ligase activity in vitro and in vivo. SUMO conjugation was increased by high temperatures, low temperatures, and abscisic acid (ABA). The ectopic expression of MdSIZ1 in Arabidopsis siz1-2 mutant plants partially complemented the morphological mutant phenotype and enhanced the levels of SUMO conjugation. Taken together, these results suggest that MdSIZ1-mediated SUMO conjugation of target proteins is an important process that regulates the adaptation of apple plants to various environmental stresses.

Dissecting the proteome dynamics of the early heat stress response leading to plant survival or death in Arabidopsis

In many plant species, an exposure to a sublethal temperature triggers an adaptative response called acclimation. This response involves an extensive molecular reprogramming that allows the plant to further survive to an otherwise lethal increase of temperature. A related response is also launched under an abrupt and lethal heat stress that, in this case, is unable to successfully promote thermotolerance and therefore ends up in plant death. Although these molecular programmes are expected to have common players, the overlapping degree and the specific regulators of each process are currently unknown. We have carried out a high-throughput comparative proteomics analysis during acclimation and during the early stages of the plant response to a severe heat stress that lead Arabidopsis seedlings either to survival or death. This analysis dissects these responses, unravels the common players and identifies the specific proteins associated with these different fates. Thermotolerance assays of mutants in genes with an uncharacterized role in heat stress demonstrate the relevance of this study to uncover both positive and negative heat regulators and pinpoint a pivotal role of JR1 and BAG6 in heat tolerance.

The Plant Heat Stress Transcription Factors (HSFs): Structure, Regulation, and Function in Response to Abiotic Stresses

Abiotic stresses such as high temperature, salinity, and drought adversely affect the survival, growth, and reproduction of plants. Plants respond to such unfavorable changes through developmental, physiological, and biochemical ways, and these responses require expression of stress-responsive genes, which are regulated by a network of transcription factors (TFs), including heat stress transcription factors (HSFs). HSFs play a crucial role in plants response to several abiotic stresses by regulating the expression of stress-responsive genes, such as heat shock proteins (Hsps). In this review, we describe the conserved structure of plant HSFs, the identification of HSF gene families from various plant species, their expression profiling under abiotic stress conditions, regulation at different levels and function in abiotic stresses. Despite plant HSFs share highly conserved structure, their remarkable diversification across plants reflects their numerous functions as well as their integration into the complex stress signaling and response networks, which can be employed in crop improvement strategies via biotechnological intervention.

Cooling water before panicle initiation increases chilling-induced male sterility and disables chilling-induced expression of genes encoding OsFKBP65 and heat shock proteins in rice spikelets

In rice (Oryza sativa L.), chilling-induced male sterility increased when plants experienced low water temperature (Tw , 18 °C for 14 d) before panicle initiation. The number of mature pollen grains after chilling at the booting stage (12 °C for 5 d) was only 45% of total pollen grains in low-Tw plants, whereas it was 71% in normal-Tw plants (Tw not controlled; approximately 23 °C under air temperature of 26 °C/21 °C, day/night). Microarray and quantitative PCR analyses showed that many stress-responsive genes (including OsFKBP65 and genes encoding the large heat shock protein OsHSP90.1, heat-stress transcription factors and many small heat shock proteins) were strongly up-regulated by chilling in normal-Tw spikelets, but were unaffected or even down-regulated by chilling in low-Tw spikelets. OsAPX2 and genes encoding some other antioxidant enzymes were also significantly down-regulated by low Tw in chilled spikelets. The levels of lipid peroxidation products (malondialdehyde equivalents) were significantly increased in low-Tw spikelets by chilling. Ascorbate peroxidase activity in chilled spikelets was significantly lower in low-Tw plants than in normal-Tw plants. Our data suggest that an OsFKBP65-related chilling response, which protects proteins from oxidative damage, is indispensable for chilling tolerance but is lost in low-Tw spikelets.

SUMOylation by a stress-specific small ubiquitin-like modifier E2 conjugase is essential for survival of Chlamydomonas reinhardtii under stress conditions

Posttranslational modification of proteins by small ubiquitin-like modifier (SUMO) is required for survival of virtually all eukaryotic organisms. Attachment of SUMO to target proteins is catalyzed by SUMO E2 conjugase. All haploid or diploid eukaryotes studied to date possess a single indispensable SUMO conjugase. We report here the unanticipated isolation of a Chlamydomonas reinhardtii (mutant5 [mut5]). in which the previously identified SUMO conjugase gene C. reinhardtii ubiquitin-conjugating enzyme9 (CrUBC9) is deleted. This surprising mutant is viable and unexpectedly, displays a pattern of protein SUMOylation at 25°C that is essentially identical to wild-type cells. However, unlike wild-type cells, mut5 fails to SUMOylate a large set of proteins in response to multiple stress conditions, a failure that results in a markedly reduced tolerance or complete lack of tolerance to these stresses. Restoration of expected stress-induced protein SUMOylation patterns as well as normal stress tolerance phenotypes in mut5 cells complemented with a CrUBC9 gene shows that CrUBC9 is an authentic SUMO conjugase and, more importantly, that SUMOylation is essential for cell survival under stress conditions. The presence of bona fide SUMOylated proteins in the mut5 mutant at 25°C can only be explained by the presence of at least one additional SUMO conjugase in C. reinhardtii, a conjugase tentatively identified as CrUBC3. Together, these results suggest that, unlike all other nonpolyploid eukaryotes, there are at least two distinct and functional SUMO E2 conjugases in C. reinhardtii, with a clear division of labor between the two sets: One (CrUBC9) is involved in essential stress-induced SUMOylations, and one (CrUBC3) is involved in housekeeping SUMOylations.

Post-transcriptional and post-translational regulations of drought and heat response in plants: a spider's web of mechanisms

Drought and heat tolerance are complex quantitative traits. Moreover, the adaptive significance of some stress-related traits is more related to plant survival than to agronomic performance. A web of regulatory mechanisms fine-tunes the expression of stress-related traits and integrates both environmental and developmental signals. Both post-transcriptional and post-translational modifications contribute substantially to this network with a pivotal regulatory function of the transcriptional changes related to cellular and plant stress response. Alternative splicing and RNA-mediated silencing control the amount of specific transcripts, while ubiquitin and SUMO modify activity, sub-cellular localization and half-life of proteins. Interactions across these modification mechanisms ensure temporally and spatially appropriate patterns of downstream-gene expression. For key molecular components of these regulatory mechanisms, natural genetic diversity exists among genotypes with different behavior in terms of stress tolerance, with effects upon the expression of adaptive morphological and/or physiological target traits.

The function of EHD2 in endocytosis and defense signaling is affected by SUMO

Post-translational modification of target proteins by the small ubiquitin-like modifier protein (SUMO) regulates many cellular processes. SUMOylation has been shown to regulate cellular localization and function of a variety of proteins, in some cases affecting nuclear import or export. We have previously characterized two EHDs (EH domain containing proteins) in Arabidospis and showed their involvement in plant endocytosis. AtEHD2 has an inhibitory effect on endocytosis of transferrin, FM-4-64, and the leucine rich repeat receptor like protein LeEix2, an effect that requires and intact coiled-coil domain. Inhibition of endocytosis of LeEix2 by EHD2 is effective in inhibiting defense responses mediated by the LeEix2 receptor in response to its ligand EIX. In the present work we demonstrate that SUMOylation of EHD2 appears to be required for EHD2-induced inhibition of LeEix2 endocytosis. Indeed, we found that a mutant form of EHD2, possessing a defective SUMOylation site, has an increased nuclear abundance, can no longer be SUMOylated and is no longer effective in inhibiting LeEix2 endocytosis or defense signaling in response to EIX.

The CNGCb and CNGCd genes from Physcomitrella patens moss encode for thermosensory calcium channels responding to fluidity changes in the plasma membrane

Land plants need precise thermosensors to timely establish molecular defenses in anticipation of upcoming noxious heat waves. The plasma membrane-embedded cyclic nucleotide-gated Ca(2+) channels (CNGCs) can translate mild variations of membrane fluidity into an effective heat shock response, leading to the accumulation of heat shock proteins (HSP) that prevent heat damages in labile proteins and membranes. Here, we deleted by targeted mutagenesis the CNGCd gene in two Physcomitrella patens transgenic moss lines containing either the heat-inducible HSP-GUS reporter cassette or the constitutive UBI-Aequorin cassette. The stable CNGCd knockout mutation caused a hyper-thermosensitive moss phenotype, in which the heat-induced entry of apoplastic Ca(2+) and the cytosolic accumulation of GUS were triggered at lower temperatures than in wild type. The combined effects of an artificial membrane fluidizer and elevated temperatures suggested that the gene products of CNGCd and CNGCb are paralogous subunits of Ca(2+)channels acting as a sensitive proteolipid thermocouple. Depending on the rate of temperature increase, the duration and intensity of the heat priming preconditions, terrestrial plants may thus acquire an array of HSP-based thermotolerance mechanisms against upcoming, otherwise lethal, extreme heat waves.

A seed preferential heat shock transcription factor from wheat provides abiotic stress tolerance and yield enhancement in transgenic Arabidopsis under heat stress environment

Reduction in crop yield and quality due to various abiotic stresses is a worldwide phenomenon. In the present investigation, a heat shock factor (HSF) gene expressing preferentially in developing seed tissues of wheat grown under high temperatures was cloned. This newly identified heat shock factor possesses the characteristic domains of class A type plant HSFs and shows high similarity to rice OsHsfA2d, hence named as TaHsfA2d. The transcription factor activity of TaHsfA2d was confirmed through transactivation assay in yeast. Transgenic Arabidopsis plants overexpressing TaHsfA2d not only possess higher tolerance towards high temperature but also showed considerable tolerance to salinity and drought stresses, they also showed higher yield and biomass accumulation under constant heat stress conditions. Analysis of putative target genes of AtHSFA2 through quantitative RT-PCR showed higher and constitutive expression of several abiotic stress responsive genes in transgenic Arabidopsis plants over-expressing TaHsfA2d. Under stress conditions, TaHsfA2d can also functionally complement the T-DNA insertion mutants of AtHsfA2, although partially. These observations suggest that TaHsfA2d may be useful in molecular breeding of crop plants, especially wheat, to improve yield under abiotic stress conditions.

Differential isolation and identification of PI(3)P and PI(3,5)P2 binding proteins from Arabidopsis thaliana using an agarose-phosphatidylinositol-phosphate affinity chromatography

A phosphatidylinositol-phosphate affinity chromatographic approach combined with mass spectrometry was used in order to identify novel PI(3)P and PI(3,5)P2 binding proteins from Arabidopsis thaliana suspension cell extracts. Most of the phosphatidylinositol-phosphate interacting candidates identified from this differential screening are characterized by lysine/arginine rich patches. Direct phosphoinositide binding was identified for important membrane trafficking regulators as well as protein quality control proteins such as the ATG18p orthologue involved in autophagosome formation and the lipid Sec14p like transfer protein. A pentatricopeptide repeat (PPR) containing protein was shown to directly bind to PI(3,5)P2 but not to PI(3)P. PIP chromatography performed using extracts obtained from high salt (0.4M and 1M NaCl) pretreated suspensions showed that the association of an S5-1 40S ribosomal protein with both PI(3)P and PI(3,5)P2 was abolished under salt stress whereas salinity stress induced an increase in the phosphoinositide association of the DUF538 domain containing protein SVB, associated with trichome size. Additional interacting candidates were co-purified with the phosphoinositide bound proteins. Binding of the COP9 signalosome, the heat shock proteins, and the identified 26S proteasomal subunits, is suggested as an indirect effect of their interaction with other proteins directly bound to the PI(3)P and the PI(3,5)P2 phosphoinositides.

BIOLOGICAL SIGNIFICANCE

PI(3,5)P2 is of special interest because of its low abundance. Furthermore, no endogenous levels have yet been detected in A. thaliana (although there is evidence for its existence in plants). Therefore the isolation of novel interacting candidates in vitro would be of a particular importance since the future study and localization of the respective endogenous proteins may indicate possible targeted compartments or tissues where PI(3,5)P2 could be enriched and thereafter identified. In addition, PI(3,5)P2 is a phosphoinositide extensively studied in mammalian and yeast systems. However, our knowledge of its role in plants as well as a list of its effectors from plants is very limited.

Protein SUMOylation and plant abiotic stress signaling: in silico case study of rice RLKs, heat-shock and Ca(2+)-binding proteins

Plants respond to stress conditions through early stress-response factors (ESRF), which serve the function of stress sensing and/or signal transduction. These mainly comprise qualitative and/or quantitative flux in the redox molecules, calcium ions (Ca(2+)), phosphatidic acid, hexose sugars and phytohormones. The role of resident proteins such as phytohormone receptors and G-proteins as first messengers under stress is well established. Yet, within the modern omics context, most of the stress response at the protein level is injudiciously attributed to substantial up- or down-regulation of expression measured at the RNA or protein level. Proteins such as kinases and transcription factors (TFs) that exhibit cascade effects are primary candidates for studies in plant stress tolerance. However, resident-protein post-translational modification (PTM), specifically in response to particular conditions such as stress, is a candidate for immediate and potent 'quick reaction force' (QRF) kind of effects. Stress-mediated SUMOylation of TFs and other proteins have been observed. SUMOylation can change the rate of activity, function or location of the modified protein. Early SUMOylation of resident proteins can act in the stress signal transduction or in adaptive response. Here, we consider brief background information on ESRFs to establish the crosstalk between these factors that impinge on PTMs. We then illustrate connections of protein SUMOylation to phytohormones and TFs. Finally, we present results of an in silico analysis of rice Receptor-Like Kinases, heat-shock and calcium-binding proteins to identify members of these gene families, whose basal expression under drought but potential SUMOylation presents them as QRF candidates for roles in stress signaling/response.

Heterologous expression of OsSIZ1, a rice SUMO E3 ligase, enhances broad abiotic stress tolerance in transgenic creeping bentgrass

Sumoylation is a posttranslational regulatory process in higher eukaryotes modifying substrate proteins through conjugation of small ubiquitin-related modifiers (SUMOs). Sumoylation modulates protein stability, subcellular localization and activity; thus, it regulates most cellular functions including response to environmental stress in plants. To study the feasibility of manipulating SUMO E3 ligase, one of the important components in the sumoylation pathway in transgenic (TG) crop plants for improving overall plant performance under adverse environmental conditions, we have analysed TG creeping bentgrass (Agrostis stolonifera L.) plants constitutively expressing OsSIZ1, a rice SUMO E3 ligase. Overexpression of OsSIZ1 led to increased photosynthesis and overall plant growth. When subjected to water deficiency and heat stress, OsSIZ1 plants exhibited drastically enhanced performance associated with more robust root growth, higher water retention and cell membrane integrity than wild-type (WT) controls. OsSIZ1 plants also displayed significantly better growth than WT controls under phosphate-starvation conditions, which was associated with a higher uptake of phosphate (Pi) and other minerals, such as potassium and zinc. Further analysis revealed that overexpression of OsSIZ1 enhanced stress-induced SUMO conjugation to substrate in TG plants, which was associated with modified expression of stress-related genes. This strongly supports a role sumoylation plays in regulating multiple molecular pathways involved in plant stress response, establishing a direct link between sumoylation and plant response to environmental adversities. Our results demonstrate the great potential of genetic manipulation of sumoylation process in TG crop species for improved resistance to broad abiotic stresses.

Regulation of the heat stress response in Arabidopsis by MPK6-targeted phosphorylation of the heat stress factor HsfA2

So far little is known on the functional role of phosphorylation in the heat stress response of plants. Here we present evidence that heat stress activates the Arabidopsis mitogen-activated protein kinase MPK6. In vitro and in vivo evidence is provided that MPK6 specifically targets the major heat stress transcription factor HsfA2. Activation of MPK6 results in complex formation with HsfA2. MPK6 phosphorylates HsfA2 on T249 and changes its intracellular localisation. Protein kinase and phosphatase inhibitor studies indicate that HsfA2 protein stability is regulated in a phosphorylation-dependent manner, but this mechanism is independent of MPK6. Overall, our data show that heat stress-induced targeting of HsfA2 by MPK6 participates in the complex regulatory mechanism how plants respond to heat stress.