Analysis of the sucrose synthase gene family in tobacco: structure, phylogeny, and expression patterns

Structure and Expression Analysis of PtrSUS, PtrINV, PtrHXK, PtrPGM, and PtrUGP Gene Families in Populus trichocarpa Torr. and Gray

Exogenous nitrogen and carbon can affect plant cell walls, which are composed of structural carbon. Sucrose synthase (SUS), invertase (INV), hexokinase (HXK), phosphoglucomutase (PGM), and UDP-glucose pyrophosphorylase (UGP) are the key enzymes of sucrose metabolism involved in cell wall synthesis. To understand whether these genes are regulated by carbon and nitrogen to participate in structural carbon biosynthesis, we performed genome-wide identification, analyzed their expression patterns under different carbon and nitrogen treatments, and conducted preliminary functional verification. Different concentrations of nitrogen and carbon were applied to poplar (Populus trichocarpa Torr. and Gray), which caused changes in cellulose, lignin, and hemicellulose contents. In poplar, 6 SUSs, 20 INVs, 6 HXKs, 4 PGMs, and 2 UGPs were identified. Moreover, the physicochemical properties, collinearity, and tissue specificity were analyzed. The correlation analysis showed that the expression levels of PtrSUS3/5, PtrNINV1/2/3/5/12, PtrCWINV3, PtrVINV2, PtrHXK5/6, PtrPGM1/2, and PtrUGP1 were positively correlated with the cellulose content. Meanwhile, the knockout of PtrNINV12 significantly reduced the cellulose content. This study could lay the foundation for revealing the functions of SUSs, INVs, HXKs, PGMs, and UGPs, which affected structural carbon synthesis regulated by nitrogen and carbon, proving that PtrNINV12 is involved in cell wall synthesis.

MYB6/bHLH13-AbSUS2 involved in sugar metabolism regulates root hair initiation of Abies beshanzuensis

Root hair is regarded as a pivotal complementary survival tactic for mycorrhizal plant like Abies beshanzuensis when symbiosis is disrupted. Relatively little is known about the mechanism underlying root hair morphogenesis in plant species that are strongly dependent on mycorrhizal symbiosis. Many of these species are endangered, and this knowledge is critical for ensuring their survival. Here, a MYB6/bHLH13-sucrose synthase 2 (AbSUS2) module was newly identified and characterized in A. beshanzuensis using bioinformatics, histochemistry, molecular biology, and transgenesis. Functional, expression pattern, and localization analysis showed that AbSUS2 participated in sucrose synthesis and was involved in root hair initiation in A. beshanzuensis. Additionally, the major enzymatic product of AbSUS2 was found to suppress root hair initiation in vitro. Our data further showed that a complex involving the transcription factors AbMYB6 and AbbHLH13 directly interacted with the promoter of AbSUS2 and strengthened its expression, thereby inhibiting root hair initiation in response to exogenous sucrose. Our findings offer novel insights into how root hair morphogenesis is regulated in mycorrhizal plants and also provide a new strategy for the preservation of endangered mycorrhizal plant species.

Exogenously applied zinc improves sugar-acid profile of loquat (Eriobotrya japonica Lindl.) by regulating enzymatic activities and expression of their metabolism-related genes

Soluble sugars and organic acids are the most abundant components in ripe fruits, and they play critical roles in the development of fruit flavor and taste. In this study, loquat trees were sprayed with 0.1, 0.2 and 0.3% zinc sulphate. The contents of soluble sugars and organic acids were determined using HPLC-RID and UPLC-MS, respectively. The activities of key enzymes involved in sugar-acid metabolism were measured and expression profiling of related genes was done using RT-qPCR. The results revealed that 0.1% zinc sulphate was a promising treatment among other Zn applications with respect to the increased levels of soluble sugars and decreased acid contents in loquats. Correlation analysis showed that the enzymes i.e., SPS, SS, FK, and HK were may be involved in the regulation of fructose and glucose metabolism in the fruit pulp of loquat. While, the activity of NADP-ME showed negative and NAD-MDH showed a positive correlation with malic acid content. Meanwhile, EjSPS1-4, EjSS2-4, EjHK1-3, and EjFK1-6 may play an important role in soluble sugar metabolism in the pulp of loquat fruits. Similarly, EjPEPC2, EjPEPC3, EjNAD-MDH1, EjNAD-MDH3-5, EjNAD-MDH6 and EjNAD-MDH13 may have a vital contribution to malic acid biosynthesis in loquat fruits. This study provides new insights for future elucidation of key mechanisms regulating soluble sugars and malic acid biosynthesis in loquats.

Genome-wide identification and analysis of the invertase gene family in tobacco (Nicotiana tabacum) reveals NtNINV10 participating the sugar metabolism

Sucrose (Suc) is directly associated with plant growth and development as well as tolerance to various stresses. Invertase (INV) enzymes played important role in sucrose metabolism by irreversibly catalyzing Suc degradation. However, genome-wide identification and function of individual members of the INV gene family in Nicotiana tabacum have not been conducted. In this report, 36 non-redundant NtINV family members were identified in Nicotiana tabacum including 20 alkaline/neutral INV genes (NtNINV1-20), 4 vacuolar INV genes (NtVINV1-4), and 12 cell wall INV isoforms (NtCWINV1-12). A comprehensive analysis based on the biochemical characteristics, the exon-intron structures, the chromosomal location and the evolutionary analysis revealed the conservation and the divergence of NtINVs. For the evolution of the NtINV gene, fragment duplication and purification selection were major factors. Besides, our analysis revealed that NtINV could be regulated by miRNAs and cis-regulatory elements of transcription factors associated with multiple stress responses. In addition, 3D structure analysis has provided evidence for the differentiation between the NINV and VINV. The expression patterns in diverse tissues and under various stresses were investigated, and qRT-PCR experiments were conducted to confirm the expression patterns. Results revealed that changes in NtNINV10 expression level were induced by leaf development, drought and salinity stresses. Further examination revealed that the NtNINV10-GFP fusion protein was located in the cell membrane. Furthermore, inhibition of the expression of NtNINV10 gene decreased the glucose and fructose in tobacco leaves. Overall, we have identified possible NtINV genes functioned in leaf development and tolerance to environmental stresses in tobacco. These findings provide a better understanding of the NtINV gene family and establish the basis for future research.

VaSUS2 confers cold tolerance in transgenic tomato and Arabidopsis by regulation of sucrose metabolism and ROS homeostasis

VaSUS2 enhances cold tolerance of transgenic tomato and Arabidopsis by regulating sucrose metabolism and improving antioxidant enzymes activity. Sucrose synthetase (SUS) is a key enzyme of sugar metabolism, and plays an important role in response to abiotic stress in plant. However, the function of VaSUS2 remains unknown in cold tolerance. Here, the cloning and functional characterization of the plasma membrane-localized VaSUS2 gene isolated from Vitis amurensis was studied. The transcript level of VaSUS2 was up-regulated under cold stress in Vitis amurensis. Heterologous expression of VaSUS2 in tomato increased SUS activity, which promoted the accumulation of glucose and fructose under cold treatment. The transgenic tomato and Arabidopsis exhibited higher levels of antioxidant enzymes activity, lower relative electrolyte leakage (REL), malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) content compared to wild type under cold stress. Importantly, the ability of scavenging reactive oxygen species (ROS) in transgenic plants was significantly improved. Moreover, yeast two-hybrid (Y2H) indicated that VaSnRK1 might be a potential interaction protein of VaSUS2. qRT-PCR showed that sucrose metabolism-related genes SlSUS, SlSPS and SlINV were significantly up-regulated in transgenic tomatoes. Meanwhile, the expression levels of antioxidant enzyme genes and cold-related genes CBF1, COR47 and ICE1 were up-regulated in transgenic plants. Taken together, these results suggested that VaSUS2 was involved in cold tolerance by increasing the levels of soluble sugars, improving the activity of antioxidant enzymes, and up-regulating the expression of cold-related genes in transgenic tomatoes and Arabidopsis.

Dynamics of the sucrose metabolism and related gene expression in tomato fruits under water deficit

The impact of water deficit on sucrose metabolism in sink organs like the fruit remains poorly known despite the need to improve fruit crops resilience to drought in the face of climate change. The present study investigated the effects of water deficit on sucrose metabolism and related gene expression in tomato fruits, aiming to identify candidate genes for improving fruit quality upon low water availability. Tomato plants were subjected to irrigated control and water deficit (-60% water supply compared to control) treatments, which were applied from the first fruit set to first fruit maturity stages. The results have shown that water deficit significantly reduced fruit dry biomass and number, among other plant physiological and growth variables, but substantially increased the total soluble solids content. The determination of soluble sugars on the basis of fruit dry weight revealed an active accumulation of sucrose and concomitant reduction in glucose and fructose levels in response to water deficit. The complete repertoire of genes encoding sucrose synthase (SUSY1-7), sucrose-phosphate synthase (SPS1-4), and cytosolic (CIN1-8), vacuolar (VIN1-2) and cell wall invertases (WIN1-4) was identified and characterized, of which SlSUSY4, SlSPS1, SlCIN3, SlVIN2, and SlCWIN2 were shown to be positively regulated by water deficit. Collectively, these results show that water deficit regulates positively the expression of certain genes from different gene families related to sucrose metabolism in fruits, favoring the active accumulation of sucrose in this organ under water-limiting conditions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01288-7.

Sugar and acid profile of loquat (Eriobotrya japonica Lindl.), enzymes assay and expression profiling of their metabolism-related genes as influenced by exogenously applied boron

Soluble sugars and organic acids are the most abundant components in ripe fruits, and they play critical roles in the development of fruit flavor and taste. Some loquat cultivars have high acid content which seriously affect the quality of fruit and reduce the value of commodity. Consequently, studying the physiological mechanism of sugar-acid metabolism in loquat can clarify the mechanism of their formation, accumulation and degradation in the fruit. Minerals application has been reported as a promising way to improve sugar-acid balance of the fruits. In this study, loquat trees were foliar sprayed with 0.1, 0.2 and 0.3% borax, and changes in soluble sugars and organic acids were recorded. The contents of soluble sugars and organic acids were determined using HPLC-RID and UPLC-MS, respectively. The activities of enzymes responsible for the metabolism of sugars and acids were quantified and expressions of related genes were determined using quantitative real-time PCR. The results revealed that 0.2% borax was a promising treatment among other B applications for the increased levels of soluble sugars and decreased acid contents in loquats. Correlation analysis showed that the enzymes i.e., SPS, SS, FK, and HK were may be involved in the regulation of fructose and glucose metabolism in the fruit pulp of loquat. While the activity of NADP-ME showed negative and NAD-MDH showed a positive correlation with malic acid content. Meanwhile, EjSPS1, EjSPS3, EjSS3, EjHK1, EjHK3, EjFK1, EjFK2, EjFK5, and EjFK6 may play an important role in soluble sugars metabolism in fruit pulp of loquat. Similarly, EjPEPC2, EjPEPC3, EjNAD-ME1, EjNAD-MDH1, EjNAD-MDH5-8, EjNAD-MDH10, and EjNAD-MDH13 may have a vital contribution to malic acid biosynthesis in loquat fruits. This study provides new insights for future elucidation of key mechanisms regulating soluble sugars and malic acid biosynthesis in loquats.

Genome-wide identification and expression profiling of sugar transporter genes in tobacco

Sugars are both nutrients and important signal molecules in higher plants. Sugar transporters (STs) are involved in sugar loading and unloading and facilitate sugar transport across membranes. Tobacco (Nicotiana tabacum) is a model plant and one of the most significant plants economically. In our research, 92 N. tabacum ST (NtST) genes were identified and classified into eight distinct subfamilies in the tobacco genome based on phylogenetic analysis. Exon-intron analysis revealed that each subfamily manifested closely associated gene architectural features based on a comparable number or length of exons. Tandem repetition and purifying selection were the main factors of NtST gene evolution. A search for cis-regulatory elements in the promoter sequences of the NtST gene families suggested that they are probably regulated by light, plant hormones, and abiotic stress factors. We performed a comprehensive expression study in different tissues, viarious abiotic and phytohormone stresses. The results revealed different expression patterns and the functional diversification of NtST genes. The resulting data showed that NtSFP1 was highly expressed all measured five tobacco tissues, and also regulated by the MeJA, and temperature stress. In addition, the virus-induced NibenSFP1 silencing in tobacco and detected dramatically enhanced glucose content, indicating the NtSFP1 might regulate the glucose content and involved in MeJA signaling way to response the temperature stress. In general, our findings provide useful information on understanding the roles of STs in phytohormone signaling way and abiotic stresses in N. tabacum.

An overview of sucrose transporter (SUT) genes family in rice

INTRODUCTION

Photosynthesis provides the energy basis for the life activities of plants by producing organic compounds, mainly sugar. As the main energy form of photosynthesis, sugar affects the growth and development of plants. During long-distance transportation, sucrose is the main form of transportation. The rate of sugar transport and the allocation of carbohydrates affect the biomass of crops and are closely related to the reproductive growth of crops.

MAIN TEXT

The transportation of sugar is divided into active transportation and passive transportation. So how does the sucrose transporters (SUT) genes, which are the main carriers of sucrose in active transportation, affect the performance of rice agronomic traits is still to be explored. In this article, we describe the structure of inflorescence and review the transport forms and metabolic processes of sucrose in rice, such as how CO₂ is fixed, carbohydrate assimilation, and transport of organic matter. Sucrose transporters exhibited remarkable effects on the development of reproductive organs in rice.

CONCLUSIONS

Here, the effects of different factors, such as the effects of anthers morphology on starch enrichment of pollen, effects of biotic and abiotic factors on sucrose transporters, effects of changes in trace elements on sucrose transporters, were discussed. Moreover, the regulation of transcription or translation level provides ideas for future research on sucrose transporters.

Genome-Wide Identification and Analysis of the Class III Peroxidase Gene Family in Tobacco (Nicotiana tabacum)

Class III peroxidases (PODs) are plant-specific enzymes that play significant roles in plant physiological processes and stress responses. However, a comprehensive analysis of the POD gene family in tobacco has not yet been conducted. In this study, 210 non-redundant POD gene members (NtPODs) were identified in tobacco (Nicotiana tabacum) and distributed unevenly throughout 24 tobacco chromosomes. Phylogenetic analysis clustered these genes into six subgroups (I-VI). Gene structure and motif analyses showed the structural and functional diversity among the subgroups. Segmental duplication and purifying selection were the main factors affecting NtPOD gene evolution. Our analyses also suggested that NtPODs might be regulated by miRNAs and cis-acting regulatory elements of transcription factors that are involved in various biological processes. In addition, the expression patterns in different tissues and under various stress treatments were investigated. The results showed that the majority of NtPODs had tissue-specific expression patterns and may be involved in many biotic and abiotic responses. qRT-PCR analyses of different tissues and stress treatments were performed to verify transcriptome patterns. Expression of a green fluorescent protein-NtPOD fusion confirmed the plasma membrane localization of NtPOD121 and NtPOD4. Furthermore, 3D structures provided evidences of membrane-bound peroxidase. These findings provide useful information to better understand the evolution of the NtPOD gene family and lay the foundation for further studies on POD gene function in tobacco.

Molecular identification and expression analysis of five sucrose synthase genes in Sorghum Bicolor

In higher plants, sucrose synthase (Susy, EC 2.4.1.13) as an enzyme with a core function, involved in the synthesis and breakdown of sugars, and plays an important role in growth and metabolism. Although, the different genes encoding Susy isozyme proteins have been cloned and functionally verified in several plant species, to date detailed information about the Susy genes is lacking in Sorghum. Here, we demonstrated the identification of five novel Susy genes from the sorghum genome database. Sequence, structure and phylogenetic analyses of these five SbSusy genes revealed evolutionary conservation through Susy gene family members across Sorghum and other crop plants. The expression of sorghum Susy genes was investigated via transcriptome database in various developmental stages and different tissues. Further qRT-PCR was performed to reveal the induction of SbSusy genes under salt, drought and sugar induction. The results indicated that all Susy genes were differentially expressed in various tissues and highly associated with sucrose metabolism. This study shows a theoretical reference of Susy genes in Sorghum, which provides new insights for the knowledge of the evolution relationships, and basic information to help clarify the molecular mechanism of Susy synthase genes in Sorghum.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01166-8.

Elshikh M, Jamil A, Nasir JA, Dvořáčková H, Dvořáček J. Genome-wide analysis of sucrose synthase family in soybean and their expression in response to abiotic stress and seed development

The sucrose synthase (SS) is an important enzyme family which play a vital role in sugar metabolism to improve the fruit quality of the plants. In many plant species, the members of SS family have been investigated but the detailed information is not available in legumes particularly and Glycine max specifically. In the present study, we found thirteen SS members (GmSS1-GmSS13) in G. max genome. High conserved regions were present in the GmSS sequences that may due to the selection pressure during evolutionary events. The segmental duplication was the major factor to increase the number of GmSS family members. The identified thirteen GmSS genes were divided into Class I, Class II and Class III with variable numbers of genes in each class. The protein interaction of GmSS gave the co-expression of sucrose synthase with glucose-1-phosphate adenylyltransferase while SLAC and REL test found number of positive sites in the coding sequences of SS family members. All the GmSS family members except GmSS7 and few of class III members, were highly expressed in all the soybean tissues. The expression of the class I members decreased during seed development, whireas, the class II members expression increased during the seed developing, may involve in sugar metabolism during seed development. Solexa sequencing libraries of acidic condition (pH 4.2) stress samples showed that the expression of class I GmSS genes increased 1- to 2-folds in treated samples than control. The differential expression pattern was observed between the members of a paralogous. This study provides detailed genome-wide analysis of GmSS family in soybean that will provide new insights for future evolutionary and soybean breeding to improve the plant growth and development.

Identification and expression analysis of the sucrose synthase gene family in pomegranate (Punica granatum L.)

BACKGROUND

Sucrose synthase (SUS, EC 2.4.1.13) is one of the major enzymes of sucrose metabolism in higher plants. It has been associated with C allocation, biomass accumulation, and sink strength. The SUS gene families have been broadly explored and characterized in a number of plants. The pomegranate (Punica granatum) genome is known, however, it lacks a comprehensive study on its SUS genes family.

METHODS

PgSUS genes were identified from the pomegranate genome using a genome-wide search method. The PgSUS gene family was comprehensively analyzed by physicochemical properties, evolutionary relationship, gene structure, conserved motifs and domains, protein structure, syntenic relationships, and cis-acting elements using bioinformatics methods. The expression pattern of the PgSUS gene in different organs and fruit development stages were assayed with RNA-seq obtained from the NCBI SRA database as well as real-time quantitative polymerase chain reaction (qPCR).

RESULTS

Five pomegranate SUS genes, located on four different chromosomes, were divided into three subgroupsaccording to the classification of other seven species. The PgSUS family was found to be highly conserved during evolution after studying the gene structure, motifs, and domain analysis. Furthermore, the predicted PgSUS proteins showed similar secondary and tertiary structures. Syntenic analysis demonstrated that four PgSUS genes showed syntenic relationships with four species, with the exception of PgSUS2. Predictive promoter analysis indicated that PgSUS genes may be responsive to light, hormone signaling, and stress stimulation. RNA-seq analysis revealed that PgSUS1/3/4 were highly expressed in sink organs, including the root, flower, and fruit, and particularly in the outer seed coats. qPCR analysis showed also that PgSUS1, PgSUS3, and PgSUS4 were remarkably expressed during fruit seed coat development. Our results provide a systematic overview of the PgSUS gene family in pomegranate, developing the framework for further research and use of functional PgSUS genes.

Transcriptome profiling of Malus sieversii under freezing stress after being cold-acclimated

Background

Freezing temperatures are an abiotic stress that has a serious impact on plant growth and development in temperate regions and even threatens plant survival. The wild apple tree (Malus sieversii) needs to undergo a cold acclimation process to enhance its freezing tolerance in winter. Changes that occur at the molecular level in response to low temperatures are poorly understood in wild apple trees.

Results

Phytohormone and physiology profiles and transcriptome analysis were used to elaborate on the dynamic response mechanism. We determined that JA, IAA, and ABA accumulated in the cold acclimation stage and decreased during freezing stress in response to freezing stress. To elucidate the molecular mechanisms of freezing stress after cold acclimation, we employed single molecular real-time (SMRT) and RNA-seq technologies to study genome-wide expression profiles in wild apple. Using the PacBio and Illumina platform, we obtained 20.79G subreads. These reads were assembled into 61,908 transcripts, and 24,716 differentially expressed transcripts were obtained. Among them, 4410 transcripts were differentially expressed during the whole process of freezing stress, and these were examined for enrichment via GO and KEGG analyses. Pathway analysis indicated that “plant hormone signal transduction”, “starch and sucrose metabolism”, “peroxisome” and “photosynthesis” might play a vital role in wild apple responses to freezing stress. Furthermore, the transcription factors DREB1/CBF, MYC2, WRKY70, WRKY71, MYB4 and MYB88 were strongly induced during the whole stress period.

Conclusions

Our study presents a global survey of the transcriptome profiles of wild apple trees in dynamic response to freezing stress after two days cold acclimation and provides insights into the molecular mechanisms of freezing adaptation of wild apple plants for the first time. The study also provides valuable information for further research on the antifreezing reaction mechanism and genetic improvement of M. sieversii after cold acclimation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07998-0.

Genome-wide identification and expression analysis of the 2OG-Fe(II) oxygenase gene family in upland cotton (Gossypium hirsutum L.)

The 2OG-Fe(II) oxygenase (RF) family of enzyme proteins can affect bulliform cells and cause leaf curling. However, there are few studies related to this family in cotton, and there has been no systematic analysis of RF genes. Here, we determined 25 RF genes in the complete genome sequence of upland cotton (Gossypium hirsutum L.) and 11 RF genes in the complete genome sequence of Arabidopsis thaliana. Cotton RF proteins can be divided into three categories. Whole genome/fragment and scattered replication events played an important role in the expansion of the RF gene family. qRT-PCR analysis results showed that RF genes respond to drought stress Pairwise comparison results showed that the expression of RF genes in Shi yuan 321 was higher than that in Kui 85-174. Overall, genome-wide identification approach was used to further analyze the related functions of the RF gene family, which may include the response to drought stress, in cotton.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01065-4.

Structure and Expression Analysis of Sucrose Phosphate Synthase, Sucrose Synthase and Invertase Gene Families in Solanum lycopersicum

Sucrose phosphate synthase (SPS), sucrose synthase (SUS) and invertase (INV) are all encoded by multigene families. In tomato (Solanum lycopersicum), a comprehensive analysis of structure characteristics of these family genes is still lacking, and the functions of individual isoforms of these families are mostly unclear under stress. Here, the structure characteristics of the three families in tomato were analyzed; moreover, as a first step toward understanding the functions of isoforms of these proteins under stress, the tissue expression pattern and stress response of these genes were also investigated. The results showed that four SPS genes, six SUS genes and nineteen INV genes were identified in tomato. The subfamily differentiation of SlSPS and SlSUS might have completed before the split of monocotyledons and dicotyledons. The conserved motifs were mostly consistent within each protein family/subfamily. These genes demonstrated differential expressions among family members and tissues, and in response to polyethylene glycerol, NaCl, H₂O₂, abscisic acid or salicylic acid treatment. Our results suggest that each isoform of these families may have different functions in different tissues and under environmental stimuli. SlSPS1, SlSPS3, SlSUS1, SlSUS3, SlSUS4, SlINVAN5 and SlINVAN7 demonstrated consistent expression responses and may be the major genes responding to exogenous stimuli.

Sucrose synthase gene family in Brassica juncea: genomic organization, evolutionary comparisons, and expression regulation

Sucrose synthase (SUS) plays an important role in sucrose metabolism and plant development. The SUS gene family has been identified in many plants, however, there is no definitive study of SUS gene in Brassica juncea. In this study, 14 SUS family genes were identified and comprehensively analyzed using bioinformatics tools. The analyzed parameters included their family member characteristics, chromosomal locations, gene structures and phylogenetic as well as transcript expression profiles. Phylogenetic analysis revealed that the 14 members could be allocated into three groups: SUS I, SUS II and SUS III. Comparisons of the exon/intron structure of the mustard SUS gene indicated that its structure is highly conserved. The conserved structure is attributed to purification selection during evolution. Expansion of the SUS gene family is associated with fragment and tandem duplications of the mustard SUS gene family. Collinearity analysis among species revealed that the SUS gene family could be lost or mutated to varying degrees after the genome was doubled, or when Brassica rapa and Brassica nigra hybridized to form Brassica juncea. The expression patterns of BjuSUSs vary among different stages of mustard stem swelling. Transcriptomics revealed that the BjuSUS01-04 expression levels were the most elevated. It has been hypothesized that they play an important role in sucrose metabolism during stem development. The expression levels of some BjuSUSs were significantly up-regulated when they were treated with plant hormones. However, when subjected to abiotic stress factors, their expression levels were suppressed. This study establishes SUS gene functions during mustard stem development and stress.

Insights Into the Molecular Evolution of AT-Hook Motif Nuclear Localization Genes in Brassica napus

AT-hook motif nuclear localization (AHL) proteins belong to a family of transcription factors, and play important roles in plant growth and development and response to various stresses through protein-DNA and protein-protein interactions. To better understand the Brassica napus AHL gene family, AHL genes in B. napus and related species were analyzed. Using Arabidopsis as a reference, 122 AHL gene family members were first identified in B. napus. According to the phylogenetic tree and gene organization, the BnaAHLs were classified into two clades (Clade-A and Clade-B) and three types (Type-I, Type-II, and Type-III). Gene organization and motif distribution analysis suggested that the AHL gene family is relatively conserved during evolution. These BnaAHLs are unevenly distributed on 38 chromosomes and expanded by whole-genome duplication (WGD) or segmental duplication. And large-scale loss events have also occurred in evolution. All types of BnaAHLs are subject to purification or neutral selection, while some positive selection sites are also identified in Type-II and Type-III groups. At the same time, the purification effect of Type-I members are stronger than that of the others. In addition, RNA-seq data and cis-acting element analysis also suggested that the BnaAHLs play important roles in B. napus growth and development, as well as in response to some abiotic and biotic stresses. Protein-protein interaction analysis identified some important BnaAHL-binding proteins, which also play key roles in plant growth and development. This study is helpful to fully understand the origin and evolution of the AHL gene in B. napus, and lays the foundation for their functional studies.

Molecular cloning and expression analysis of sucrose phosphate synthase genes in cassava (Manihot esculenta Crantz)

Sucrose phosphate synthase (SPS), a key rate-limiting enzyme in the sucrose biosynthesis pathway in plants, is encoded by a multi-gene family. Until recently, the identification and characterization of the SPS gene family have been performed for dozens of plant species; however, few studies have involved a comprehensive analysis of the SPS family members in tropical crops, such as cassava (Manihot esculenta Crantz). In the current study, five SPS genes (MeSPS1, MeSPS2, MeSPS3, MeSPS4, and MeSPS5) were isolated from cassava, and their sequence characteristics were comprehensively characterized. These MeSPS genes were found distributed on five chromosomes (Chr2, Chr14, Chr15, Chr16, and Chr18). Phylogenetic analysis showed that the MeSPS protein sequences were clustered into three families, together with other SPS sequences from both dicot and monocot species (families A, B, and C). The spatio-temporal expression pattern analysis of MeSPS genes showed a tissue-specific and partially overlapping expression pattern, with the genes mainly expressed in source tissues during cassava growth and development. Correlation analysis revealed that the expression of MeSPS genes correlated positively with root starch content, indicating that the expression of MeSPS genes might accelerate the rate of starch accumulation in the roots of cassava plants.

Impact of Drought on Soluble Sugars and Free Proline Content in Selected Arabidopsis Mutants

Simple Summary

Drought has severe effects on plants, negatively impacting economic, agricultural and environmental processes. Depending on the duration and the strength of water stress conditions, plants adjust a series of physiological, cellular, and molecular mechanisms aimed at providing a correct stress response and, if possible, at establishing stress tolerance. The model plant Arabidopsis thaliana is one of the best tools for analyzing the involvement of specific genes in the response to drought. Thanks to this tool, the role of two genes encoding enzymes involved in sugars metabolism, and one gene encoding an enzyme involved in proline synthesis, have been investigated. In addition to suggesting an interaction between the metabolism of proline and that of soluble sugars, results broaden our understanding of the predominant role played by the accumulation of soluble sugars in counteracting mild osmotic stress.

Abstract

Water shortage is an increasing problem affecting crop yield. Accumulation of compatible osmolytes is a typical plant response to overcome water stress. Sucrose synthase 1 (SUS1), and glucan, water dikinase 2 (GWD2) and δ¹-pyrroline-5-carboxylate synthetase 1 (P5CS1) are members of small protein families whose role in the response of Arabidopsis thaliana plants to mild osmotic stress has been studied in this work. Comparative analysis between wild-type and single loss-of-function T-DNA plants at increasing times following exposure to drought showed no differences in the content of water-insoluble carbohydrate (i.e., transitory starch and cell wall carbohydrates) and in the total amount of amino acids. On the contrary, water-soluble sugars and proline contents were significantly reduced compared to wild-type plants regardless of the metabolic pathway affected by the mutation. The present results contribute to assigning a physiological role to GWD2, the least studied member of the GWD family; strengthening the involvement of SUS1 in the response to osmotic stress; showing a greater contribution of soluble sugars than proline in osmotic adjustment of Arabidopsis in response to drought. Finally, an interaction between proline and soluble sugars emerged, albeit its nature remains speculative and further investigations will be required for complete comprehension.

Genome-wide identification and analysis of the sucrose synthase gene family in cassava (Manihot esculenta Crantz)

Sucrose synthase (SUS), a key enzyme of the sucrose metabolism pathway, is encoded by a multi-gene family in plants. To date, dozens of SUS gene families have been characterized in various plant genomes. However, only a few studies have performed comprehensive analyses in tropical crops like cassava (Manihot esculenta Crantz). In the present study, seven non-redundant members of the SUS gene family (MeSUS1-7) were identified and characterized from the cassava genome. The MeSUS genes were distributed on five chromosomes (Chr1, Chr2, Chr3, Chr14, and Chr16) and the encoded proteins could be classified into three major groups with other SUS proteins from both dicot and monocot species (SUS I, SUS II, and SUS III). The spatio-temporal expression profiles of MeSUS genes showed a developmental stage-dependent, partially overlapping pattern, mainly expressed in the source and sink tissues. Cold and drought treatments significantly induced the expressions of MeSUS2, MeSUS4, MeSUS6, and MeSUS7 and the activities of the encoded enzymes, indicating that these genes may play crucial roles in resistance against abiotic stresses. These results provide new insights into the multifaceted role of the SUS gene family members in various physiological processes, especially sucrose transport and starch accumulation in cassava roots.

Sucrose phosphate synthase (SPS), sucrose synthase (SUS) and their products in the leaves of Miscanthus × giganteus and Zea mays at low temperature

The changes in the expression of key sugar metabolism enzymes (SPS and SUS), sucrose content and arrangement of chloroplast starch may play a significant role in the cold response in M. giganteus and maize plants. To understand the mechanism of the chilling-response of two closely-related C₄ plants, we investigated the changes in the expression of sucrose phosphate synthase (SPS) and sucrose synthase (SUS) as well as changes in their potential products: sucrose, cellulose and starch in the leaves of Miscanthus × giganteus and Zea mays. Low temperature (12-14 °C) increased SPS content in Miscanthus (MG) and chilling-sensitive maize line (Zm-S), but not in chilling-tolerant one (Zm-T). In Zm-S line, chilling also caused the higher intensity of labelling of SPS in the cytoplasm of mesophyll cells, as demonstrated by electron microscopy. SUS labelling was also increased by cold stress only in MG plants what was observed in the secondary wall between mesophyll and bundle sheath cells, as well as in the vacuoles of companion cells. Cold led to a marked increase in total starch grain area in the chloroplasts of Zm-S line. In turn, Fourier transform infrared spectroscopy (FTIR) showed a slight shift in the cellulose band position, which may indicate the formation of more compact cellulose arrangement in Zm-T maize line. In conclusion, this work presents new findings supporting diversified cold-response, not only between two C₄ plant species but also within one species of maize.

The involvement of wheat U-box E3 ubiquitin ligase TaPUB1 in salt stress tolerance

U-box E3 ubiquitin ligases play important roles in the ubiquitin/26S proteasome machinery and in abiotic stress responses. TaPUB1-overexpressing wheat (Triticum aestivum L.) were generated to evaluate its function in salt tolerance. These plants had more salt stress tolerance during seedling and flowering stages, whereas the TaPUB1-RNA interference (RNAi)-mediated knock-down transgenic wheat showed more salt stress sensitivity than the wild type (WT). TaPUB1 overexpression upregulated the expression of genes related to ion channels and increased the net root Na⁺ efflux, but decreased the net K⁺ efflux and H⁺ influx, thereby maintaining a low cytosolic Na⁺ /K⁺ ratio, compared with the WT. However, RNAi-mediated knock-down plants showed the opposite response to salt stress. TaPUB1 could induce the expression of some genes that improved the antioxidant capacity of plants under salt stress. TaPUB1 also interacted with TaMP (Triticum aestivum α-mannosidase protein), a regulator playing an important role in salt response in yeast and in plants. Thus, low cytosolic Na⁺ /K⁺ ratios and better antioxidant enzyme activities could be maintained in wheat with overexpression of TaPUB1 under salt stress. Therefore, we conclude that the U-box E3 ubiquitin ligase TaPUB1 positively regulates salt stress tolerance in wheat.

Mining of Sucrose Synthases from Glycyrrhiza uralensis and Their Application in the Construction of an Efficient UDP-Recycling System

Sucrose synthase (SUS) plays an important role in carbohydrate metabolism in plants. The SUS genes in licorice remain unknown. To reveal the sucrose metabolic pathway in licorice, all the 12 putative SUS genes of Glycyrrhiza uralensis were systematically identified by genome mining, and two novel SUSs (GuSUS1 and GuSUS2) were isolated and characterized for the first time. Furthermore, we found that the flexible N-terminus was responsible for the low stability of plant SUSs, and deletion of redundant N-terminus improved the stability of GuSUS1 and GuSUS2. The half-life of both GuSUS1 and GuSUS2 mutants was increased by 2-fold. Finally, the GuSUS1 mutant was coupled with UGT73C11 for the glycosylation of glycyrrhetinic acid (GA) with uridine 5'-diphosphate disodium salt hydrate (UDP) in situ recycling, and GA conversion was increased by 7-fold. Our study not only identified the SUS genes in licorice but also provided a stable SUS mutant for the construction of an efficient UDP-recycling system for GA glycosylation.

Distinct cellulose and callose accumulation for enhanced bioethanol production and biotic stress resistance in OsSUS3 transgenic rice

Genetic modification of plant cell walls is an effective approach to reduce lignocellulose recalcitrance in biofuel production, but it may affect plant stress response. Hence, it remains a challenge to reduce biomass recalcitrance and simultaneously enhance stress resistance. In this study, the OsSUS3-transgenic plants exhibited increased cell wall polysaccharides deposition and reduced cellulose crystallinity and xylose/arabinose proportion of hemicellulose, resulting in largely enhanced biomass saccharification and bioethanol production. Additionally, strengthening of the cell wall also contributed to plant biotic resistance. Notably, the transgenic plants increased stress-induced callose accumulation, and promoted the activation of innate immunity, leading to greatly improved multiple resistances to the most destructive diseases and a major pest. Hence, this study demonstrates a significant improvement both in bioethanol production and biotic stress resistance by regulating dynamic carbon partitioning for cellulose and callose biosynthesis in OsSUS3-transgenic plants. Meanwhile, it also provides a potential strategy for plant cell wall modification.

Analysis of the diversity and tissue specificity of sucrose synthase genes in the long read transcriptome of sugarcane

BACKGROUND

Sugarcane accumulates very high levels of sucrose in the culm. Elucidation of the molecular mechanisms that allows such high sucrose synthesis and accumulation (up to 650 mM) is made difficult by the complexity of the highly polyploid genome. Here we report the use of RNA Seq data to characterize the sucrose synthase (SuSy) genes expressed in the transcriptome of the mature sugarcane plant.

RESULTS

Four SuSy gene families were identified in the sugarcane Iso-Seq long read transcriptome (SUGIT) through gene annotation of transcripts that mapped to reference SuSy genes from sorghum and maize. In total, 38, 19, 14, and 2 transcripts were identified for the four corresponding SuSy genes 1, 2, 4 and 7, respectively. Comparative studies using available SuSy genes from sorghum (1, 2, 4, 6, 7) and maize (1-7) revealed that the sugarcane SuSy genes were interrupted by multiple introns and that they share a highly conserved gene structure. Spatial expression of the four SuSy genes in sugarcane genotypes and in the progenitor species, Saccharum spontaneum and Saccharum officinarum, was studied in the leaf and root tissues and also in three regions of the culm tissue; top, middle and bottom internodes. Expression profiles indicated that all SuSy transcripts were differentially expressed between the top and bottom tissues, with high expression in the top tissues, lower expression in the bottom and moderate expression in the middle, indicating a gradient of SuSy activity in the sugarcane culm. Further, the root tissue had similar expression levels to that of the top internodes while leaf tissues showed lower expression. In the progenitors, SuSy7 was found to be highly expressed in S. officinarum while the other three SuSy genes had moderate expression in both the progenitors.

CONCLUSIONS

The high expression of the SuSy genes in sink tissues, the top internodes and the roots suggests functional roles in sucrose utilization to support growth. The SuSy7 gene has not been previously reported in sugarcane. As sugarcane is unique in storing such high amounts of sucrose, it is possible that there are more SuSy genes/isoforms with specific expression patterns to be discovered in this complex system.

Genome-wide identification and expression analysis of sucrose synthase genes in allotetraploid Brassica juncea

Sucrose plays pivotal role in energy metabolism and regulating gene expression of several physiological processes in higher plants. Here, fourteen sucrose synthase (SUS) genes have been identified in the allotetraploid genome of Indian mustard, Brassica juncea. The identified SUS genes in B. juncea (BjSUS) were derived from the two-progenitor species, B. rapa and B. nigra. Intron-exon analysis indicated loss or gain of 1-3 introns in diversification of SUS gene family. Phylogenetic analysis revealed discrete evolutionary paths for the BjSUS genes, originating from three ancestor groups, SUS I, SUS II and SUS III. Gene expression study revealed significant variability in expression of the BjSUS paralogs across the different tissues. BjSUS genes showed transcriptional activation in response to defense hormones and a late response to wounding. Tissue and temporal specificity of expression revealed importance of specific SUS paralogs at different developmental stages and under different stress conditions. The study highlighted differential involvement of SUS paralogs in sucrose metabolism across the tissues and stress-responses, in a major oilseed crop B. juncea.

Down-Regulating Cucumber Sucrose Synthase 4 (CsSUS4) Suppresses the Growth and Development of Flowers and Fruits

Sucrose synthase (SUS), which catalyzes the reversible conversion of sucrose and uridine diphosphate (UDP) into fructose and UDP-glucose, is a key enzyme in sucrose metabolism in higher plants. In this study, we used reverse genetic approaches and carbohydrate analysis to investigate the role of cucumber sucrose synthase gene 4 (CsSUS4) in the growth and development of sink organs. Transcript analyses showed that CsSUS4 was predominantly expressed in sink organs, particularly in flowers, fruits and roots, and that CsSUS4 protein was localized to companion cells and phloem parenchyma cells. Down-regulation of CsSUS4 expression resulted in a decrease in SUS activity in conjunction with lower hexose, starch and cellulose contents in fruits, and led to an overall reduction in the size and weight of flowers and fruits. Furthermore, CsSUS4 overexpression (OE) lines exhibited increased carbohydrate content, and larger and heavier flowers and fruits. The numbers of multi-petal flowers and multi-carpel fruits were greater in CsSUS4-OE plants compared with wild type and were regulated by MADS-box transcription factor. These results demonstrate that CsSUS4 plays important roles in the growth and development of cucumber flowers and fruits.

An Overview of Sucrose Synthases in Plants

Sucrose is the end product of photosynthesis and the primary sugar transported in the phloem of most plants. Sucrose synthase (SuSy) is a glycosyl transferase enzyme that plays a key role in sugar metabolism, primarily in sink tissues. SuSy catalyzes the reversible cleavage of sucrose into fructose and either uridine diphosphate glucose (UDP-G) or adenosine diphosphate glucose (ADP-G). The products of sucrose cleavage by SuSy are available for many metabolic pathways, such as energy production, primary-metabolite production, and the synthesis of complex carbohydrates. SuSy proteins are usually homotetramers with an average monomeric molecular weight of about 90 kD (about 800 amino acids long). Plant SuSy isozymes are mainly located in the cytosol or adjacent to plasma membrane, but some SuSy proteins are found in the cell wall, vacuoles, and mitochondria. Plant SUS gene families are usually small, containing between four to seven genes, with distinct exon-intron structures. Plant SUS genes are divided into three separate clades, which are present in both monocots and dicots. A comprehensive phylogenetic analysis indicates that a first SUS duplication event may have occurred before the divergence of the gymnosperms and angiosperms and a second duplication event probably occurred in a common angiosperm ancestor, leading to the existence of all three clades in both monocots and dicots. Plants with reduced SuSy activity have been shown to have reduced growth, reduced starch, cellulose or callose synthesis, reduced tolerance to anaerobic-stress conditions and altered shoot apical meristem function and leaf morphology. Plants overexpressing SUS have shown increased growth, increased xylem area and xylem cell-wall width, and increased cellulose and starch contents, making SUS high-potential candidate genes for the improvement of agricultural traits in crop plants. This review summarizes the current knowledge regarding plant SuSy, including newly discovered possible developmental roles for SuSy in meristem functioning that involve sugar and hormonal signaling.

Identification and analysis of the chloride channel gene family members in tobacco (Nicotiana tabacum)

The chloride channel (CLC) protein family, which includes both chloride (Cl-) channels and chloride/proton (Cl-/H+) antiporters, is present in all domains of life, from prokaryotes to eukaryotes. However, there are no reported studies about this gene family in tobacco, an economically important global crop plant. In this study, we identified seventeen CLC genes in the genome of Nicotiana tabacum. A multiple sequence alignment showed that all of the predicted proteins shared a high sequence similarity and had a highly conserved GKxGPxxH motif. A gene structure analysis revealed that the NtCLC genes had highly divergent intron-exon patterns. A phylogenetic and conserved motif analysis revealed that the NtCLC family was divided into two clades, in a manner similar to other plants. We also evaluated the expression patterns of these NtCLC genes in different tissues and in plants treated with salt stress. The NtCLC genes had highly variable expression patterns, for example, the largely stem- and bud-specific expression patterns of NtCLC6 and NtCLC8, respectively. Salt stress treatment (300 mM NaCl) induced the expression of NtCLC2, NtCLC3, and NtCLC12, suggesting that these genes might play a role in tobacco responses to salt stress. Furthermore, the concentration of Cl- in the NtCLC2- and NtCLC13-silenced plants showed an obvious lower and higher level, respectively, than the control plants. Thus, we indicated that NtCLC2 or NtCLC13 might play an important role in chloride transport or metabolism in tobacco. Together, these findings establish an empirical foundation for the further functional characterization of the NtCLC genes in tobacco.

Expression of TaGF14b, a 14-3-3 adaptor protein gene from wheat, enhances drought and salt tolerance in transgenic tobacco

MAIN CONCLUSION

TaGF14b enhances tolerance to multiple stresses through ABA signaling pathway by altering physiological and biochemical processes, including ROS-scavenging system, stomatal closure, compatible osmolytes, and stress-related gene expressions in tobaccos. The 14-3-3 proteins are involved in plant growth, development, and in responding to abiotic stresses. However, the precise functions of 14-3-3s in responding to drought and salt stresses remained unclear, especially in wheat. In this study, a 14-3-3 gene from wheat, designated TaGF14b, was cloned and characterized. TaGF14b was upregulated by polyethylene glycol 6000, sodium chloride, hydrogen peroxide, and abscisic acid (ABA) treatments. Ectopic expression of TaGF14b in tobacco conferred enhanced tolerance to drought and salt stresses. Transgenic tobaccos had longer root, better growth status, and higher relative water content, survival rate, photosynthetic rate, and water use efficiency than control plants under drought and salt stresses. The contribution of TaGF14b to drought and salt tolerance relies on the regulations of ABA biosynthesis and ABA signaling, as well as stomatal closure and stress-related gene expressions. Moreover, TaGF14b expression could significantly enhance the reactive oxygen species (ROS) scavenging system to ameliorate oxidative damage to cells. In addition, TaGF14b increased tolerance to osmotic stress evoked by drought and salinity through modifying water conservation and compatible osmolytes in plants. In conclusion, TaGF14b enhances tolerance to multiple abiotic stresses through the ABA signaling pathway in transgenic tobaccos by altering physiological and biochemical processes.

The Sucrose Synthase Gene Family in Chinese Pear (Pyrus bretschneideri Rehd.): Structure, Expression, and Evolution

Sucrose synthase (SS) is a key enzyme involved in sucrose metabolism that is critical in plant growth and development, and particularly quality of the fruit. Sucrose synthase gene families have been identified and characterized in plants various plants such as tobacco, grape, rice, and Arabidopsis. However, there is still lack of detailed information about sucrose synthase gene in pear. In the present study, we performed a systematic analysis of the pear (Pyrus bretschneideri Rehd.) genome and reported 30 sucrose synthase genes. Subsequently, gene structure, phylogenetic relationship, chromosomal localization, gene duplications, promoter regions, collinearity, RNA-Seq data and qRT-PCR were conducted on these sucrose synthase genes. The transcript analysis revealed that 10 PbSSs genes (30%) were especially expressed in pear fruit development. Additionally, qRT-PCR analysis verified the RNA-seq data and shown that PbSS30, PbSS24, and PbSS15 have a potential role in the pear fruit development stages. This study provides important insights into the evolution of sucrose synthase gene family in pear and will provide assistance for further investigation of sucrose synthase genes functions in the process of fruit development, fruit quality and resistance to environmental stresses.

Genome-Wide Analysis of the Sucrose Synthase Gene Family in Grape (Vitis vinifera): Structure, Evolution, and Expression Profiles

Sucrose synthase (SS) is widely considered as the key enzyme involved in the plant sugar metabolism that is critical to plant growth and development, especially quality of the fruit. The members of SS gene family have been identified and characterized in multiple plant genomes. However, detailed information about this gene family is lacking in grapevine (Vitis vinifera L.). In this study, we performed a systematic analysis of the grape (V. vinifera) genome and reported that there are five SS genes (VvSS1-5) in the grape genome. Comparison of the structures of grape SS genes showed high structural conservation of grape SS genes, resulting from the selection pressures during the evolutionary process. The segmental duplication of grape SS genes contributed to this gene family expansion. The syntenic analyses between grape and soybean (Glycine max) demonstrated that these genes located in corresponding syntenic blocks arose before the divergence of grape and soybean. Phylogenetic analysis revealed distinct evolutionary paths for the grape SS genes. VvSS1/VvSS5, VvSS2/VvSS3 and VvSS4 originated from three ancient SS genes, which were generated by duplication events before the split of monocots and eudicots. Bioinformatics analysis of publicly available microarray data, which was validated by quantitative real-time reverse transcription PCR (qRT-PCR), revealed distinct temporal and spatial expression patterns of VvSS genes in various tissues, organs and developmental stages, as well as in response to biotic and abiotic stresses. Taken together, our results will be beneficial for further investigations into the functions of SS gene in the processes of grape resistance to environmental stresses.

Guard cell-specific upregulation of sucrose synthase 3 reveals that the role of sucrose in stomatal function is primarily energetic

Isoform 3 of sucrose synthase (SUS3) is highly expressed in guard cells; however, the precise function of SUS3 in this cell type remains to be elucidated. Here, we characterized transgenic Nicotiana tabacum plants overexpressing SUS3 under the control of the stomatal-specific KST1 promoter, and investigated the changes in guard cell metabolism during the dark to light transition. Guard cell-specific SUS3 overexpression led to increased SUS activity, stomatal aperture, stomatal conductance, transpiration rate, net photosynthetic rate and growth. Although only minor changes were observed in the metabolite profile in whole leaves, an increased fructose level and decreased organic acid levels and sucrose to fructose ratio were observed in guard cells of transgenic lines. Furthermore, guard cell sucrose content was lower during light-induced stomatal opening. In a complementary approach, we incubated guard cell-enriched epidermal fragments in (13) C-NaHCO3 and followed the redistribution of label during dark to light transitions; this revealed increased labeling in metabolites of, or associated with, the tricarboxylic acid cycle. The results suggest that sucrose breakdown is a mechanism to provide substrate for the provision of organic acids for respiration, and imply that manipulation of guard cell metabolism may represent an effective strategy for plant growth improvement.

Characterization of Rice Homeobox Genes, OsHOX22 and OsHOX24, and Over-expression of OsHOX24 in Transgenic Arabidopsis Suggest Their Role in Abiotic Stress Response

Homeobox transcription factors are well known regulators of plant growth and development. In this study, we carried out functional analysis of two candidate stress-responsive HD-ZIP I class homeobox genes from rice, OsHOX22, and OsHOX24. These genes were highly up-regulated under various abiotic stress conditions at different stages of rice development, including seedling, mature and reproductive stages. The transcript levels of these genes were enhanced significantly in the presence of plant hormones, including abscisic acid (ABA), auxin, salicylic acid, and gibberellic acid. The recombinant full-length and truncated homeobox proteins were found to be localized in the nucleus. Electrophoretic mobility shift assay established the binding of these homeobox proteins with specific DNA sequences, AH1 (CAAT(A/T)ATTG) and AH2 (CAAT(C/G)ATTG). Transactivation assays in yeast revealed the transcriptional activation potential of full-length OsHOX22 and OsHOX24 proteins. Homo- and hetero-dimerization capabilities of these proteins have also been demonstrated. Further, we identified putative novel interacting proteins of OsHOX22 and OsHOX24 via yeast-two hybrid analysis. Over-expression of OsHOX24 imparted higher sensitivity to stress hormone, ABA, and abiotic stresses in the transgenic Arabidopsis plants as revealed by various physiological and phenotypic assays. Microarray analysis revealed differential expression of several stress-responsive genes in transgenic lines as compared to wild-type. Many of these genes were found to be involved in transcriptional regulation and various metabolic pathways. Altogether, our results suggest the possible role of OsHOX22/OsHOX24 homeobox proteins as negative regulators in abiotic stress responses.

Transcriptome-wide identification of sucrose synthase genes in Ornithogalum caudatum

A transcriptome-wide discovery and functional identification of a sucrose synthase family was presented. Importantly, OcSus1 and OcSus2 were first verified to be responsible for the biosynthesis of glucose-containing polysaccharides inO. caudatum.

New insight into the catalytic properties of rice sucrose synthase

Sucrose synthase (SuS), which catalyzes the reversible conversion of sucrose and uridine diphosphate (UDP) into fructose and UDP-glucose, is a key enzyme in sucrose metabolism in higher plants. SuS belongs to family 4 of the glycosyltransferases (GT4) and contains an E-X7-E motif that is conserved in members of GT4 and two other GT families. To gain insight into the roles of this motif in rice sucrose synthase 3 (RSuS3), the two conserved glutamate residues (E678 and E686) in this motif and a phenylalanine residue (F680) that resides between the two glutamate residues were changed by site-directed mutagenesis. All mutant proteins maintained their tetrameric conformation. The mutants E686D and F680Y retained partial enzymatic activity and the mutants E678D, E678Q, F680S, and E686Q were inactive. Substrate binding assays indicated that UDP and fructose, respectively, were the leading substrates in the sucrose degradation and synthesis reactions of RSuS3. Mutations on E678, F680, and E686 affected the binding of fructose, but not of UDP. The results indicated that E678, F680, and E686 in the E-X7-E motif of RSuS3 are essential for the activity of the enzyme and the sequential binding of substrates. The sequential binding of the substrates implied that the reaction catalyzed by RSuS can be controlled by the availability of fructose and UDP, depending on the metabolic status of a tissue.