Citrus heat shock transcription factor CitHsfA7-mediated citric acid degradation in response to heat stress

Generation of suspension cell cultures with high syringin content and anti-inflammatory activity through overexpressing glycotransferase SiUGT72BZ2 in Saussurea involucrata

The snow lotus species Saussurea involucrata (Kar. & Kir.) Sch.Bip., an endangered traditional Chinese herb, belongs to a genus of the Asteraceae family. Syringin present in S. involucrata stands as one of the predominant bioactive compounds. However, the biosynthetic pathway of syringin remains largely elusive. Here, S. involucrata suspension cell culture was subjected to methyl jasmonate (MeJA) treatment, which stimulated the synthesis of syringin, increasing its content by up to 3.9-fold. Comparative transcriptome analysis revealed that genes involved in syringin biosynthesis were generally upregulated in response to MeJA. Furthermore, two candidate UDP-glycosyltransferase genes, SiUGT72BZ2 and SiUGT72CY1, were identified through phylogenetic tree and expression profiling analyses. Overexpression of SiUGT72BZ2 (BZ2_OE) and SiUGT72CY1 (CY1_OE) in S. involucrata suspension cell cultures led to 15.2- and 5.9-fold higher syringin levels than empty vector control cultures, respectively. Notably, upregulation of SiUGT72BZ2 enhanced the biosynthesis of coniferin as well. In contrast, only trace amounts of coniferin were present in control and CY1_OE cell cultures. Subsequent anti-inflammatory assays using lipopolysaccharide (LPS)-stimulated RAW264.7 cells demonstrated that the extracts from these cell cultures possessed remarkable anti-inflammatory properties. Most strikingly, the BZ2_OE cultures exhibited superior anti-inflammatory effects compared to the control and CY1_OE. In conclusion, our research has not only identified the key enzymes in syringin synthesis but also, through genetic engineering, has generated novel cell culture resources enriched with syringin and coniferin, and enhanced anti-inflammatory activities, highlighting the potential of S. involucrata cell culture as an alternative for wild snow lotus resources.

Rapid degradation of ACLA, a subunit of ATP citrate lyase, via autophagy and 26S proteasome pathways to promote pepper growth-to-tolerance transition under heat stress

Citric acid in plant cells is crucial for growth as it serves as a precursor to multiple essential compounds. It also helps plants tolerate high temperatures. However, the mechanisms remain unclear regarding how citric acid balances its role in promoting growth and protecting against stress. We identified an ACLA protein, a subunit of ATP citrate lyase (ACL) in pepper (Capsicum annuum), that converts cytosolic citric acid into acetyl-CoA. Silencing ACLA reduced citric acid metabolites, leading to stunted growth and decreased heat tolerance. Conversely, ACLA-2 overexpression increased acetyl-CoA metabolites but reduced citric acid levels, which also led to reduced heat tolerance. However, applying exogenous citrate significantly improved the heat tolerance of ACLA-overexpressing plants compared with wild-types. This suggests that citric acid plays a dual role in the synthesis of structural components and in enhancing heat stress resistance. When plants are subjected to heat stress, ACL is rapidly degraded within 1 min. Treatments with E64d and MG132 demonstrated that autophagy and the 26S proteasome pathway contribute to this degradation. This dynamic degradation precisely regulates the dual role of ACL in growth and stress responses, indicating a novel mechanism by which plant cells rapidly adapt to environmental changes through the degradation of key enzymes.

Phosphorylation of the strawberry MADS-box CMB1 regulates ripening via the catabolism of abscisic acid

Research on the ripening of fleshy fruits has relied on techniques that measure transcriptional changes. How ripening is linked to posttranslational modifications such as protein phosphorylation remains less studied. Here, we characterize the MADS-box SEPALLATA 4 (SEP4) subfamily transcription factor FaCMB1, a key negative regulator controlling strawberry ripening, whose transcript and protein abundance decrease progressively with fruit development and are repressed by abscisic acid (ABA). Transient RNAi or overexpression of FaCMB1 significantly altered the fruit ripening process and affected the content of endogenous ABA and ripening-related quality. Transcriptome sequencing (RNA-seq) analysis suggested that manipulation of FaCMB1 expression levels affected the transcription of FaASR (ABA-, stress-, ripening-induced), while FaCMB1 can repress the gene expression of FaASR by directly binding to its promoter. Furthermore, FaASR inhibited the transcriptional activity of FaCYP707A4, a key ABA 8'-hydroxylase enzyme involved in ABA catabolism. We show that FaCMB1 can be phosphorylated by the kinase FaSTPK, and Phos-tag assays indicated that the phosphorylation level of FaCMB1 increases during fruit ripening. This phosphorylation of FaCMB1 affects the binding ability of FaCMB1 to the FaASR promoter and alleviates its transcriptional repression. In conclusion, we elucidated a feedback regulatory path involving FaCMB1-FaASR-FaCYP707A4-ABA. During the fruit ripening process, an increase in ABA content led to a decrease in FaCMB1 transcript and protein levels, which, combined with increased phosphorylation levels, collectively impaired the transcriptional repression of FaASR by FaCMB1. Meanwhile, the increased transcriptional level of FaASR further repressed the expression level of FaCYP707A4, leading to ABA accumulation and fruit ripening.

Whole-genome identification of HSF family genes in Cerasus humilis and expression analysis under high-temperature stress

The heat shock factors (HSFs) play important roles in activating heat stress responses in plants. Cerasus humilis (Ch) is a nutrient-rich fruit tree that can resist various abiotic and biotic stressors. However, the HSFs in C. humilis have not yet been characterized and their roles remain unclear. In this study, 21 ChHSF gene members were identified after searching the entire genome of C. humilis. Gene structure and motif composition analysis revealed that 16 ChHSF genes had only one intron and the motif3 was highly conserved in family of ChHSFs. Furthermore, the cis-acting elements analysis indicated that they most ChHSFs participate in plant growth and development, abiotic stress responses, and plant hormone regulations. By analyzing the tissue specific transcriptomes, it was found that most ChHSF genes had higher expression levels in leaves than in other tissues of C.humilis. Notably, the ChHSF04 gene exhibited a striking 115.5-, 14.4-, and 16.0-fold higher expression in leaves relative to seeds, roots, and fruits, respectively. The high temperature (40 °C) treated C. humilis seedlings quantitative real-time polymerase chain reaction (qRT-PCR) was conducted on all ChHSF gene members. The results show that the expression of most ChHSF genes in the leaves was significantly upregulated and peaked at 12 h under the heat stress and the expression levels of ChHSF04, ChHSF05, ChHSF12, ChHSF13, ChHSF15 and ChHSF16 exhibited 53-, 33-, 24-, 22-, 43- and 65-fold upregulation, indicating that these genes may play important roles in early response to heat stress in C. humilis. These results provide valuable insights into the evolutionary relationship of the ChHSF gene family and its role in high temperature stress responses.

Mutagenesis of AcSQBP9 in kiwifruit results in reduction of malate via alteration of the expression of a plastidial malate dehydrogenase

Organic acids are major contributors to the flavor of fleshy fruits. In kiwifruit, the Al-ACTIVATED MALATE TRANSPORTER gene (AcALMT1) is key to the accumulation of citrate, while factors driving malate metabolism remain largely unknown. During kiwifruit (Actinidia chinensis cv "Hongyang") development, a rapid decline of malate content was observed between 6 and 12 weeks after full bloom (WAFB), which was studied using RNA-seq analysis. Co-expression network analysis indicated that expression of the chloroplast localized AcPNAD-MDH1 (Plastid-Localized NAD-Dependent Malate Dehydrogenase) negatively correlated with malate content. Overexpression of AcPNAD-MDH1 in kiwifruit resulted lower malate and citrate content in leaves. Among 15 transcription factors that are highly correlated with the expression of AcPNAD-MDH1, AcSQBP9 (SQUAMOSA PROMOTER-BINDING PROTEIN) was shown to directly bind the promoter of AcPNAD-MDH1 to repress transcriptional activity. Moreover, targeted CRISPR-Cas9-induced mutagenesis of AcSQBP9 in kiwifruit produced a significant decrease in malate and citrate, accompanied by an increase in AcPNAD-MDH1 expression. Both PNAD-MDH and SQBP have not been widely studied in fruit metabolism, so the present omics-oriented study provides insights for both kiwifruit and general plant organic acid metabolism.

CitGATA7 interact with histone acetyltransferase CitHAG28 to promote citric acid degradation by regulating the glutamine synthetase pathway in citrus

Organic acid is a crucial indicator of fruit quality traits. Citric acid, the predominant organic acid in citrus fruit, directly influences its edible quality and economic value. While the transcriptional regulatory mechanisms of citric acid metabolism have been extensively studied, the understanding about the transcriptional and epigenetic co-regulation mechanisms is limited. This study characterized a transcription factor, CitGATA7, which directly binds to and activates the expression of genes associated with the glutamine synthetase pathway regulating citric acid degradation. These genes include the aconitase encoding gene CitACO3, the isocitrate dehydrogenase encoding gene CitIDH1, and the glutamine synthetase encoding gene CitGS1. Furthermore, CitGATA7 physically interacts with the histone acetyltransferase CitHAG28 to enhance histone 3 acetylation levels near the transcription start site of CitACO3, CitIDH1, and CitGS1, thereby increasing their transcription and promoting citric acid degradation. The findings demonstrate that the CitGATA7-CitHAG28 protein complex transcriptionally regulate the expression of the GS pathway genes, i.e., CitACO3, CitIDH1, and CitGS1, via histone acetylation, thus promoting citric acid catabolism. This study establishes a direct link between transcriptional regulation and histone acetylation regarding citric acid metabolism, providing insights for strategies to manipulate organic acid accumulation in fruit.

Comparative Transcriptome Analysis Reveals Potential Molecular Regulation of Organic Acid Metabolism During Fruit Development in Late-Maturing Hybrid Citrus Varieties

Late-maturing hybrid citrus is a significant fruit that combines the best traits of both parents and is highly prized for its unique flavor. Not only can organic acids alter the flavor of citrus pulp, but they are also essential for cellular metabolism, energy conversion, and maintaining the acidbase balance in plant tissues. Although organic acids play a key role in the quality formation of citrus fruits, there is still insufficient research on the metabolic processes of organic acids in late-maturing hybrid citrus varieties. In this study, three late-maturing citrus varieties with different acidity levels, namely 'Huangjinjia' (HJ), 'Kiyomi' (QJ), and 'Harumi' (CJ), were selected to systematically investigate the metabolic regulation mechanism of organic acids in late-maturing citrus through transcriptome sequencing technology, combined with physiological and biochemical analyses. This study revealed gene expression differences related to organic acid synthesis and degradation. Through gene expression profiling, several genes closely associated with organic acid metabolism were identified, and a preliminary gene network related to the regulation of organic acid metabolism was constructed. The results showed that there were significant differences in the organic acid metabolic pathways between different varieties and growth stages of the fruit. Specifically, HJ had a higher TA content than QJ and CJ, primarily due to the significantly higher citric acid and malic acid contents in HJ compared to the other two varieties. Further analysis revealed that four gene modules showed a high correlation with the levels of major organic acids in the fruits. The genes involved in these modules are closely related to organic acid synthesis, degradation, and transport. Additionally, we also identified several key genes (AS1, BZP44, COL4, TCP4, IDD10, YAB2, and GAIPB) that might be involved in the regulation of organic acid metabolism. The functions of these genes could have a significant impact on the expression levels changes of enzymes related to organic acid metabolism. This study provides a foundation for exploring the intrinsic mechanisms regulating the organic acid content in late-maturing hybrid citrus fruits and contributes to the functional research of organic acids in late-maturing hybrid citrus and the molecular design of high-quality varieties.

AUXIN RESPONSE FACTOR 2 mediates repression of strawberry receptacle ripening via auxin-ABA interplay

Cultivated strawberry (Fragaria × ananassa) is a popular, economically important fruit. The ripening of the receptacle (pseudocarp), the main edible part, depends on endogenously produced abscisic acid (ABA) and is suppressed by the high level of auxin produced from achenes (true fruit) during early development. However, the mechanism whereby auxin regulates receptacle ripening through inhibiting ABA biosynthesis remains unclear. Here, we identified AUXIN RESPONSE FACTOR 2 (FaARF2), which showed decreased expression with reduced auxin content in the receptacle, leading to increased ABA levels and accelerated ripening. Dual-luciferase, yeast one-hybrid, and electrophoretic mobility shift assays demonstrated that FaARF2 could bind to the AuxRE element in the promoter of 9-CIS-EPOXYCAROT-ENOID DIOXYGENASE 1 (FaNCED1), a key ABA biosynthetic gene, to suppress its transcriptional activity. Transiently overexpressing FaARF2 in the receptacles decreased FaNCED1 expression and ABA levels, resulting in inhibition of receptacle ripening and of development of quality attributes, such as pigmentation, aroma, and sweetness. This inhibition caused by overexpressing FaARF2 was partially recovered by the injection of exogenous ABA; conversely, transient silencing of FaARF2 using RNA interference produced the opposite results. The negative targeting of FaNCED1 by FaARF2 is a key link between auxin-ABA interactions and regulation of strawberry ripening.

The LpHsfA2-molecular module confers thermotolerance via fine tuning of its transcription in perennial ryegrass (Lolium perenne L.)

Temperature sensitivity and tolerance play a key role in plant survival and production. Perennial ryegrass (Lolium perenne L.), widely cultivated in cool-season for forage supply and turfgrass, is extremely susceptible to high temperatures, therefore serving as an excellent grass for dissecting the genomic and genetic basis of high-temperature adaptation. In this study, expression analysis revealed that LpHsfA2, an important gene associated with high-temperature tolerance in perennial ryegrass, is rapidly and substantially induced under heat stress. Additionally, heat-tolerant varieties consistently display elevated expression levels of LpHsfA2 compared with heat-sensitive ones. Comparative haplotype analysis of the LpHsfA2 promoter indicated an uneven distribution of two haplotypes (HsfA2Hap1 and HsfA2Hap2) across varieties with differing heat tolerance. Specifically, the HsfA2Hap1 allele is predominantly present in heat-tolerant varieties, while the HsfA2Hap2 allele exhibits the opposite pattern. Overexpression of LpHsfA2 confers enhanced thermotolerance, whereas silencing of LpHsfA2 compromises heat tolerance. Furthermore, LpHsfA2 orchestrates its protective effects by directly binding to the promoters of LpHSP18.2 and LpAPX1 to activate their expression, preventing the non-specific misfolding of intracellular protein and the accumulation of reactive oxygen species in cells. Additionally, LpHsfA4 and LpHsfA5 were shown to engage directly with the promoter of LpHsfA2, upregulating its expression as well as the expression of LpHSP18.2 and LpAPX1, thus contributing to enhanced heat tolerance. Markedly, LpHsfA2 possesses autoregulatory ability by directly binding to its own promoter to modulate the self-transcription. Based on these findings, we propose a model for modulating the thermotolerance of perennial ryegrass by precisely regulating the expression of LpHsfA2. Collectively, these findings provide a scientific basis for the development of thermotolerant perennial ryegrass cultivars.

Vacuolar proteomic analysis reveals tonoplast transporters for accumulation of citric acid and sugar in citrus fruit

Vacuole largely dictates the fruit taste and flavor, as most of the sugars and organic acids are stored in the vacuoles of the fruit. However, difficulties associated with vacuole separation severely hinder identification and characterization of vacuolar proteins in fruit species. In this study, we established an effective approach for separating vacuoles and successfully purified vacuolar protein from six types of citrus fruit with varying patterns of sugar and organic acid contents. By using label-free LC-MS/MS proteomic analysis, 1443 core proteins were found to be associated with the essential functions of vacuole in citrus fruit. Correlation analysis of metabolite concentration with proteomic data revealed a transporter system for the accumulation of organic acid and soluble sugars in citrus. Furthermore, we characterized the physiological roles of selected key tonoplast transporters, ABCG15, Dict2.1, TMT2, and STP7 in the accumulation of citric acid and sugars. These findings provide a novel perspective and practical solution for investigating the transporters underlying the formation of citrus taste and flavor.

The transcription factor CitZAT5 modifies sugar accumulation and hexose proportion in citrus fruit

Sugars are fundamental to plant developmental processes. For fruits, the accumulation and proportion of sugars play crucial roles in the development of quality and attractiveness. In citrus (Citrus reticulata Blanco.), we found that the difference in sweetness between mature fruits of "Gongchuan" and its bud sport "Youliang" is related to hexose contents. Expression of a SuS (sucrose synthase) gene CitSUS5 and a SWEET (sugars will eventually be exported transporter) gene CitSWEET6, characterized by transcriptome analysis at different developmental stages of these 2 varieties, revealed higher expression levels in "Youliang" fruit. The roles of CitSUS5 and CitSWEET6 were investigated by enzyme activity and transient assays. CitSUS5 promoted the cleavage of sucrose to hexoses, and CitSWEET6 was identified as a fructose transporter. Further investigation identified the transcription factor CitZAT5 (ZINC FINGER OF ARABIDOPSIS THALIANA) that contributes to sucrose metabolism and fructose transportation by positively regulating CitSUS5 and CitSWEET6. The role of CitZAT5 in fruit sugar accumulation and hexose proportion was investigated by homologous transient CitZAT5 overexpression, -VIGS, and -RNAi. CitZAT5 modulates the hexose proportion in citrus by mediating CitSUS5 and CitSWEET6 expression, and the molecular mechanism explained the differences in sugar composition of "Youliang" and "Gongchuan" fruit.

Transcriptome sequencing analyses uncover mechanisms of citrus rootstock seedlings under waterlogging stress

Citrus plants are sensitive to waterlogging, which can cause yield reduction. Their production heavily depends on the rootstock being used for grafting of scion cultivars, and the rootstock is the first organ to be affected by waterlogging stress. However, the underlying molecular mechanisms of waterlogging stress tolerance remain elusive. In this study we investigated the stress response of two waterlogging-tolerant citrus varieties (Citrus junos Sieb ex Tanaka cv. Pujiang Xiangcheng and Ziyang Xiangcheng), and one waterlogging-sensitive variety (red tangerine) at the morphological, physiological, and genetic levels in leaf and root tissues of partially submerged plants. The results showed that waterlogging stress significantly decreased the SPAD value and root length but did not obviously affect the stem length and new root numbers. The malondialdehyde (MDA) content and the enzyme activities of superoxide dismutase (SOD), guaiacol peroxidase (POD), and catalase (CAT) were enhanced in the roots. The RNA-seq analysis revealed that the differentially expressed genes (DEGs) were mainly linked to 'cutin, suberine, and wax biosynthesis', 'diterpenoid biosynthesis', and 'glycerophospholipid metabolism' in the leaves, whereas were linked to 'flavonoid biosynthesis', 'biosynthesis of secondary metabolites and metabolic pathways' in the roots. Finally, we developed a working model based on our results to elucidate the molecular basis of waterlogging-responsive in citrus. Therefore, our data obtained in this study provided valuable genetic resources that will facilitate the breeding of citrus varieties with improved waterlogging tolerance.

Antioxidant response and quality of sunburn Beurré D'Anjou pears (Pyrus communis L.)

Sunburn is a physiological fruit disorder induced by exposure to excessive solar radiation. This disorder leads to significant losses in the yield of marketable fruits by negatively affecting quality parameters such as maturity and external color of the fruits. The purpose of this work was to characterize the physiological and biochemical aspects related to oxidative metabolism in Beurré D'Anjou pear fruit with different sunburn levels. Fruits were collected and classified into three sunburn levels at harvest: no sunburn (S0), mild sunburn (S1), and moderate sunburn (S2). On sunburned area, the maturity indices were measured on the fruit flesh, while external color, photosynthetic and photoprotective pigments, total phenols, electrolyte leakage, lipid peroxidation, antioxidant capacity and antioxidant enzymatic activities were determined on fruit peel. The hue angle and saturation of peel color of pears with different sunburn levels showed significant reduction with increasing damage. These changes in peel color were associated with a reduction in chlorophyll content and variations in carotenoid and anthocyanin levels. Due to metabolic changes resulting from defense and adaptive responses to high solar radiation, sunburned tissues showed significantly increased firmness, soluble solids content, and starch degradation, and lower acidity compared to undamaged fruits. We observed also increased antioxidant capacity in the peel of S1 and S2 fruit, related to higher phenolic contents and increased SOD and APX activities. Consistent with previous reports in apple, our study demonstrates that sunburn affects pear fruit quality traits and maturity state by enhancing oxidative metabolism.

Abscisic acid mediated strawberry receptacle ripening involves the interplay of multiple phytohormone signaling networks

As a canonical non-climacteric fruit, strawberry (Fragaria spp.) ripening is mainly mediated by abscisic acid (ABA), which involves multiple other phytohormone signalings. Many details of these complex associations are not well understood. We present an coexpression network, involving ABA and other phytohormone signalings, based on weighted gene coexpression network analysis of spatiotemporally resolved transcriptome data and phenotypic changes of strawberry receptacles during development and following various treatments. This coexpression network consists of 18,998 transcripts and includes transcripts related to phytohormone signaling pathways, MADS and NAC family transcription factors and biosynthetic pathways associated with fruit quality. Members of eight phytohormone signaling pathways are predicted to participate in ripening and fruit quality attributes mediated by ABA, of which 43 transcripts were screened to consist of the hub phytohormone signalings. In addition to using several genes reported from previous studies to verify the reliability and accuracy of this network, we explored the role of two hub signalings, small auxin up-regulated RNA 1 and 2 in receptacle ripening mediated by ABA, which are also predicted to contribute to fruit quality. These results and publicly accessible datasets provide a valuable resource to elucidate ripening and quality formation mediated by ABA and involves multiple other phytohormone signalings in strawberry receptacle and serve as a model for other non-climacteric fruits.

MdMYB110a, directly and indirectly, activates the structural genes for the ALA-induced accumulation of anthocyanin in apple

5-Aminolevulinic acid (ALA), an essential biosynthetic precursor of tetrapyrrole compounds, promotes the anthocyanin accumulation in many plant species. However, the underlying mechanism of ALA-induced accumulation is not yet fully understood. In this study, we identified an important regulator of the anthocyanin accumulation, MdMYB110a, which plays an important role in the ALA-induced anthocyanin accumulation. MdMYB110a activated the expression of MdGSTF12 by binding to its promoter. Additionally, two interacting MdMYB110a proteins, MdWD40-280 and MdHsfB3a, were isolated and confirmed as positive regulators of the ALA-induced anthocyanin accumulation. Both MdWD40-280 and MdHsfB3a enhanced the ability of MdMYB110a to transcribe MdGSTF12. A yeast one-hybrid assay revealed that MdWD40-280 did not bind to most structural genes in the anthocyanin biosynthetic and transport pathways, thus promoting anthocyanin accumulation by MdWD40-280 to depend on MdMYB110a. However, MdHsfB3a could bind to both the MdDFR and MdANS promoters, thereby directly regulating anthocyanin biosynthesis. Collectively, these results provide new insight into the mechanism of ALA-induced anthocyanin accumulation.

ERF49 mediates brassinosteroid regulation of heat stress tolerance in Arabidopsis thaliana

Background

Heat stress is a major abiotic stress affecting the growth and development of plants, including crop species. Plants have evolved various adaptive strategies to help them survive heat stress, including maintaining membrane stability, encoding heat shock proteins (HSPs) and ROS-scavenging enzymes, and inducing molecular chaperone signaling. Brassinosteroids (BRs) are phytohormones that regulate various aspects of plant development, which have been implicated also in plant responses to heat stress, and resistance to heat in Arabidopsis thaliana is enhanced by adding exogenous BR. Brassinazole resistant 1 (BZR1), a transcription factor and positive regulator of BR signal, controls plant growth and development by directly regulating downstream target genes. However, the molecular mechanism at the basis of BR-mediated heat stress response is poorly understood. Here, we report the identification of a new factor critical for BR-regulated heat stress tolerance.

Results

We identified ERF49 in a genetic screen for proteins required for BR-regulated gene expression. We found that ERF49 is the direct target gene of BZR1 and that overexpressing ERF49 enhanced sensitivity of transgenic plants to heat stress. The transcription levels of heat shock factor HSFA2, heat stress-inducible gene DREB2A, and three heat shock protein (HSP) were significantly reduced under heat stress in ERF49-overexpressed transgenic plants. Transcriptional activity analysis in protoplast revealed that BZR1 inhibits ERF49 expression by binding to the promoter of ERF49. Our genetic analysis showed that dominant gain-of-function brassinazole resistant 1-1D mutant (bzr1-1D) exhibited lower sensitivity to heat stress compared with wild-type. Expressing ERF49-SRDX (a dominant repressor reporter of ERF49) in bzr1-1D significantly decreased the sensitivity of ERF49-SRDX/bzr1-1D transgenic plants to heat stress compared to bzr1-1D.

Conclusions

Our data provide clear evidence that BR increases thermotolerance of plants by repressing the expression of ERF49 through BZR1, and this process is dependent on the expression of downstream heat stress-inducible genes. Taken together, our work reveals a novel molecular mechanism mediating plant response to high temperature stress.

The Effect of Short-Term Temperature Pretreatments on Sugars, Organic Acids, and Amino Acids Metabolism in Valencia Orange Fruit

Temperature pretreatment is one of the most important factors which significantly affects the postharvest quality of citrus fruit. In this study, late-ripening Valencia orange (citrus sinensis) fruits were used to investigate the effect of short-term treatment at low (6°C), room (20°C), and high (40°C) temperatures on fruit quality. Our results revealed that both low and room-temperature treatments maintained the content of sugars and organic acids, whereas high-temperature treatments elevated the accumulation of sugars but decreased the content of citric acid. In fruit peel (flavedo and albedo), the accumulation of sugars and organic acids responding to temperatures was diverse and mostly different from that in the pulp. Meanwhile, GABA and several amino acids were upregulated under short-term high-temperature treatment but downregulated in response to low-temperature treatment in both peel and pulp. Furthermore, PCA and correlation analysis revealed that the short-term temperature treatments changed the metabolic flow, and GABA was positively correlated with sugars and organic acids. Our study analyzed the metabolic changes of fruit peel and pulp in response to short-term temperature treatments and revealed that GABA may act as a signaling molecular involved in temperature-controlled quality changes.

Genome-wide identification of heat shock transcription factor families in perennial ryegrass highlights the role of LpHSFC2b in heat stress response

Perennial ryegrass (Lolium perenne) is a cool-season turf and forage grass. Heat shock transcription factors (HSFs) play an important role in regulating plant abiotic stress. However, HSFs in perennial ryegrass have rarely been characterized. Here, 25 LpHSFs were identified from the perennial ryegrass genome. Phylogenetic analysis showed that the LpHSFs could be classified into 12 subclasses. Gene structure analysis showed that 22 LpHSFs have only one intron. Cis-acting elements analysis revealed that the promoter of 15 LpHSFs contained hormone-responsive and abiotic stress-responsive elements. Expression profile analysis indicated that 24 LpHSFs were differentially expressed under submerge, drought, heat, and cold stresses. In addition, a subclass C2 gene, LpHSFC2b, was significantly induced by abiotic stresses. The LpHSFC2b protein is localized to the nucleus, and heterologous expression of LpHSFC2b in Arabidopsis improves plant thermotolerance. This study provides insights useful for the breeding of stress tolerance in perennial ryegrass.

Functional Characterization of Heat Shock Factor (CrHsf) Families Provide Comprehensive Insight into the Adaptive Mechanisms of Canavalia rosea (Sw.) DC. to Tropical Coral Islands

Heat shock transcription factors (Hsfs) are key regulators in plant heat stress response, and therefore, they play vital roles in signal transduction pathways in response to environmental stresses, as well as in plant growth and development. Canavalia rosea (Sw.) DC. is an extremophile halophyte with good adaptability to high temperature and salt-drought tolerance, and it can be used as a pioneer species for ecological reconstruction on tropical coral islands. To date, very little is known regarding the functions of Hsfs in the adaptation mechanisms of plant species with specialized habitats, especially in tropical leguminous halophytes. In this study, a genome-wide analysis was performed to identify all the Hsfs in C. rosea based on whole-genome sequencing information. The chromosomal location, protein domain or motif organization, and phylogenetic relationships of 28 CrHsfs were analyzed. Promoter analyses indicated that the expression levels of different CrHsfs were precisely regulated. The expression patterns also revealed clear transcriptional changes among different C. rosea tissues, indicating that the regulation of CrHsf expression varied among organs in a developmental or tissue-specific manner. Furthermore, the expression levels of most CrHsfs in response to environmental conditions or abiotic stresses also implied a possible positive regulatory role of this gene family under abiotic stresses, and suggested roles in adaptation to specialized habitats such as tropical coral islands. In addition, some CrHsfAs were cloned and their possible roles in abiotic stress tolerance were functionally characterized using a yeast expression system. The CrHsfAs significantly enhanced yeast survival under thermal and oxidative stress challenges. Our results contribute to a better understanding of the plant Hsf gene family and provide a basis for further study of CrHsf functions in environmental thermotolerance. Our results also provide valuable information on the evolutionary relationships among CrHsf genes and the functional characteristics of the gene family. These findings are beneficial for further research on the natural ecological adaptability of C. rosea to tropical environments.

R2R3-MYB transcription factor FaMYB5 is involved in citric acid metabolism in strawberry fruits

The citrate content of strawberry fruits affects their organoleptic quality. However, little is known about the transcriptional regulatory mechanisms of citric acid metabolism in strawberry fruits. In this study, the R2R3-MYB transcription factor FaMYB5 was identified and placed in the R2R3-MYB subfamily. FaMYB5 is found in the nucleus and shows tissue- and stage-specific expression levels. Citric acid content was positively correlated with FaMYB5 transcript levels. Upregulated FaMYB5 increased citric acid accumulation in transient FaMYB5-overexpressing strawberry fruits, whereas transient RNA silencing of FaMYB5 in strawberry fruits resulted in a reduction of citric acid content. The role of FaMYB5 was verified using stable transgenic NC89 tobacco. Furthermore, a yeast one-hybrid assay revealed that FaMYB5 influences citric acid accumulation by binding to the FaACO (aconitase), FaGAD (glutamate decarboxylase), and FaCS2 (citrate synthase) promoters. Dual-luciferase assays were used to demonstrate that FaMYB5 could activate FaCS2 expression and repress the transcription levels of FaACO and FaGAD. This study identified important roles of FaMYB5 in the regulation of citric acid metabolism and provided a potential target for improving strawberry fruit taste in horticultural crops.

The Interaction Between CitMYB52 and CitbHLH2 Negatively Regulates Citrate Accumulation by Activating CitALMT in Citrus Fruit

Citric acid plays significant roles in numerous physiological processes in plants, including carbon metabolism, signal transduction, and tolerance to environmental stress. For fruits, it has a major effect on fruit organoleptic quality by directly influencing consumer taste. Citric acid in citrus is mainly regulated by the balance between synthesis, degradation, and vacuolar storage. The genetic and molecular regulations of citric acid synthesis and degradation have been comprehensively elucidated. However, the transporters for citric acid in fruits are less well understood. Here, an aluminum-activated malate transporter, CitALMT, was characterized. Transient overexpression and stable transformation of CitALMT significantly reduced citrate concentration in citrus fruits and transgenic callus. Correspondingly, transient RNA interference-induced silencing of CitALMT and increased citrate significantly, indicating that CitALMT plays an important role in regulating citrate concentration in citrus fruits. In addition, dual-luciferase assays indicated that CitMYB52 and CitbHLH2 could trans-activate the promoter of CitALMT. EMSA analysis showed that CitbHLH2 could physically interact with the E-box motif in the CitALMT promoter. Bimolecular fluorescence complementation assays, yeast two-hybrid, coimmunoprecipitation and transient overexpression, and RNAi assay indicated that the interaction between CitMYB52 and CitbHLH2 could synergistically trans-activate CitALMT to negatively regulate citrate accumulation.

Genome-Wide Identification of Petunia HSF Genes and Potential Function of PhHSF19 in Benzenoid/Phenylpropanoid Biosynthesis

Volatile benzenoids/phenylpropanoids are the main flower scent compounds in petunia (Petunia hybrida). Heat shock factors (HSFs), well known as the main regulator of heat stress response, have been found to be involved in the biosynthesis of benzenoid/phenylpropanoid and other secondary metabolites. In order to figure out the potential function of HSFs in the regulation of floral scent in petunia, we systematically identified the genome-wide petunia HSF genes and analyzed their expression and then the interaction between the key petunia HSF gene with target gene involved in benzenoid/phenylpropanoid biosynthesis. The results revealed that 34 HSF gene family members were obtained in petunia, and most petunia HSFs contained one intron. The phylogenetic analysis showed that 23 petunia HSFs were grouped into the largest subfamily HSFA, while only two petunia HSFs were in HSFC subfamily. The DBD domain and NLS motif were well conserved in most petunia HSFs. Most petunia HSF genes’ promoters contained STRE motifs, the highest number of cis-acting element. PhHSF19 is highly expressed in petal tubes, followed by peduncles and petal limbs. During flower development, the expression level of PhHSF19 was dramatically higher at earlier flower opening stages than that at the bud stage, suggesting that PhHSF19 may have potential roles in regulating benzenoid/phenylpropanoid biosynthesis. The expression pattern of PhHSF19 is positively related with PhPAL2, which catalyzes the first committed step in the phenylpropanoid pathway. In addition, there are three STRE elements in the promoter of PhPAL2. PhHSF19 was proven to positively regulate the expression of PhPAL2 according to the yeast one hybrid and dual luciferase assays. These results lay a theoretical foundation for further studies of the regulation of HSFs on plant flower scent biosynthesis.