Chrysanthemum CmHSFA4 gene positively regulates salt stress tolerance in transgenic chrysanthemum

Integrative physiology and transcriptome sequencing reveal differences between G. hirsutum and G. barbadense in response to salt stress and the identification of key salt tolerance genes

BACKGROUND

Soil salinity is one of the major abiotic stresses that threatens crop growth. Cotton has some degree of salt tolerance, known as the "pioneer crop" of saline-alkali land. Cultivation of cotton is of great significance to the utilization of saline-alkali land and the development of cotton industry. Gossypium hirsutum and G. barbadense, as two major cotton species, are widely cultivated worldwide. However, until recently, the regulatory mechanisms and specific differences of their responses to salt stress have rarely been reported.

RESULTS

In this study, we comprehensively compared the differences in the responses of G. hirsutum acc. TM-1 and G. barbadense cv. Hai7124 to salt stress. The results showed that Hai7124 exhibited better growth than did TM-1 under salt stress, with greater PRO content and antioxidant capability, whereas TM-1 only presented greater K+ content. Transcriptome analysis revealed significant molecular differences between the two cotton species in response to salt stress. The key pathways of TM-1 induced by salt are mainly related to growth and development, such as porphyrin metabolism, DNA replication, ribosome and photosynthesis. Conversely, the key pathways of Hai7124, such as plant hormone signal transduction, MAPK signaling pathway-plant, and phenylpropanoid biosynthesis, are mainly related to plant defense. Further comparative analyses of differentially expressed genes (DEGs) revealed that antioxidant metabolism, abscisic acid (ABA) and jasmonic acid (JA) signalling pathways were more strongly activated in Hai7124, whereas TM-1 was more active in K+ transporter-related genes and ethylene (ETH) signalling pathway. These differences underscore the various molecular strategies adopted by the two cotton species to navigate through salt stress, and Hai7124 responded more strongly to salt stress, which explains the potential reasons for the greater salt tolerance of Hai7124. Finally, we identified 217 potential salt tolerance-related genes, 167 of which overlapped with the confidence intervals of significant SNPs identified in previous genome-wide association studies (GWASs), indicating the high reliability of these genes.

CONCLUSIONS

These findings provide new insights into the differences in the regulatory mechanisms of salt tolerance between G. hirsutum and G. barbadense, and identify key candidate genes for salt tolerance molecular breeding in cotton.

Transcription factor CmHSFA4-CmMYBS3 complex enhances salt tolerance in chrysanthemum by repressing CmMYB121 expression

Excessive soil salinity not only hampers plant growth and development but can also lead to plant death. Previously, we found that heat-shock factor A4 (CmHSFA4) enhances the tolerance of chrysanthemum (Chrysanthemum morifolium) to salt. However, the underlying molecular mechanism remains unclear. In this study, we identified a candidate MYB transcription factor, CmMYB121, which responded to salt stress. We observed that the CmMYB121 transcription is suppressed by CmHSFA4. Moreover, overexpression of CmMYB121 exacerbated chrysanthemum sensitivity to salt stress. CmHSFA4 directly bound to the promoter of CmMYB121 at the heat-shock element. Protein-protein interaction assays identified an interaction between CmHSFA4 and CmMYBS3, a transcriptional repressor, and recruited the corepressor TOPLESS (CmTPL) to inhibit CmMYB121 transcription by impairing the H3 and H4 histone acetylation levels of CmMYB121. Our study demonstrated that a CmHSFA4-CmMYBS3-CmTPL complex modulates CmMYB121 expression, consequently regulating the tolerance of chrysanthemum to salt. The findings shed light on the responses of plants to salt stress.

Differential gene expression of salt-tolerant alfalfa in response to salinity and inoculation by Ensifer meliloti

BACKGROUND

Alfalfa (Medicago sativa L.) experiences many negative effects under salinity stress, which may be mediated by recurrent selection. Salt-tolerant alfalfa may display unique adaptations in association with rhizobium under salt stress.

RESULTS

To elucidate inoculation effects on salt-tolerant alfalfa under salt stress, this study leveraged a salt-tolerant alfalfa population selected through two cycles of recurrent selection under high salt stress. After experiencing 120-day salt stress, mRNA was extracted from 8 random genotypes either grown in 0 or 8 dS/m salt stress with or without inoculation by Ensifer meliloti. Results showed 320 and 176 differentially expressed genes (DEGs) modulated in response to salinity stress or inoculation x salinity stress, respectively. Notable results in plants under 8 dS/m stress included upregulation of a key gene involved in the Target of Rapamycin (TOR) signaling pathway with a concomitant decrease in expression of the SNrK pathway. Inoculation of salt-stressed plants stimulated increased transcription of a sulfate-uptake gene as well as upregulation of the Lysine-27-trimethyltransferase (EZH2), Histone 3 (H3), and argonaute (AGO, a component of miRISC silencing complexes) genes related to epigenetic and post-transcriptional gene control.

CONCLUSIONS

Salt-tolerant alfalfa may benefit from improved activity of TOR and decreased activity of SNrK1 in salt stress, while inoculation by rhizobiumstimulates production of sulfate uptake- and other unique genes.

Genome-wide identification and role of HSFs in antioxidant response of hot water treated zucchini fruit during cold storage

Zucchini squashes are cold-sensitive and vulnerable to chilling injury (CI) resulting from reactive oxygen species (ROS) and hot water (HW) immersing effectively reduce CI symptoms during cold storage. However, mechanism involved in reduced ROS due to HW treatment has not been characterized well. In this study, tender green zucchini fruit were treated with HW for 15 min at 45 ± 1 °C and stored for 15 d at 4 ± 1 °C and above 90 % relative humidity. Results showed substantial reduction in CI index, electrolyte leakage, malonaldehyde (MDA) contents and ROS accumulation along with increased activity of ROS-scavenging enzymes due to HW treatment. To gain insight into the molecular mechanism involved in antioxidant defense system, transcriptomic analysis revealed that heat shock factors (HSF) accumulated due to HW treatment regulated the ROS pathway during cold stress. CpHSFA4a was one of the highly expressed transcription factors (TF) due to HW treatment that regulated the transcription of ROS enzymes related genes. CpHSFA4a bind actively with heat shock element (HSE) in promoter regions of CpSOD, CpCAT, CpAPX1, CpAPX2, and CpAPX3, activated and increased the expression of these genes. In conclusion, HW treatment alleviated the CI by maintaining ROS homeostasis through CpHSFA4a mediated ROS pathway in zucchini squashes during cold storage.

Transcriptome Analysis of the Regulatory Mechanisms of Holly (Ilex dabieshanensis) under Salt Stress Conditions

The holly Ilex dabieshanensis K. Yao & M. B. Deng, a tree endemic to the Dabieshan Mountains region in China, is a commonly used landscaping plant. Like other crops, its growth is affected by salt stress. The molecular mechanism underlying salt tolerance in holly is still unclear. In this study, we used NaCl treatment and RNA sequencing (RNA-seq) at different times to identify the salt stress response genes of holly. A total of 4775 differentially expressed genes (DEGs) were identified. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of the DEGs obtained at different salt treatment times (3, 6, 9, 12, and 24 h), as compared to control (ck, 0 h), showed that plant hormone signal transduction and carotenoid biosynthesis were highly enriched. The mechanism by which holly responds to salt stress involves many plant hormones, among which the accumulation of abscisic acid (ABA) and its signal transduction may play an important role. In addition, ion homeostasis, osmotic metabolism, accumulation of antioxidant enzymes and nonenzymatic antioxidant compounds, and transcription factors jointly regulate the physiological balance in holly, providing important guarantees for its growth and development under conditions of salt stress. These results lay the foundation for studying the molecular mechanisms of salt tolerance in holly and for the selection of salt-tolerant varieties.

B. subtilis CNBG-PGPR-1 induces methionine to regulate ethylene pathway and ROS scavenging for improving salt tolerance of tomato

Soil salinity severely threatens plant growth and crop yields. The utilization of PGPR is an effective strategy for enhancing plant salt tolerance, but the mechanisms involved in this process have rarely been reported. In this study, we investigated the effects of Bacillus subtilis CNBG-PGPR-1 on improving plant salt tolerance and elucidated the molecular pathways involved. The results showed that CNBG-PGPR-1 significantly improved the cellular homeostasis and photosynthetic efficiency of leaves and reduced ion toxicity and osmotic stress caused by salt in tomato. Transcriptome analysis uncovered that CNBG-PGPR-1 enhanced plant salt tolerance through the activation of complex molecular pathways, with plant hormone signal transduction playing an important role. Comparative analysis and pharmacological experiments confirmed that the ethylene pathway was closely related to the beneficial effect of CNBG-PGPR-1 on improving plant salt tolerance. Furthermore, we found that methionine, a precursor of ethylene synthesis, significantly accumulated in response to CNBG-PGPR-1 in tomato. Exogenous L-methionine largely mimicked the beneficial effects of CNBG-PGPR-1 and activated the expression of ethylene pathway-related genes, indicating CNBG-PGPR-1 induces methionine accumulation to regulate the ethylene pathway in tomato. Finally, CNBG-PGPR-1 reduced salt-induced ROS by activating ROS scavenger-encoding genes, mainly involved in GSH metabolism and POD-related genes, which were also closely linked to methionine metabolism. Overall, our studies demonstrate that CNBG-PGPR-1-induced methionine is a key regulator in enhancing plant salt tolerance through the ethylene pathway and ROS scavenging, providing a novel understanding of the mechanism by which beneficial microbes improve plant salt tolerance.

The IAA17.1/HSFA5a module enhances salt tolerance in Populus tomentosa by regulating flavonol biosynthesis and ROS levels in lateral roots

Auxin signaling provides a promising approach to controlling root system architecture and improving stress tolerance in plants. However, how the auxin signaling is transducted in this process remains unclear. The Aux indole-3-acetic acid (IAA) repressor IAA17.1 is stabilized by salinity, and primarily expressed in the lateral root (LR) primordia and tips in poplar. Overexpression of the auxin-resistant form of IAA17.1 (IAA17.1m) led to growth inhibition of LRs, markedly reduced salt tolerance, increased reactive oxygen species (ROS) levels, and decreased flavonol content. We further identified that IAA17.1 can interact with the heat shock protein HSFA5a, which was highly expressed in roots and induced by salt stress. Overexpression of HSFA5a significantly increased flavonol content, reduced ROS accumulation, enhanced LR growth and salt tolerance in transgenic poplar. Moreover, HSFA5a could rescue the defective phenotypes caused by IAA17.1m. Expression analysis showed that genes associated with flavonol biosynthesis were altered in IAA17.1m- and HAFA5a-overexpressing plants. Furthermore, we identified that HSFA5a directly activated the expression of key enzyme genes in the flavonol biosynthesis pathway, while IAA17.1 suppressed HSFA5a-mediated activation of these genes. Collectively, the IAA17.1/HSFA5a module regulates flavonol biosynthesis, controls ROS accumulation, thereby modulating the root system of poplar to adapt to salt stress.

A LlWRKY33-LlHSFA4-LlCAT2 module confers resistance to Botrytis cinerea in lily

Gray mold caused by Botrytis cinerea is one of the major threats in lily production. However, limited information is available about the underlying defense mechanism against B. cinerea in lily. Here, we characterized a nuclear-localized class A heat stress transcription factor (HSF)-LlHSFA4 from lily (Lilium longiflorum), which positively regulated the response to B. cinerea infection. LlHSFA4 transcript and its promoter activity were increased by B. cinerea infection in lily, indicating its involvement in the response to B. cinerea. Virus-induced gene silencing (VIGS) of LlHSFA4 impaired the resistance of lily to B. cinerea. Consistent with its role in lily, overexpression of LlHSFA4 in Arabidopsis (Arabidopsis thaliana) enhanced the resistance of transgenic Arabidopsis to B. cinerea infection. Further analysis showed that LlWRKY33 directly activated LlHSFA4 expression. We also found that both LlHSFA4 and LlWRKY33 positively regulated plant response to B. cinerea through reducing cell death and H2O2 accumulation and activating the expression of the reactive oxygen species (ROS) scavenging enzyme gene LlCAT2 (Catalase 2) by binding its prompter, which might contribute to reducing H2O2 accumulation in the infected area. Taken together, our data suggested that there may be a LlWRKY33-LlHSFA4-LlCAT2 regulatory module which confers B. cinerea resistance via reducing cell death and the ROS accumulation.

Genome-wide analysis of the MdZR gene family revealed MdZR2.2-induced salt and drought stress tolerance in apple rootstock

The DNL-type zinc finger protein constitutes a zinc ribbon protein (ZR) family, which belongs to a branch of zinc finger protein and plays an essential role in response to abiotic stress. Here, we identified six apple (Malus domestica) MdZR genes. Based on their phylogenetic relationship and gene structure, the MdZR genes were divided into three categories, including MdZR1, MdZR2, and MdZR3. Subcellular results showed that the MdZRs are located on the nuclear and membrane. The transcriptome data showed that MdZR2.2 is expressed in various tissues. The expression analysis results showed that MdZR2.2 was significantly upregulated under salt and drought treatments. Thus, we selected MdZR2.2 for further research. Overexpression of MdZR2.2 in apple callus improved their tolerance to drought and salt stress and ability to scavenge reactive oxygen species (ROS). In contrast, transgenic apple roots with silenced MdZR2.2 grew more poorly than the wild type when subjected to salt and drought stress, which reduced their ability to scavenge ROS. To our knowledge, this is the first study to analyze the MdZR protein family. This study identified a gene that responds to drought and salt stress. Our findings lay a foundation for a comprehensive analysis of the MdZR family members.

Cucumis sativus PHLOEM PROTEIN 2-A1 like gene positively regulates salt stress tolerance in cucumber seedlings

PHLOEM PROTEIN 2-A1 like (PP2-A1) gene is a member of the PP2 multigene family, and the protein encoded by which has the function of stress defense. Based on our previous proteomic study of cucumber phloem sap, CsPP2-A1 protein expression was significantly enriched under salt stress. In this paper, we obtained CsPP2-A1 interfering (CsPP2-A1-RNAi) cucumber by Agrobacterium tumefaciens-mediated method. The phenotypic changes of wild-type (WT) cucumber, CsPP2-A1-overexpressing (OE) cucumber, and CsPP2-A1-RNAi cucumber under salt treatment were observed and compared. Furthermore, physiological indicators were measured in four aspects: osmoregulation, membrane permeability, antioxidant system, and photosynthetic system. The analysis of contribution and correlation for each variable were conducted by principal component analysis (PCA) and Pearson's correlation coefficient. The above results showed that CsPP2-A1-RNAi cucumber plants exhibited weaker salt tolerance compared to WT cucumber and CsPP2-A1-OE cucumber plants in terms of phenotype and physiological indicators in response to salt stress, while CsPP2-A1-OE cucumber always showed the robust salt tolerance. Together, these results indicated that CsPP2-A1 brought a salinity tolerance ability to cucumber through osmoregulation and reactive oxygen species (ROS) homeostasis. The results of the study provided evidence for the function of CsPP2-A1 in plant salt tolerance enhancement, and they will serve as a reference for future salt-tolerant cucumber genetic manipulation.

The phloem lectin PP2-A1 enhances aphid resistance by affecting aphid behavior and maintaining ROS homeostasis in cucumber plants

Aphid (Aphis gossypii Glover) attack frequently results in a significant loss of output and deterioration of fruit quality in cucumber (Cucumis sativus L.). Phloem protein 2 (PP2) is conserved as a phloem lectin in plants, and few studies have been conducted on the regulatory mechanism of PP2. Based on our previous study of CsPP2-A1 in cucumber, to further investigate the biological function of CsPP2-A1, we compared the changes of selectivity, non-selectivity, colonization, reproductions of aphids, and the phenotype in wild type (WT), CsPP2-A1 overexpressing (CsPP2-A1-OE), and CsPP2-A1 interfering (CsPP2-A1-RNAi) cucumber plants after inoculation with aphids. We found that CsPP2-A1-OE cucumber plants generated resistance to aphids. The aphid colonization rate and number of reproductions of CsPP2-A1-OE cucumber plants were significantly lower than that of WT and CsPP2-A1-RNAi cucumber plants. Through Pearson's correlation and principal component analysis (PCA), it was found that CsPP2-A1 played a crucial role in the balance of reactive oxygen species (ROS) in plants. Overexpression of the CsPP2-A1 resulted in increased levels of antioxidant enzyme, eliminating ROS and preventing the damage by ROS in cucumber. Furthermore, nutritional imbalance for aphids and content of secondary metabolites were increased in overexpressed CsPP2-A1 cucumber plants, and thus preventing aphid attack. These together may improve cucumber resistance against aphids and the mechanism of CsPP2-A1 defense against aphids was preliminarily explored.

Genome-wide analysis of HSF family and overexpression of PsnHSF21 confers salt tolerance in Populus simonii × P. nigra

Heat shock transcription factor (HSF) is an important TF that performs a dominant role in plant growth, development, and stress response network. In this study, we identified a total of 30 HSF members from poplar, which are unevenly distributed on 17 chromosomes. The poplar HSF family can be divided into three subfamilies, and the members of the same subfamily share relatively conserved domains and motifs. HSF family members are acidic and hydrophilic proteins that are located in the nucleus and mainly carry out gene expansion through segmental replication. In addition, they have rich collinearity across plant species. Based on RNA-Seq analysis, we explored the expression pattern of PtHSFs under salt stress. Subsequently, we cloned the significantly upregulated PtHSF21 gene and transformed it into Populus simonii × P. nigra. Under salt stress, the transgenic poplar overexpressing PtHSF21 had a better growth state and higher reactive oxygen scavenging ability. A yeast one-hybrid experiment indicated PtHSF21 could improve salt tolerance by specifically binding to the anti-stress cis-acting element HSE. This study comprehensively profiled the fundamental information of poplar HSF family members and their responses to salt stress and specifically verified the biological function of PtHSF21, which provides clues for understanding the molecular mechanism of poplar HSF members in response to salt stress.

The TIFY family protein CmJAZ1-like negatively regulates petal size via interaction with the bHLH transcription factor CmBPE2 in Chrysanthemum morifolium

Petals are the second floral whorl of angiosperms, exhibiting astonishing diversity in their size between and within species. This variation is essential for protecting their inner reproductive organs and attracting pollinators for fertilization. However, currently, the genetic and developmental control of petal size remains unexplored. Chrysanthemum (Chrysanthemum morifolium) belongs to the Asteraceae family, the largest group of angiosperms, and the extraordinary diversity of petal size in chrysanthemums makes it an ideal model for exploring the regulation mechanism of petal size. Here, we reveal that overexpression of a JAZ repressor CmJAZ1-like exhibits decreased petal size compared to that of the wild-type as a result of repressed cell expansion. Through further in-depth exploration, we confirm an interaction pair between CmJAZ1-like and the bHLH transcription factor CmBPE2. The inhibition of CmBPE2 expression negatively regulates petal size by downregulating the expression of genes involved in cell expansion. Furthermore, CmJAZ1-like significantly reduced the activation ability of CmBPE2 on its target gene CmEXPA7 by directly interacting with it, thus participating in the regulation of petal size development in chrysanthemum. Our results will provide insights into the molecular mechanisms of petal size regulation in flowering plants.

Towards the Improvement of Ornamental Attributes in Chrysanthemum: Recent Progress in Biotechnological Advances

Incessant development and introduction of novel cultivars with improved floral attributes are vital in the dynamic ornamental industry. Chrysanthemum (Chrysanthemum morifolium) is a highly favored ornamental plant, ranking second globally in the cut flower trade, after rose. Development of new chrysanthemum cultivars with improved and innovative modifications in ornamental attributes, including floral color, shape, plant architecture, flowering time, enhanced shelf life, and biotic and abiotic stress tolerance, is a major goal in chrysanthemum breeding. Despite being an economically important ornamental plant, the application of conventional and molecular breeding approaches to various key traits of chrysanthemum is hindered owing to its genomic complexity, heterozygosity, and limited gene pool availability. Although classical breeding of chrysanthemum has resulted in the development of several hundreds of cultivars with various morphological variations, the genetic and transcriptional control of various important ornamental traits remains unclear. The coveted blue colored flowers of chrysanthemums cannot be achieved through conventional breeding and mutation breeding due to technical limitations. However, blue-hued flower has been developed by genetic engineering, and transgenic molecular breeding has been successfully employed, leading to substantial progress in improving various traits. The recent availability of whole-genome sequences of chrysanthemum offers a platform to extensively employ MAS to identify a large number of markers for QTL mapping, and GWAS to dissect the genetic control of complex traits. The combination of NGS, multi-omic platforms, and genome editing technologies has provided a tremendous scope to decipher the molecular and regulatory mechanisms. However, the application and integration of these technologies remain inadequate for chrysanthemum. This review, therefore, details the significance of floral attributes, describes the efforts of recent advancements, and highlights the possibilities for future application towards the improvement of crucial ornamental traits in the globally popular chrysanthemum plant.

The transcriptional coactivator CmMBF1c is required for waterlogging tolerance in Chrysanthemum morifolium

Waterlogging is one of the most serious abiotic stressors affecting Chrysanthemum morifolium during its lifespan. However, the molecular mechanisms underlying the waterlogging tolerance of chrysanthemum remain unclear. In this study, we discovered that the transcriptional coactivator MULTIPROTEIN BRIDGING FACTOR 1c (CmMBF1c) was significantly induced by waterlogging stress in chrysanthemums. Promoter sequence analysis and transient dual-luciferase assay using chrysanthemum protoplasts showed that the waterlogging-tolerant cultivar 'Nannongxuefeng' carried more response elements involved in waterlogging and hypoxia stress compared with the waterlogging-sensitive cultivar 'Qinglu', conferring on 'Nannongxuefeng' a stronger hypoxia responsive activity and higher CmMBF1c expression under waterlogging conditions. Subcellular localization and transcriptional activity assays showed that CmMBF1c protein was localized to the nucleus and had no transcriptional activation activity. Overexpression of CmMBF1c in 'Qinglu' enhanced its waterlogging tolerance by promoting its reactive oxygen species (ROS) scavenging ability and maintaining low ROS levels. However, RNAi-mediated knockdown of CmMBF1c in cultivar 'Nannongxuefeng' resulted in the opposite tendency. Yeast two-hybrid screening and tobacco bimolecular fluorescence complementation assays revealed that CmHRE2, a pivotal regulator of hypoxia response, could interact with CmMBF1c. In summary, this study demonstrates that CmMBF1c improves chrysanthemum waterlogging tolerance by regulating its ROS signaling pathway and interacting with CmHRE2. These findings together offer, to our knowledge, new mechanistic insights into chrysanthemum waterlogging tolerance and provide a rational foundation for future research on the genetic improvement of horticultural crops for waterlogging stress tolerance.

Heat shock-induced cold acclimation in cucumber through CsHSFA1d-activated JA biosynthesis and signaling

Cucumber (Cucumis sativus) originated in tropical areas and is very sensitive to low temperatures. Cold acclimation is a universal strategy that improves plant resistance to cold stress. In this study, we report that heat shock induces cold acclimation in cucumber seedlings, via a process involving the heat-shock transcription factor HSFA1d. CsHSFA1d expression was improved by both heat shock and cold treatment. Moreover, CsHSFA1d transcripts accumulated more under cold treatment after a heat-shock pre-treatment than with either heat shock or cold treatment alone. After exposure to cold, cucumber lines overexpressing CsHSFA1d displayed stronger tolerance for cold stress than the wild type, whereas CsHSFA1d knockdown lines obtained by RNA interference were more sensitive to cold stress. Furthermore, both the overexpression of CsHSFA1d and heat-shock pre-treatment increased the endogenous jasmonic acid (JA) content in cucumber seedlings after cold treatment. Exogenous application of JA rescued the cold-sensitive phenotype of CsHSFA1d knockdown lines, underscoring that JA biosynthesis is key for CsHSFA1d-mediated cold tolerance. Higher JA content is likely to lead to the degradation of CsJAZ5, a repressor protein of the JA pathway. We also established that CsJAZ5 interacts with CsICE1. JA-induced degradation of CsJAZ5 would be expected to release CsICE1, which would then activate the ICE-CBF-COR pathway. After cold treatment, the relative expression levels of ICE-CBF-COR signaling pathway genes, such as CsICE1, CsCBF1, CsCBF2 and CsCOR1, in CsHSFA1d overexpression lines were significantly higher than in the wild type and knockdown lines. Taken together, our results help to reveal the mechanism underlying heat shock-induced cold acclimation in cucumber.

Cassava (Manihot esculenta) Slow Anion Channel (MeSLAH4) Gene Overexpression Enhances Nitrogen Assimilation, Growth, and Yield in Rice

Nitrogen is one of the most important nutrient elements required for plant growth and development, which is also immensely related to the efficient use of nitrogen by crop plants. Therefore, plants evolved sophisticated mechanisms and anion channels to extract inorganic nitrogen (nitrate) from the soil or nutrient solutions, assimilate, and recycle the organic nitrogen. Hence, developing crop plants with a greater capability of using nitrogen efficiently is the fundamental research objective for attaining better agricultural productivity and environmental sustainability. In this context, an in-depth investigation has been conducted into the cassava slow type anion channels (SLAHs) gene family, including genome-wide expression analysis, phylogenetic relationships with other related organisms, chromosome localization, and functional analysis. A potential and nitrogen-responsive gene of cassava (MeSLAH4) was identified and selected for overexpression (OE) analysis in rice, which increased the grain yield and root growth related performance. The morpho-physiological response of OE lines was better under low nitrogen (0.01 mm NH₄NO₃) conditions compared to the wild type (WT) and OE lines under normal nitrogen (0.5 mm NH₄NO₃) conditions. The relative expression of the MeSLAH4 gene was higher (about 80-fold) in the OE line than in the wild type. The accumulation and flux assay showed higher accumulation of NO 3 - and more expansion of root cells and grain dimension of OE lines compared to the wild type plants. The results of this experiment demonstrated that the MeSLAH4 gene may play a vital role in enhancing the efficient use of nitrogen in rice, which could be utilized for high-yielding crop production.

Molecular insights into sensing, regulation and improving of heat tolerance in plants

Climate-change-mediated increase in temperature extremes has become a threat to plant productivity. Heat stress-induced changes in growth pattern, sensitivity to pests, plant phonologies, flowering, shrinkage of maturity period, grain filling, and increased senescence result in significant yield losses. Heat stress triggers multitude of cellular, physiological and molecular responses in plants beginning from the early sensing followed by signal transduction, osmolyte synthesis, antioxidant defense, and heat stress-associated gene expression. Several genes and metabolites involved in heat perception and in the adaptation response have been isolated and characterized in plants. Heat stress responses are also regulated by the heat stress transcription factors (HSFs), miRNAs and transcriptional factors which together form another layer of regulatory circuit. With the availability of functionally validated candidate genes, transgenic approaches have been applied for developing heat-tolerant transgenic maize, tobacco and sweet potato. In this review, we present an account of molecular mechanisms of heat tolerance and discuss the current developments in genetic manipulation for heat tolerant crops for future sustainable agriculture.

The Heat Stress Transcription Factor LlHsfA4 Enhanced Basic Thermotolerance through Regulating ROS Metabolism in Lilies (Lilium Longiflorum)

Heat stress severely affects the annual agricultural production. Heat stress transcription factors (HSFs) represent a critical regulatory juncture in the heat stress response (HSR) of plants. The HsfA1-dependent pathway has been explored well, but the regulatory mechanism of the HsfA1-independent pathway is still under-investigated. In the present research, HsfA4, an important gene of the HsfA1-independent pathway, was isolated from lilies (Lilium longiflorum) using the RACE method, which encodes 435 amino acids. LlHsfA4 contains a typical domain of HSFs and belongs to the HSF A4 family, according to homology comparisons and phylogenetic analysis. LlHsfA4 was mainly expressed in leaves and was induced by heat stress and H₂O₂ using qRT-PCR and GUS staining in transgenic Arabidopsis. LlHsfA4 had transactivation activity and was located in the nucleus and cytoplasm through a yeast one hybrid system and through transient expression in lily protoplasts. Over expressing LlHsfA4 in Arabidopsis enhanced its basic thermotolerance, but acquired thermotolerance was not achieved. Further research found that heat stress could increase H₂O₂ content in lily leaves and reduced H₂O₂ accumulation in transgenic plants, which was consistent with the up-regulation of HSR downstream genes such as Heat stress proteins (HSPs), Galactinol synthase1 (GolS1), WRKY DNA binding protein 30 (WRKY30), Zinc finger of Arabidopsis thaliana 6 (ZAT6) and the ROS-scavenging enzyme Ascorbate peroxidase 2 (APX2). In conclusion, these results indicate that LlHsfA4 plays important roles in heat stress response through regulating the ROS metabolism in lilies.

The heat shock factor GhHSFA4a positively regulates cotton resistance to Verticillium dahliae

Heat shock factors (HSFs) play a crucial role in the environmental stress responses of numerous plant species, including defense responses to pathogens; however, their role in cotton resistance to Verticillium dahliae remains unclear. We have previously identified several differentially expressed genes (DEGs) in Arabidopsis thaliana after inoculation with V. dahliae. Here, we discovered that GhHSFA4a in Gossypium hirsutum (cotton) after inoculation with V. dahliae shares a high identity with a DEG in A. thaliana in response to V. dahliae infection. Quantitative real-time PCR (qRT-PCR) analysis indicated that GhHSFA4a expression was rapidly induced by V. dahliae and ubiquitous in cotton roots, stems, and leaves. In a localization analysis using transient expression, GhHSFA4a was shown to be localized to the nucleus. Virus-induced gene silencing (VIGS) revealed that downregulation of GhHSFA4a significantly increased cotton susceptibility to V. dahliae. To investigate GhHSFA4a-mediated defense, 814 DEGs were identified between GhHSFA4a-silenced plants and controls using comparative RNA-seq analysis. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that DEGs were enriched in "flavonoid biosynthesis", "sesquiterpenoid and triterpenoid biosynthesis", "linoleic acid metabolism" and "alpha-linolenic acid metabolism". The expression levels of marker genes for these four pathways were triggered after inoculation with V. dahliae. Moreover, GhHSFA4a-overexpressing lines of A. thaliana displayed enhanced resistance against V. dahliae compared to that of the wild type. These results indicate that GhHSFA4a is involved in the synthesis of secondary metabolites and signal transduction, which are indispensable for innate immunity against V. dahliae in cotton.

Heterografted chrysanthemums enhance salt stress tolerance by integrating reactive oxygen species, soluble sugar, and proline

Chrysanthemum, one of the most important commercial ornamental crops, is susceptible to salinity, which limits its cultivation and application in coastal and inland saline areas. Grafting is widely used to improve the salt tolerance of horticultural crops, but the mechanisms of grafted chrysanthemum responses to salt stress remain unclear. In this study, we showed that heterografted chrysanthemums with Artemisia annua as rootstock exhibited increased salt tolerance compared with self-grafted and self-rooted chrysanthemums. Under high salt stress, the roots of heterografted chrysanthemums enrich Na⁺, resulting in a reduction of Na⁺ toxicity in the scion, with only a small amount of Na⁺ being transported to the leaves. On the other hand, the roots of heterografted chrysanthemums alleviated high Na⁺ stress via enhanced catalase enzyme activity, downregulation of the expression of reactive oxygen species (ROS) accumulation-related genes, massive accumulation of soluble sugars and proline, and upregulation of the expression of heat shock protein-related genes to enhance salt tolerance. In addition, the leaves of heterografted chrysanthemums respond to low Na⁺ stress by increasing peroxidase enzyme activity and soluble sugar and proline contents, to maintain a healthy state. However, self-grafted and self-rooted plants could not integrate ROS, soluble sugars, and proline in response to salt stress, and thus exhibited a salt-sensitive phenotype. Our research reveals the mechanisms underlying the increased salt tolerance of heterografted chrysanthemums and makes it possible to have large-scale cultivation of chrysanthemums in saline areas.

The First De Novo Transcriptome Assembly and Transcriptomic Dynamics of the Mangrove Tree Rhizophora stylosa Griff. (Rhizophoraceae)

Mangroves are salt-tolerant plant species that grow in coastal saline water and are adapted to harsh environmental conditions. In this study, we de novo assembled and functionally annotated the transcriptome of Rhizophora stylosa, the widely distributed mangrove from the largest mangrove family (Rhizophoraceae). The final transcriptome consists of 200,491 unigenes with an average length, and N50 of 912.7 and 1334 base pair, respectively. We then compared the genome-wide expression profiles between the two morphologically distinct natural populations of this species growing under different levels of salinity depending on their distance from the ocean. Among the 200,491 unigenes, 40,253 were identified as differentially expressed between the two populations, while 15,741 and 24,512 were up- and down-regulated, respectively. Functional annotation assigned thousands of upregulated genes in saline environment to the categories related to abiotic stresses such as response to salt-, osmotic-, and oxidative-stress. Validation of those genes may contribute to a better understanding of adaptation in mangroves species. This study reported, for the first time, the transcriptome of R. stylosa, and the dynamic of it in response to salt stress and provided a valuable resource for elucidation of the molecular mechanism underlying the salt stress response in mangroves and other plants that live under stress.

De Novo Transcriptome Assembly, Functional Annotation, and Transcriptome Dynamics Analyses Reveal Stress Tolerance Genes in Mangrove Tree (Bruguiera gymnorhiza)

Their high adaptability to difficult coastal conditions makes mangrove trees a valuable resource and an interesting model system for understanding the molecular mechanisms underlying stress tolerance and adaptation of plants to the stressful environmental conditions. In this study, we used RNA sequencing (RNA-Seq) for de novo assembling and characterizing the Bruguiera gymnorhiza (L.) Lamk leaf transcriptome. B. gymnorhiza is one of the most widely distributed mangrove species from the biggest family of mangroves; Rhizophoraceae. The de novo assembly was followed by functional annotations and identification of individual transcripts and gene families that are involved in abiotic stress response. We then compared the genome-wide expression profiles between two populations of B. gymnorhiza, growing under different levels of stress, in their natural habitats. One population living in high salinity environment, in the shore of the Pacific Ocean- Japan, and the other population living about one kilometre farther from the ocean, and next to the estuary of a river; in less saline and more brackish condition. Many genes involved in response to salt and osmotic stress, showed elevated expression levels in trees growing next to the ocean in high salinity condition. Validation of these genes may contribute to future salt-resistance research in mangroves and other woody plants. Furthermore, the sequences and transcriptome data provided in this study are valuable scientific resources for future comparative transcriptome research in plants growing under stressful conditions.

ThHSFA1 Confers Salt Stress Tolerance through Modulation of Reactive Oxygen Species Scavenging by Directly Regulating ThWRKY4

Heat shock transcription factors (HSFs) play critical roles in several types of environmental stresses. However, the detailed regulatory mechanisms in response to salt stress are still largely unknown. In this study, we examined the salt-induced transcriptional responses of ThHSFA1-ThWRKY4 in Tamarix hispida and their functions and regulatory mechanisms in salt tolerance. ThHSFA1 protein acts as an upstream regulator that can directly activate ThWRKY4 expression by binding to the heat shock element (HSE) of the ThWRKY4 promoter using yeast one-hybrid (Y1H), chromatin immunoprecipitation (ChIP), and dual-luciferase reporter assays. ThHSFA1 and ThWRKY4 expression was significantly induced by salt stress and abscisic acid (ABA) treatment in the roots and leaves of T. hispida. ThHSFA1 is a nuclear-localized protein with transactivation activity at the C-terminus. Compared to nontransgenic plants, transgenic plants overexpressing ThHSFA1 displayed enhanced salt tolerance and exhibited reduced reactive oxygen species (ROS) levels and increased antioxidant enzyme activity levels under salt stress. Therefore, we further concluded that ThHSFA1 mediated the regulation of ThWRKY4 in response to salt stress in T. hispida.

The response mechanism to salt stress in Arabidopsis transgenic lines over-expressing of GmG6PD

Glucose-6-phosphate dehydrogenase (G6PD or G6PDH) plays an important role in response to salt stress in plants. However, much less is known about G6PD proteins in soybean (Glycine max L.). Here, we found that a soybean cytosolic G6PD gene, GmG6PD7, was induced by NaCl. We generated Arabidopsis transgenic lines overexpressing GmG6PD7. The seed germination rate and primary root length of Arabidopsis thaliana over-expressing GmG6PD7 under NaCl treatment were enhanced. Salt stress induced an obvious increase of the total and cytosolic G6PD activity and the marked decrease of ROS levels in the transgenic plants. At the same time, over-expressing GmG6PD7 in Arabidopsis affected the glutathione and NADPH level and activated ROS scavengers, suggesting that GmG6PD7 contributes to increase salinity tolerance by decreasing ROS accumulation. What's more, we found GmG6PD7 overexpression led to the up-regulation of abscisic acid (ABA) degradation gene and the down-regulation of ABA synthesis and ABA-responsive genes, which finally reduced ABA content to improve seed germination rate under salinity stress. It was noteworthy that GmG6PD7 can rescue the seed and root phenotype of Arabidopsis cytosolic G6PD mutant (Atg6pd5 and Atg6pd6) under salt stress, suggesting cytosolic G6PD may have a conserved function in soybean and Arabidopsis.

Earthworm and arbuscular mycorrhiza interactions: Strategies to motivate antioxidant responses and improve soil functionality

Earthworms and arbuscular mycorrhizal fungi (AMF) act synergistically in the rhizosphere and may increase host plant tolerance to Cd. However, mechanisms by which earthworm-AMF-plant partnerships counteract Cd phytotoxicity are unknown. Thus, we evaluated individual and interactive effects of these soil organisms on photosynthesis, antioxidant capacity, and essential nutrient uptake by Solanum nigrum, as well as on soil quality following Cd exposure (0-120 mg kg^-1). Decreases in biomass and photosynthetic activity, as well as nutrient imbalances were observed in Cd-stressed plants; however, the addition of AMF and earthworms reversed these effects. Cd exposure increased superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, whereas inoculation with Rhizophagus intraradices decreased those. Soil enzymatic activity decreased by 15-60% with increasing Cd concentrations. However, Cd-mediated toxicity was partially reversed by soil organisms. Earthworms and AMF ameliorated soil quality based on soil enzyme activity. At 120 mg kg^-1 Cd, the urease, catalase, and acid phosphatase activities were 1.6-, 1.4-, and 1.2-fold higher, respectively, in soils co-incubated with earthworms and AMF than in uninoculated soil. Cd inhibited shoot Fe and Ca phytoaccumulation, whereas AMF and earthworms normalized the status of essential elements in plants. Cd detoxification by earthworm-AMF-S. nigrum symbiosis was manifested by increases in plant biomass accumulation (22-117%), chlorophyll content (17-63%), antioxidant levels (SOD 10-18%, POD 9-25%, total polyphenols 17-22%, flavonoids 15-29%, and glutathione 7-61%). It also ameliorated the photosynthetic capacity, and macro- and micronutrient statuses of plants; markedly reduced the levels of malondialdehyde (20-27%), superoxide anion (29-36%), and hydrogen peroxide (19-30%); and upregulated the transcription level of FeSOD. Thus, the combined action of earthworms and AMF feasibly enhances metal tolerance of hyperaccumulating plants and improves the quality of polluted soil.

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.

Genetic elucidation of hydrogen signaling in plant osmotic tolerance and stomatal closure via hydrogen sulfide

Although ample evidence showed that exogenous hydrogen gas (H₂) controls a diverse range of physiological functions in both animals and plants, the selective antioxidant mechanism, in some cases, is questioned. Importantly, most of the experiments on the function of H₂ in plants were based on pharmacological approaches due to the synthesis pathway(s) in plants are still unclear. Here, we observed that the seedling growth inhibition of Arabidopsis caused by low doses of mannitol could progressively recover by recuperation, accompanied with the increased hydrogenase activity and H₂ synthesis. To investigate the functions of endogenous H₂, a hydrogenase gene (CrHYD1) for H₂ biosynthesis from Chlamydomonas reinhardtii was expressed in Arabidopsis. Transgenic plants could intensify higher H₂ synthesis compared with wild type and Arabidopsis transformed with the empty vector, and exhibited enhanced osmotic tolerance in both germination and post-germination stages. In response to mannitol, transgenic plants enhanced _L-Cys desulfhydrase (DES)-dependent hydrogen sulfide (H₂S) synthesis in guard cells and thereafter stomatal closure. The application of des mutant further highlights H₂S acting as a downstream molecule of endogenous H₂ control of stomatal closure. These results thus open a new window for increasing plant tolerance to osmotic stress.

CaHsfA1d Improves Plant Thermotolerance via Regulating the Expression of Stress- and Antioxidant-Related Genes

Heat shock transcription factor (Hsf) plays an important role in regulating plant thermotolerance. The function and regulatory mechanism of CaHsfA1d in heat stress tolerance of pepper have not been reported yet. In this study, phylogenetic tree and sequence analyses confirmed that CaHsfA1d is a class A Hsf. CaHsfA1d harbored transcriptional function and predicted the aromatic, hydrophobic, and acidic (AHA) motif mediated function of CaHsfA1d as a transcription activator. Subcellular localization assay showed that CaHsfA1d protein is localized in the nucleus. The CaHsfA1d was transcriptionally up-regulated at high temperatures and its expression in the thermotolerant pepper line R9 was more sensitive than that in thermosensitive pepper line B6. The function of CaHsfA1d under heat stress was characterized in CaHsfA1d-silenced pepper plants and CaHsfA1d-overexpression Arabidopsis plants. Silencing of the CaHsfA1d reduced the thermotolerance of the pepper, while CaHsfA1d-overexpression Arabidopsis plants exhibited an increased insensitivity to high temperatures. Moreover, the CaHsfA1d maintained the H2O2 dynamic balance under heat stress and increased the expression of Hsfs, Hsps (heat shock protein), and antioxidant gene AtGSTU5 (glutathione S-transferase class tau 5) in transgenic lines. Our findings clearly indicate that CaHsfA1d improved the plant thermotolerance via regulating the expression of stress- and antioxidant-related genes.

Chitosan regulates metabolic balance, polyamine accumulation, and Na+ transport contributing to salt tolerance in creeping bentgrass

BACKGROUND

Chitosan (CTS), a natural polysaccharide, exhibits multiple functions of stress adaptation regulation in plants. However, effects and mechanism of CTS on alleviating salt stress damage are still not fully understood. Objectives of this study were to investigate the function of CTS on improving salt tolerance associated with metabolic balance, polyamine (PAs) accumulation, and Na+ transport in creeping bentgrass (Agrostis stolonifera).

RESULTS

CTS pretreatment significantly alleviated declines in relative water content, photosynthesis, photochemical efficiency, and water use efficiency in leaves under salt stress. Exogenous CTS increased endogenous PAs accumulation, antioxidant enzyme (SOD, POD, and CAT) activities, and sucrose accumulation and metabolism through the activation of sucrose synthase and pyruvate kinase activities, and inhibition of invertase activity. The CTS also improved total amino acids, glutamic acid, and γ-aminobutyric acid (GABA) accumulation. In addition, CTS-pretreated plants exhibited significantly higher Na+ content in roots and lower Na+ accumulation in leaves then untreated plants in response to salt stress. However, CTS had no significant effects on K+/Na+ ratio. Importantly, CTS enhanced salt overly sensitive (SOS) pathways and also up-regulated the expression of AsHKT1 and genes (AsNHX4, AsNHX5, and AsNHX6) encoding Na+/H+ exchangers under salt stress.

CONCLUSIONS

The application of CTS increased antioxidant enzyme activities, thereby reducing oxidative damage to roots and leaves. CTS-induced increases in sucrose and GABA accumulation and metabolism played important roles in osmotic adjustment and energy metabolism during salt stress. The CTS also enhanced SOS pathway associated with Na+ excretion from cytosol into rhizosphere, increased AsHKT1 expression inhibiting Na+ transport to the photosynthetic tissues, and also up-regulated the expression of AsNHX4, AsNHX5, and AsNHX6 promoting the capacity of Na+ compartmentalization in roots and leaves under salt stress. In addition, CTS-induced PAs accumulation could be an important regulatory mechanism contributing to enhanced salt tolerance. These findings reveal new functions of CTS on regulating Na+ transport, enhancing sugars and amino acids metabolism for osmotic adjustment and energy supply, and increasing PAs accumulation when creeping bentgrass responds to salt stress.

Genome-Wide Analysis of the Heat Shock Transcription Factor Gene Family in Brassica juncea: Structure, Evolution, and Expression Profiles

Heat shock transcription factor (HSF) is ubiquitous in the whole biological world and plays an important role in regulating growth and development and responses to environment stress. In this study, a total of 60 HSF transcription factors in Brassica juncea genome were identified and analyzed. Phylogenetic analysis showed that HSF genes were divided into three groups namely: A, B, and C, of which group A was further divided into nine subgroups (A1-A9). The analysis of gene structure and conserved motifs showed that some homologous genes are highly conserved. There was strong conservative microcollinearity among Brassica rapa, B. juncea, and Brassica oleracea, which provides a basis for studying the replication of gene families. Moreover, the results revealed that the promoter regions of BjuHSF genes were rich in cis-elements related to growth and development, hormone signal, and stress response. The prediction of protein interaction results showed that HSFs could interact with multiple transcription factors and proteins in the genome, while functional annotation revealed that BjuHSF genes were involved in many biological processes. The expression patterns of BjuHSF genes were analyzed by qPCR, and the results showed that these genes were closely linked to stress response, hormones, and development process. These results are a foundation for further analysis of the regulation mechanism of HSF gene family.

Overexpression of Maize ZmMYB59 Gene Plays a Negative Regulatory Role in Seed Germination in Nicotiana tabacum and Oryza sativa

MYB transcription factors are involved in many biological processes, including metabolism, stress response and plant development. In our previous work, ZmMYB59 was down-regulated by deep sowing during maize seed germination. However, there are few reports on seed germination regulated by MYB proteins. In this study, to examine its functions during seed germination, Agrobacterium-mediated transformation was exploited to generate ZmMYB59 overexpression (OE) tobacco and rice. In T2 generation transgenic tobacco, germination rate, germination index, vigor index and hypocotyl length were significantly decreased by 25.0-50.9, 34.5-54.4, 57.5-88.3, and 21.9-31.3% compared to wild-type (WT) lines. In T2 generation transgenic rice, above corresponding parameters were notably reduced by 39.1-53.8, 51.4-71.4, 52.5-74.0, and 28.3-41.5%, respectively. On this basis, antioxidant capacity and endogenous hormones were determined. The activities of catalase, peroxidase, superoxide dismutase, ascorbate peroxidase of OE lines were significantly lower than those of WT, suggesting that ZmMYB59 reduced their oxidation resistance. As well, ZmMYB59 overexpression extremely inhibited the synthesis of gibberellin A1 (GA1) and cytokinin (CTK), and promoted the synthesis of abscisic acid (ABA) concurrently. Taken together, it proposed that ZmMYB59 was a negative regulator during seed germination in tobacco and rice, which also contributes to illuminate the molecular mechanisms regulated by MYB transcription factors.

Genome-Wide Identification and Functional Characterization of the Heat Shock Factor Family in Eggplant (Solanum melongena L.) under Abiotic Stress Conditions

Plant heat shock factors (Hsfs) play crucial roles in various environmental stress responses. Eggplant (Solanum melongena L.) is an agronomically important and thermophilic vegetable grown worldwide. Although the functions of Hsfs under environmental stress conditions have been characterized in the model plant Arabidopsis thaliana and tomato, their roles in responding to various stresses remain unclear in eggplant. Therefore, we characterized the eggplant SmeHsf family and surveyed expression profiles mediated by the SmeHsfs under various stress conditions. Here, using reported Hsfs from other species as queries to search SmeHsfs in the eggplant genome and confirming the typical conserved domains, we identified 20 SmeHsf genes. The SmeHsfs were further classified into 14 subgroups on the basis of their structure. Additionally, quantitative real-time PCR revealed that SmeHsfs responded to four stresses—cold, heat, salinity and drought—which indicated that SmeHsfs play crucial roles in improving tolerance to various abiotic stresses. The expression pattern of SmeHsfA6b exhibited the most immediate response to the various environmental stresses, except drought. The genome-wide identification and abiotic stress-responsive expression pattern analysis provide clues for further analysis of the roles and regulatory mechanism of SmeHsfs under environmental stresses.

Genome-wide characterization of tea plant (Camellia sinensis) Hsf transcription factor family and role of CsHsfA2 in heat tolerance

BACKGROUND

Heat stress factors (Hsfs) play vital roles in signal transduction pathways operating in responses to environmental stresses. However, Hsf gene family has not been thoroughly explored in tea plant (Camellia sinensis L.).

RESULTS

In this study, we identified 25 CsHsf genes in C. sinensis that were separated by phylogenetic analysis into three sub-families (i.e., A, B, and C). Gene structures, conserved domains and motifs analyses indicated that the CsHsf members in each class were relatively conserved. Various cis-acting elements involved in plant growth regulation, hormone responses, stress responses, and light responses were located in the promoter regions of CsHsfs. Furthermore, degradome sequencing analysis revealed that 7 CsHsfs could be targeted by 9 miRNAs. The expression pattern of each CsHsf gene was significantly different in eight tissues. Many CsHsfs were differentially regulated by drought, salt, and heat stresses, as well as exogenous abscisic acid (ABA) and Ca²⁺. In addition, CsHsfA2 was located in the nucleus. Heterologous expression of CsHsfA2 improved thermotolerance in transgenic yeast, suggesting its potential role in the regulation of heat stress response.

CONCLUSIONS

A comprehensive genome-wide analysis of Hsf in C. sinensis present the global identification and functional prediction of CsHsfs. Most of them were implicated in a complex gene regulatory network controlling various abiotic stress responses and signal transduction pathways in tea plants. Additionally, heterologous expression of CsHsfA2 increased thermotolerance of transgenic yeast. These findings provide new insights into the functional divergence of CsHsfs and a basis for further research on CsHsfs functions.

SaHsfA4c From Sedum alfredii Hance Enhances Cadmium Tolerance by Regulating ROS-Scavenger Activities and Heat Shock Proteins Expression

The heat shock transcription factor (Hsf) family, an important member in plant stress response, affects cadmium (Cd) tolerance in plants. In this study, we identified and functionally characterized a transcript of the Hsf A4 subgroup from Sedum alfredii. Designated as SaHsfA4c, the open reading frame was 1,302 bp long and encoded a putative protein of 433 amino acids containing a complete DNA-binding domain (DBD). Heterologous expression of SaHsfA4c in yeast enhanced Cd stress tolerance and accumulation, whereas expression of the alternatively spliced transcript InSaHsfA4c which contained an intron and harbored an incomplete DBD, resulted in relatively poor Cd stress tolerance and low Cd accumulation in transgenic yeast. The function of SaHsfA4c under Cd stress was characterized in transgenic Arabidopsis and non-hyperaccumulation ecotype S. alfredii. SaHsfA4c was able to rescue the Cd sensitivity of the Arabidopsis athsfa4c mutant. SaHsfA4c reduced reactive oxygen species (ROS) accumulation and increased the expression of ROS-scavenging enzyme genes and Hsps in transgenic lines. The present results suggest that SaHsfA4c increases plant resistance to stress by up-regulating the activities of ROS-scavenging enzyme and the expression of Hsps.

Methyl-coenzyme M reductase-dependent endogenous methane enhances plant tolerance against abiotic stress and alters ABA sensitivity in Arabidopsis thaliana

Our study firstly elaborated the underlying mechanism of endogenous CH₄-induced abiotic tolerance, along with an alteration of ABA sensitivity by mimicking the endogenous CH₄ production in MtMCR transgenic Arabidopsis. Endogenous methane (CH₄) production and/or emission have been ubiquitously observed in stressed plants. However, their physiological roles remain unclear. Here, the methyl-coenzyme M reductase gene from Methanobacterium thermoautotrophicum (MtMCR), encoding the enzyme of methanogenesis, was expressed in Arabidopsis thaliana, to mimic the production of endogenous CH₄. In response to salinity and osmotic stress, MtMCR expression was up-regulated in transgenic plants, resulting in significant increase of endogenous CH₄ levels. Similar results were observed in abscisic acid (ABA) treatment. The functions of endogenous CH₄ were characterized by the changes in plant phenotypes related to stress and ABA sensitivity during the germination and post-germination periods. When challenged with osmotic stress, a reduction in water loss and stomatal closure, were observed. Redox homeostasis was reestablished during osmotic and salinity stress, and ion imbalance was also restored in salinity conditions. The expression of several stress/ABA-responsive genes was up-regulated, and ABA sensitivity, in particularly, was significantly altered in the MtMCR transgenic plants. Together, our genetic study for the first time elaborated the possible mechanism of endogenous CH₄-enhanced salinity and osmotic tolerance, along with an alteration of ABA sensitivity. These findings thus provided novel cues for understanding the possible roles of endogenous CH₄ in plants.

Current achievements and future prospects in the genetic breeding of chrysanthemum: a review

Chrysanthemum (Chrysanthemum morifolium Ramat.) is a leading flower with applied value worldwide. Developing new chrysanthemum cultivars with novel characteristics such as new flower colors and shapes, plant architectures, flowering times, postharvest quality, and biotic and abiotic stress tolerance in a time- and cost-efficient manner is the ultimate goal for breeders. Various breeding strategies have been employed to improve the aforementioned traits, ranging from conventional techniques, including crossbreeding and mutation breeding, to a series of molecular breeding methods, including transgenic technology, genome editing, and marker-assisted selection (MAS). In addition, the recent extensive advances in high-throughput technologies, especially genomics, transcriptomics, proteomics, metabolomics, and microbiomics, which are collectively referred to as omics platforms, have led to the collection of substantial amounts of data. Integration of these omics data with phenotypic information will enable the identification of genes/pathways responsible for important traits. Several attempts have been made to use emerging molecular and omics methods with the aim of accelerating the breeding of chrysanthemum. However, applying the findings of such studies to practical chrysanthemum breeding remains a considerable challenge, primarily due to the high heterozygosity and polyploidy of the species. This review summarizes the recent achievements in conventional and modern molecular breeding methods and emerging omics technologies and discusses their future applications for improving the agronomic and horticultural characteristics of chrysanthemum.

The mitogen-activated protein kinase 4-phosphorylated heat shock factor A4A regulates responses to combined salt and heat stresses

Heat shock factors regulate responses to high temperature, salinity, water deprivation, or heavy metals. Their function in combinations of stresses is, however, not known. Arabidopsis HEAT SHOCK FACTOR A4A (HSFA4A) was previously reported to regulate responses to salt and oxidative stresses. Here we show, that the HSFA4A gene is induced by salt, elevated temperature, and a combination of these conditions. Fast translocation of HSFA4A tagged with yellow fluorescent protein from cytosol to nuclei takes place in salt-treated cells. HSFA4A can be phosphorylated not only by mitogen-activated protein (MAP) kinases MPK3 and MPK6 but also by MPK4, and Ser309 is the dominant MAP kinase phosphorylation site. In vivo data suggest that HSFA4A can be the substrate of other kinases as well. Changing Ser309 to Asp or Ala alters intramolecular multimerization. Chromatin immunoprecipitation assays confirmed binding of HSFA4A to promoters of target genes encoding the small heat shock protein HSP17.6A and transcription factors WRKY30 and ZAT12. HSFA4A overexpression enhanced tolerance to individually and simultaneously applied heat and salt stresses through reduction of oxidative damage. Our results suggest that this heat shock factor is a component of a complex stress regulatory pathway, connecting upstream signals mediated by MAP kinases MPK3/6 and MPK4 with transcription regulation of a set of stress-induced target genes.

cDNA Library for Mining Functional Genes in Sedum alfredii Hance Related to Cadmium Tolerance and Characterization of the Roles of a Novel SaCTP2 Gene in Enhancing Cadmium Hyperaccumulation

Heavy metal contamination presents serious threats to living organisms. Functional genes related to cadmium (Cd) hypertolerance or hyperaccumulation must be explored to enhance phytoremediation. Sedum alfredii Hance is a Zn/Cd cohyperaccumulator exhibiting abundant genes associated with Cd hypertolerance. Here, we developed a method for screening genes related to Cd tolerance by expressing a cDNA-library for S. alfredii Hance. Yeast functional complementation validated 42 of 48 full-length genes involved in Cd tolerance, and the majority of them were strongly induced in roots and exhibited diverse expression profiles across tissues. Coexpression network analysis suggested that 15 hub genes were connected with genes involved in metabolic processes, response to stimuli, and metal transporter and antioxidant activity. The functions of a novel SaCTP2 gene were validated by heterologous expression in Arabidopsis, responsible for retarding chlorophyll content decrease, maintaining membrane integrity, promoting reactive oxygen species (ROS) scavenger activities, and reducing ROS levels. Our findings suggest a highly complex network of genes related to Cd hypertolerance in S. alfredii Hance, accomplished via the antioxidant system, defense genes induction, and the calcium signaling pathway. The proposed cDNA-library method is an effective approach for mining candidate genes associated with Cd hypertolerance to develop genetically engineered plants for use in phytoremediation.

Genome-wide transcriptomic analysis of a desert willow, Salix psammophila, reveals the function of hub genes SpMDP1 and SpWRKY33 in drought tolerance

BACKGROUND

Drought is a major environmental constraint to plant growth, development and productivity. Compared with most willows that are generally susceptible to drought, the desert willow Salix psammophila has extraordinary adaptation to drought stress. However, its molecular basis of drought tolerance is still largely unknown.

RESULTS

During polyethylene glycol 6000 (PEG 6000)-simulated drought stress, we found that the osmotic adjustment substances were accumulated and the antioxidant enzyme activities were enhanced in S. psammophila roots. A total of 8172 differentially expressed genes were identified in roots of S. psammophila through RNA-Sequencing. Based on K-means clustering, their expression patterns were classified into nine clusters, which were enriched in several stress-related processes including transcriptional regulation, response to various stresses, cell death, etc. Moreover, 672 transcription factors from 45 gene families were differentially expressed under drought stress. Furthermore, a weighted gene co-expression network was constructed, and eight genes were identified as hub genes. We demonstrated the function of two hub genes, magnesium-dependent phosphatase 1 (SpMDP1) and SpWRKY33, through overexpression in Arabidopsis thaliana. Overexpression of the two hub genes enhanced the drought tolerance in transgenic plants, suggesting that the identification of candidate drought tolerance genes in this study was highly efficient and credible.

CONCLUSIONS

Our study analyzed the physiological and molecular responses to drought stress in S. psammophila, and these results contribute to dissect the mechanism of drought tolerance of S. psammophila and facilitate identification of critical genes involved in drought tolerance for willow breeding.

PtHSFA4a gene play critical roles in the adaptation of Arabidopsis thaliana plants to high-Zinc stress

Heat shock transcription factors (HSFs) play pivotal roles in various abiotic stresses. However, only one of the studies on HSFs that participated in excess zinc (Zn) stress in our previous study in Populus ussuriensis. Here, overexpression of P. trichocarpa PtHSFA4a gene in Arabidopsis thaliana significantly improved excess Zn tolerance. It was found that PtHSFA4a-OE lines have higher seed germination rate than wild type (WT) when exposed to excess Zn. Also, PtHSFA4a-OE lines exhibit high viability and stronger root growth than WT in soil. PtHSFA4a reduced the intracellular concentration of free zinc ion of roots when overexpressed in A. thaliana. Our data indicate PtHSFA4a is the candidate gene to act as positive regulators in the resistance to excess Zn, extending our knowledge of excess Zn tolerance transcription factors.

PuHSFA4a Enhances Tolerance To Excess Zinc by Regulating Reactive Oxygen Species Production and Root Development in Populus

Zinc (Zn) is an essential micronutrient but in excess is highly toxic to plants. Plants regulate Zn homeostasis and withstand excess Zn through various pathways; these pathways are generally tightly regulated by a specific set of genes. However, the transcription factors involved in excess Zn tolerance have yet to be identified. Here, we characterized a Populus ussuriensis heat shock transcription factor A4a (PuHSFA4a) that acts as a positive regulator of excess Zn tolerance in P ussuriensis We used overexpression (PuHSFA4a-OE) and chimeric dominant repressor (PuHSFA4a-SRDX) lines to identify the targets of PuHSFA4a PuHSFA4a transcription is specifically induced in roots by high Zn. Overexpression of PuHSFA4a conferred excess Zn tolerance and a dominant repressor version of PuHSFA4a increased excess Zn sensitivity in P ussuriensis by regulating the antioxidant system in roots. PuHSFA4a coordinately activates genes related to abiotic stress responses and root development and directly binds to the promoter regions of glutathione-s-transferase U17 (PuGSTU17) and phospholipase A2 (PuPLA2 ). PuGSTU17 overexpression significantly increased GST activity and reduced reactive oxygen species levels in roots while PuGSTU17-RNA interference lines exhibited the opposite phenotype. Furthermore, PuPLA2 overexpression promoted root growth under high Zn stress. Taken together, we provide evidence that PuHSFA4a coordinately activates the antioxidant system and root development-related genes and directly targets PuGSTU17 and PuPLA, thereby promoting excess Zn tolerance in P ussuriensis roots.

Overexpression of CmMYB15 provides chrysanthemum resistance to aphids by regulating the biosynthesis of lignin

MYB transcription factors are widely involved in the development of and physiological processes in plants. Here, we isolated the chrysanthemum R2R3-MYB family transcription factor CmMYB15, a homologous gene of AtMYB15. It was demonstrated that CmMYB15 expression was induced by aphids and that CmMYB15 could bind to AC elements, which usually exist in the promoter of lignin biosynthesis genes. Overexpression of CmMYB15 in chrysanthemum enhanced the resistance of aphids. Additionally, the content of lignin and the expression of several lignin biosynthesis genes increased. In summary, the results indicate that CmMYB15 regulates lignin biosynthesis genes that enhance the resistance of chrysanthemum to aphids.

Progress in Understanding the Physiological and Molecular Responses of Populus to Salt Stress

Salt stress (SS) has become an important factor limiting afforestation programs. Because of their salt tolerance and fully sequenced genomes, poplars (Populus spp.) are used as model species to study SS mechanisms in trees. Here, we review recent insights into the physiological and molecular responses of Populus to SS, including ion homeostasis and signaling pathways, such as the salt overly sensitive (SOS) and reactive oxygen species (ROS) pathways. We summarize the genes that can be targeted for the genetic improvement of salt tolerance and propose future research areas.

Characterization of the Populus Rab family genes and the function of PtRabE1b in salt tolerance

BACKGROUND

Rab proteins form the largest family of the Ras superfamily of small GTP-binding proteins and regulate intracellular trafficking pathways. However, the function of the Rab proteins in woody species is still an open question.

RESULTS

Here, a total of 67 PtRabs were identified in Populus trichocarpa and categorized into eight subfamilies (RabA-RabH). Based on their chromosomal distribution and duplication blocks in the Populus genome, a total of 27 PtRab paralogous pairs were identified and all of them were generated by whole-genome duplication events. Combined the expression correlation and duplication date, the PtRab paralogous pairs that still keeping highly similar expression patterns were generated around the latest large-scale duplication (~ 13 MYA). The cis-elements and co-expression network of unique expanded PtRabs suggest their potential roles in poplar development and environmental responses. Subcellular localization of PtRabs from each subfamily indicates each subfamily shows a localization pattern similar to what is revealed in Arabidopsis but RabC shows a localization different from their counterparts. Furthermore, we characterized PtRabE1b by overexpressing its constitutively active mutant PtRabE1b(Q74L) in poplar and found that PtRabE1b(Q74L) enhanced the salt tolerance.

CONCLUSIONS

These findings provide new insights into the functional divergence of PtRabs and resources for genetic engineering resistant breeding in tree species.

Identification, Expression Analysis of the Hsf Family, and Characterization of Class A4 in Sedum Alfredii Hance under Cadmium Stress

Sedum alfredii Hance, a cadmium (Cd)/zinc (Zn)/lead (Pb) co-hyperaccumulating species, is a promising phytoremediation candidate because it accumulates substantial amounts of heavy metal ions without showing any obvious signs of poisoning. The heat shock transcription factor (Hsf) family plays crucial roles in plant growth, development, and stress responses. Although the roles of some Hsfs in abiotic stress have been well studied in model plants, the Hsf family has not been systematically investigated in heavy metal hyperaccumulators. Here, we comprehensively analyzed the Hsf gene family in S. alfredii based on a transcriptome under Cd stress. There were 22 Hsf members that were identified and phylogenetically clustered into three classes, namely, SaHsfA, SaHsfB, and SaHsfC. All of the three classes shared similar motifs. The expression profiles of the 22 Hsf members showed significant differences: 18 SaHsfs were responsive to Cd stress, as were multiple SaHsp genes, including SaHsp18.1, SaHsp22, SaHsp26.5, SaHsp70, SaHsp90, and SaHsp101. Two class A4 members, SaHsfA4a and SaHsfA4c, exhibited transcriptional activation activities. Overexpression of SaHsfA4a and SaHsfA4c in transgenic yeast indicated an improved tolerance to Cd stress and Cd accumulation. Our results suggest SaHsfs play important regulatory roles in heavy metal stress responses, and provide a reference for further studies on the mechanism of heavy metal stress regulation by SaHsfs.

Overexpression of Phosphate Transporter Gene CmPht1;2 Facilitated Pi Uptake and Alternated the Metabolic Profiles of Chrysanthemum Under Phosphate Deficiency

Low availability of phosphorus (P) in the soil is the principal limiting factor for the growth of cut chrysanthemum. Plant phosphate transporters (PTs) facilitate acquisition of inorganic phosphate (Pi) and its homeostasis within the plant. In the present study, CmPht1;2 of the Pht1 family was cloned from chrysanthemum. CmPht1;2 is composed of 12 transmembrane domains and localized to the plasma membrane. Expression of CmPht1;2 in roots was induced by Pi starvation. Chrysanthemum plants with overexpression of CmPht1;2 (Oe) showed higher Pi uptake, as compared to the wild type (WT), both under Pi-starvation and Pi-sufficient conditions, and also showed a higher root biomass compared to WT in the Pi-starvation conditions. Seven days after the P-deficiency treatment, 85 distinct analytes were identified in the roots and 27 in the shoots between the Oe1 plant and WT, in which sophorose, sorbitol (sugars), hydroxybutyric acid (organic acids), and ornithine (amino acid) of CmPht1;2 overexpressing chrysanthemum are specific responses to P-starvation.