Photoperiodic regulation of the sucrose transporter StSUT4 affects the expression of circadian-regulated genes and ethylene production

Photosynthetic adaptation strategies in peppers under continuous lighting: insights into photosystem protection

Energy efficient lighting strategies have received increased interest from controlled environment producers. Long photoperiods (up to 24 h - continuous lighting (CL)) of lower light intensities could be used to achieve the desired daily light integral (DLI) with lower installed light capacity/capital costs and low electricity costs in regions with low night electricity prices. However, plants grown under CL tend to have higher carbohydrate and reactive oxygen species (ROS) levels which may lead to leaf chlorosis and down-regulation of photosynthesis. We hypothesize that the use of dynamic CL using a spectral change and/or light intensity change between day and night can negate CL-injury. In this experiment we set out to assess the impact of CL on pepper plants by subjecting them to white light during the day and up to 150 µmol m-2 s-1 of monochromatic blue light at night while controlling the DLI at the same level. Plants grown under all CL treatments had similar cumulative fruit number and weight compared to the 16h control indicating no reduction in production. Plants grown under CL had higher carbohydrate levels and ROS-scavenging capacity than plants grown under the 16h control. Conversely, the amount of photosynthetic pigment decreased with increasing nighttime blue light intensity. The maximum quantum yield of photosystem II (Fv/Fm), a metric often used to measure stress, was unaffected by light treatments. However, when light-adapted, the operating efficiency of photosystem II (ΦPSII) decreased and non-photochemical quenching (NPQ) increased with increasing nighttime blue light intensity. This suggests that both acclimated and instantaneous photochemistry during CL can be altered and is dependent on the nighttime light intensity. Furthermore, light-adapted chlorophyll fluorescence measurements may be more adept at detecting altered photochemical states than the conventional stress metric using dark-adapted measurements.

Sucrose homeostasis: Mechanisms and opportunity in crop yield improvement

Sugar homeostasis is a critical feature of biological systems. In humans, raised and dysregulated blood sugar is a serious health issue. In plants, directed changes in sucrose homeostasis and allocation represent opportunities in crop improvement. Plant tissue sucrose varies more than blood glucose and is found at higher concentrations (cytosol and phloem ca. 100 mM v 3.9-6.9 mM for blood glucose). Tissue sucrose varies with developmental stage and environment, but cytosol and phloem exhibit tight sucrose control. Sucrose homeostasis is a consequence of the integration of photosynthesis, synthesis of storage end-products such as starch, transport of sucrose to sinks and sink metabolism. Trehalose 6-phosphate (T6P)-SnRK1 and TOR play central, still emerging roles in regulating and coordinating these processes. Overall, tissue sucrose levels are more strongly related to growth than to photosynthesis. As a key sucrose signal, T6P regulates sucrose levels, transport and metabolic pathways to coordinate source and sink at a whole plant level. Emerging evidence shows that T6P interacts with meristems. With careful targeting, T6P manipulation through exploiting natural variation, chemical intervention and genetic modification is delivering benefits for crop yields. Regulation of cereal grain set, filling and retention may be the most strategically important aspect of sucrose allocation and homeostasis for food security.

The tuber-specific StbHLH93 gene regulates proplastid-to-amyloplast development during stolon swelling in potato

In potato, stolon swelling is a complex and highly regulated process, and much more work is needed to fully understand the underlying mechanisms. We identified a novel tuber-specific basic helix-loop-helix (bHLH) transcription factor, StbHLH93, based on the high-resolution transcriptome of potato tuber development. StbHLH93 is predominantly expressed in the subapical and perimedullary region of the stolon and developing tubers. Knockdown of StbHLH93 significantly decreased tuber number and size, resulting from suppression of stolon swelling. Furthermore, we found that StbHLH93 directly binds to the plastid protein import system gene TIC56 promoter, activates its expression, and is involved in proplastid-to-amyloplast development during the stolon-to-tuber transition. Knockdown of the target TIC56 gene resulted in similarly problematic amyloplast biogenesis and tuberization. Taken together, StbHLH93 functions in the differentiation of proplastids to regulate stolon swelling. This study highlights the critical role of proplastid-to-amyloplast interconversion during potato tuberization.

Tomato sucrose transporter SlSUT4 participates in flowering regulation by modulating gibberellin biosynthesis

The functions of sucrose transporters (SUTs) differ among family members. The physiological function of SUT1 has been studied intensively, while that of SUT4 in various plant species including tomato (Solanum lycopersicum) is less well-understood. In this study, we characterized the function of tomato SlSUT4 in the regulation of flowering using a combination of molecular and physiological analyses. SlSUT4 displayed transport activity for sucrose when expressed in yeast (Saccharomyces cerevisiae), and it localized at both the plasma membrane and tonoplast. SlSUT4 interacted with SlSUT1, causing partial internalization of the latter, the main phloem loader of sucrose in tomato. Silencing of SlSUT4 promoted SlSUT1 localization to the plasma membrane, contributing to increased sucrose export and thus increased sucrose level in the shoot apex, which promoted flowering. Both silencing of SlSUT4 and spraying with sucrose suppressed gibberellin biosynthesis through repression of ent-kaurene oxidase and gibberellin 20-oxidase-1 (2 genes encoding key enzymes in gibberellin biosynthesis) expression by SlMYB76, which directly bound to their promoters. Silencing of SlMYB76 promoted gibberellin biosynthesis. Our results suggest that SlSUT4 is a functional SUT in tomato; downregulation of SlSUT4 expression enhances sucrose transport to the shoot apex, which promotes flowering by inhibiting gibberellin biosynthesis.

Sugar concentrations and expression of SUTs suggest active phloem loading in tall trees of Fagus sylvatica and Quercus robur

Phloem loading and sugar distribution are key steps for carbon partitioning in herbaceous and woody species. Although the phloem loading mechanisms in herbs are well studied, less is known for trees. It was shown for saplings of Fagus sylvatica L. and Quercus robur L. that the sucrose concentration in the phloem sap was higher than in the mesophyll cells, which suggests that phloem loading of sucrose involves active steps. However, the question remains whether this also applies for tall trees. To approach this question, tissue-specific sugar and starch contents of small and tall trees of F. sylvatica and Q. robur as well as the sugar concentration in the subcellular compartments of mesophyll cells were examined. Moreover, sucrose uptake transporters (SUTs) were analyzed by heterology expression in yeast and the tissue-specific expressions of SUTs were investigated. Sugar content in leaves of the canopy (11 and 26 m height) was up to 25% higher compared with that of leaves of small trees of F. sylvatica and Q. robur (2 m height). The sucrose concentration in the cytosol of mesophyll cells from tall trees was between 120 and 240 mM and about 4- to 8-fold lower than the sucrose concentration in the phloem sap of saplings. The analyzed SUT sequences of both tree species cluster into three types, similar to SUTs from other plant species. Heterologous expression in yeast confirmed that all analyzed SUTs are functional sucrose transporters. Moreover, all SUTs were expressed in leaves, bark and wood of the canopy and the expression levels in small and tall trees were similar. The results show that the phloem loading in leaves of tall trees of F. sylvatica and Q. robur probably involves active steps, because there is an uphill concentration gradient for sucrose. SUTs may be involved in phloem loading.

Identification and molecular characterization of novel sucrose transporters in the hexaploid wheat (Triticum aestivum L.)

Common wheat (Triticum aestivum) is a major cereal crop grown and consumed globally. Recent advances in sequencing technology have facilitated the exploration of large and repetitive genomes. Plant sucrose transporter (SUT) genes are vital components of energy transport systems that play prominent roles in various plant functions, such as signaling and stress regulation. In this study, we identified and analyzed five novel sucrose transporter genes in wheat. The wheat sucrose transporter genes were divided into five clades based on their phylogenetic relationships. Synteny analysis revealed that synteny in the genome is highly conserved between wheat and rye, barley, and Brachypodium. Furthermore, the cis-element analysis indicated that sucrose transporter genes might be regulated by light and some phytohormone-related transcriptional factors. Overall, plant tissue-specific gene expression revealed enhanced expression of the transporter genes in the root and stem, whereas they were differentially expressed under abiotic stress treatments (cold, heat, NaCl, PEG-6000, and sucrose). These results indicate that each TaSUT gene may play a crucial role in stabilizing plants under stress by actively regulating the energy demands of cells. The findings of this study may provide a basis for further research on sucrose transporters and their significant roles in plant energy metabolism as well as in abiotic stress response, signaling, and regulation.

Genome-wide identification and expression analysis of SUT gene family members in sugar beet (Beta vulgaris L.)

Sucrose transporters (SUTs) play an important role in the transmembrane transport and distribution of sucrose, and their activity has an important impact on plant growth and crop yield. In this study, the SUT gene family was identified in the whole beet genome using bioinformatics methods, and gene characteristics, subcellular localization prediction, phylogenetic evolution, promoter cis-elements and expression patterns were systematically analyzed. A total of 9 SUT gene family members were identified from in beet genome and divided into 3 different groups (group 1, group 2, and Group 3), which were unevenly distributed on 4 chromosomes. Most SUT family members contained photoresponsive and hormone-regulated response elements. Subcellular localization prediction showed that the BvSUT genes are all located in the inner membrane, and most of the terms identified through GO enrichment analysis are classified as "membrane" related. The results of RT-qPCR showed that the expression level of the BvSUT gene was significantly higher in the tuber enlargement stage (100-140 d) than in other stages. This study is the first to analyze the BvSUT gene family in sugar beet, and it provides a theoretical basis for the functional exploration and application of SUT genes in crop improvement, especially in sugar crops.

Sucrose Transporter StSUT2 Affects Potato Plants Growth, Flowering Time, and Tuber Yield

BACKGROUND

Sucrose transporters (SUTs) mediate sucrose phloem loading in source tissue and sucrose unloading into sink tissue in potatoes and higher plants, thus playing a crucial role in plant growth and development. In potatoes, the physiological function of the sucrose transporters StSUT1 and StSUT4 has been clarified, whereas the physiological role of StSUT2 is not yet fully understood.

METHODS AND RESULTS

This study analyzed the relative expression of StSUT2 compared to that of StSUT1 and StSUT4 in different tissues from potatoes and its impact on different physiological characteristics by using StSUT2-RNA interference lines. Here, we report a negative effect of StSUT2-RNA interference on plant height, fresh weight, internodes number, leaf area, flowering time, and tuber yield. However, our data indicate that StSUT2 is not involved in carbohydrate accumulation in potato leaves and tubers. In addition, the data of the RNA-seq between the StSUT2-RNA interference line and WT showed that 152 genes were differentially expressed, of which 128 genes were upregulated and 24 genes were downregulated, and the GO and KEGG analyses revealed that differentially expressed genes were mainly related to cell wall composition metabolism.

CONCLUSIONS

Thus, StSUT2 functions in potato plant growth, flowering time, and tuber yield without affecting carbohydrate accumulation in the leaves and tubers but may be involved in cell wall composition metabolism.

Elucidation of the interactome of the sucrose transporter StSUT4: sucrose transport is connected to ethylene and calcium signalling

Sucrose transporters of the SUT4 clade show dual targeting to both the plasma membrane as well as to the vacuole. Previous investigations revealed a role for the potato sucrose transporter StSUT4 in flowering, tuberization, shade avoidance response, and ethylene production. Down-regulation of StSUT4 expression leads to early flowering, tuberization under long days, far-red light insensitivity, and reduced diurnal ethylene production. Sucrose export from leaves was increased and a phase-shift of soluble sugar accumulation in source leaves was observed, arguing for StSUT4 to be involved in the entrainment of the circadian clock. Here, we show that StSUT4, whose transcripts are highly unstable and tightly controlled at the post-transcriptional level, connects components of the ethylene and calcium signalling pathway. Elucidation of the StSUT4 interactome using the split ubiquitin system helped to prove direct physical interaction between the sucrose transporter and the ethylene receptor ETR2, as well as with the calcium binding potato calmodulin-1 (PCM1) protein, and a calcium-load activated calcium channel. The impact of calcium ions on transport activity and dual targeting of the transporter was investigated in detail. For this purpose, a reliable esculin-based transport assay was established for SUT4-like transporters. Site-directed mutagenesis helped to identify a diacidic motif within the seventh transmembrane spanning domain that is essential for sucrose transport activity and targeting, but not required for calcium-dependent inhibition. A link between sucrose, calcium and ethylene signalling has been previously postulated with respect to pollen tube growth, shade avoidance response, or entrainment of the circadian clock. Here, we provide experimental evidence for the direct interconnection of these signalling pathways at the molecular level by direct physical interaction of the main players.

Keeping time in the dark: Potato diel and circadian rhythmic gene expression reveals tissue-specific circadian clocks

The circadian clock is an internal molecular oscillator and coordinates numerous physiological processes through regulation of molecular pathways. Tissue-specific clocks connected by mobile signals have previously been found to run at different speeds in Arabidopsis thaliana tissues. However, tissue variation in circadian clocks in crop species is unknown. In this study, leaf and tuber global gene expression in cultivated potato under cycling and constant environmental conditions was profiled. In addition, we used a circadian-regulated luciferase reporter construct to study tuber gene expression rhythms. Diel and circadian expression patterns were present among 17.9% and 5.6% of the expressed genes in the tuber. Over 500 genes displayed differential tissue specific diel phases. Intriguingly, few core circadian clock genes had circadian expression patterns, while all such genes were circadian rhythmic in cultivated tomato leaves. Furthermore, robust diel and circadian transcriptional rhythms were observed among detached tubers. Our results suggest alternative regulatory mechanisms and/or clock composition is present in potato, as well as the presence of tissue-specific independent circadian clocks. We have provided the first evidence of a functional circadian clock in below-ground storage organs, holding important implications for other storage root and tuberous crops.

Subcellular dynamics and protein-protein interactions of plant sucrose transporters

Although extensively studied for their role in long distance transport, plant sucrose transporters are active not only in the phloem but throughout the plant body. Sucrose transporters of the SUT family were first described to be plasma membrane-resident proteins, but recent investigations revealed that subcellular dynamics of these transporters were part of complex regulatory mechanisms. The yeast two-hybrid split-ubiquitin system, tandem-affinity purification, and bimolecular-fluorescence complementation aided in identification of a complex network of SUT-interacting proteins that led to answers to many open questions. We found, for example, interacting proteins localized to other subcellular compartments. Although sucrose transporters were assumed to be localized mainly on the plasma membrane, and the tonoplast in the case of SUT4, the interaction partners were not exclusively predicted to be plasma membrane proteins, but belonged to the extracellular space (cell wall), intracellular vesicles, the ER, tonoplast, nuclei, and peroxisomes, among other cellular compartments. A subset of the SUT-interacting proteins localized exclusively to plasmodesmata. We conclude that (transient) protein-protein interactions of integral membrane proteins help to sequester SUTs to subcellular compartments, such as membrane microdomains, with specific functions to enable subcellular transport and cell-to-cell trafficking via plasmodesmata. Identification of SNARE proteins (soluble N-ethylmaleimide-sensitive factor protein attachment protein receptors) and protein disulfide isomerases support the assumption that the protein-protein interaction plays an important role for the subcellular movement of sugar transporters. It becomes apparent that the interaction partners provide a substantial impact on how and where the transporter is localized or processed for either targeting to a specific cellular or extracellular location, or tagging for degradation or recycling. In this review, interacting proteins, as well as the role of oligomeric complex formation, post-translational modification, and stress responses are summarized for SUTs of higher plants.

Genome-wide analyses of the Nodulin-like gene family in bread wheat revealed its potential roles during arbuscular mycorrhizal symbiosis

Nodulin-like (NL) genes are involved in transporting of various substances and may play key roles during the establishment of symbiosis in legumes plants. However, basic biological information of NL genes in the wheat genome is still largely unknown. Here, we identified and characterized NL genes in wheat via integrating genomic information, collinearity analysis, co-expression network analysis (WGCNA) and transcriptome analysis. In addition, we analyzed the polymorphisms and the roles of NL genes during arbuscular mycorrhizal (AM) symbiosis using a large wheat panel consists of 259 wheat genotypes. We identified 181 NL genes in the wheat genome, which were classified into SWEET, Early Nodulin-Like (ENODL), Major Facilitator Superfamily-Nodulin (MFS), Vacuolar Iron Transporter (VIT) and Early nodulin 93 (ENOD93) subfamily. The expansion of NL genes was mainly driven by segmental duplication. The bHLH genes are potential unrecognized transcription factors regulating NL genes. Moreover, two NL genes were more sensitive than other NL genes to AM colonization. The polymorphisms of NL genes are mainly due to random drift, and the natural mutation of NL genes led to significant differences in the mycorrhizal dependence of wheat in phosphorus uptake. The results concluded that NL genes potentially play important roles during AM symbiosis with wheat.

Hetero/Homo-Complexes of Sucrose Transporters May Be a Subtle Mode to Regulate Sucrose Transportation in Grape Berries

The sugar distribution mechanism in fruits has been the focus of research worldwide; however, it remains unclear. In order to elucidate the relevant mechanisms in grape berries, the expression, localization, function, and regulation of three sucrose transporters were studied in three representative Vitis varieties. Both SUC11 and SUC12 expression levels were positively correlated with sugar accumulation in grape berries, whereas SUC27 showed a negative relationship. The alignment analysis and sucrose transport ability of isolated SUCs were determined to reflect coding region variations among V. vinifera, V. amurensis Ruper, and V. riparia, indicating that functional variation existed in one SUT from different varieties. Furthermore, potentially oligomerized abilities of VvSUCs colocalized in the sieve elements of the phloem as plasma membrane proteins were verified. The effects of oligomerization on transport properties were characterized in yeast. VvSUC11 and VvSUC12 are high-affinity/low-capacity types of SUTs that stimulate each other by upregulating V_max and K_m, inhibiting sucrose transport, and downregulating the K_m of VvSUC27. Thus, changes in the distribution of different SUTs in the same cell govern functional regulation. The activation and inhibition of sucrose transport could be achieved in different stages and tissues of grape development to achieve an effective distribution of sugar.

Sugar transporters in grasses: Function and modulation in source and storage tissues

Carbohydrate partitioning, the process of transporting carbohydrates from photosynthetic (source) tissues, such as leaves, to non-photosynthetic (sink) tissues, such as stems, roots, and reproductive structures, is vital not only for the growth and development of plants but also for withstanding biotic and abiotic stress. In many plants, sucrose is the primary form of carbohydrate loaded into the phloem for long-distance transport and unloaded into the sink tissues for utilization or storage. We highlight recent findings about 1) phloem loading in grasses, 2) the principal families of sugar transporters involved in sucrose transport, and 3) novel mechanisms by which the activities of sugar transporters are modulated. We discuss exciting discoveries from eudicot species that provide valuable insights regarding the regulation of these sugar transporters, which may be translatable to monocot species. As we better understand the intricate pathways that control the activities of various sugar transporters, we can utilize this knowledge for developing improved crop varieties.

Phylogenetic relationships of sucrose transporters (SUTs) in plants and genome-wide characterization of SUT genes in Orchidaceae reveal roles in floral organ development

Sucrose is the primary form of photosynthetically produced carbohydrates transported long distance in many plant species and substantially affects plant growth, development and physiology. Sucrose transporters (SUTs or SUCs) are a group of membrane proteins that play vital roles in mediating sucrose allocation within cells and at the whole-plant level. In this study, we investigated the relationships among SUTs in 24 representative plant species and performed an analysis of SUT genes in three sequenced Orchidaceae species: Dendrobium officinale, Phalaenopsis equestris, and Apostasia shenzhenica. All the SUTs from the 24 plant species were classified into three groups and five subgroups, subgroups A, B1, B2.1, B2.2, and C, based on their evolutionary relationships. A total of 22 SUT genes were identified among Orchidaceae species, among which D. officinale had 8 genes (DoSUT01-08), P. equestris had eight genes (PeqSUT01-08) and A. shenzhenica had 6 genes (AsSUT01-06). For the 22 OrchidaceaeSUTs, subgroups A, B2.2 and C contained three genes, whereas the SUT genes were found to have significantly expanded in the monocot-specific subgroup B2.1, which contained 12 genes. To understand sucrose partitioning and the functions of sucrose transporters in Orchidaceae species, we analyzed the water-soluble sugar content and performed RNA sequencing of different tissues of D. officinale, including leaves, stems, flowers and roots. The results showed that although the total content of water-soluble polysaccharides was highest in the stems of D. officinale, the sucrose content was highest in the flowers. Moreover, gene expression analysis showed that most of the DoSUTs were expressed in the flowers, among which DoSUT01,DoSUT07 and DoSUT06 had significantly increased expression levels. These results indicated that stems are used as the main storage sinks for photosynthetically produced sugar in D. officinale and that DoSUTs mainly function in the cellular machinery and development of floral organs. Our findings provide valuable information on sucrose partitioning and the evolution and functions of SUT genes in Orchidaceae and other species.

Seasonal changes in temperate woody plant phloem anatomy and physiology: implications for long-distance transport

Seasonal changes in climate are accompanied by shifts in carbon allocation and phenological changes in woody angiosperms, the timing of which can have broad implications for species distributions, interactions and ecosystem processes. During critical transitions from autumn to winter and winter to spring, physiological and anatomical changes within the phloem could impose a physical limit on the ability of woody angiosperms to transport carbon and signals. There is a paucity of the literature that addresses tree (floral or foliar) phenology, seasonal phloem anatomy and seasonal phloem physiology together, so our knowledge of how carbon transport could fluctuate seasonally, especially in temperate climates is limited. We review phloem phenology focussing on how sieve element anatomy and phloem sap flow could affect carbon availability throughout the year with a focus on winter. To investigate whether flow is possible in the winter, we construct a simple model of phloem sap flow and investigate how changes to the sap concentration, pressure gradient and sieve plate pores could influence flow during the winter. Our model suggests that phloem transport in some species could occur year-round, even in winter, but current methods for measuring all the parameters surrounding phloem sap flow make it difficult to test this hypothesis. We highlight outstanding questions that remain about phloem functionality in the winter and emphasize the need for new methods to address gaps in our knowledge about phloem function.

Environmental and developmental factors driving xylem anatomy and micro-density in black spruce

Wood density is the product of carbon allocation for structural growth and reflects the trade-off between mechanical support and water conductivity. We tested a conceptual framework based on the assumption that micro-density depends on direct and indirect relationships with endogenous and exogenous factors. The dynamics of wood formation, including timings and rates of cell division, cell enlargement, and secondary wall deposition, were assessed from microcores collected weekly between 2002 and 2016 from five black spruce stands located along a latitudinal gradient in Quebec, Canada. Cell anatomy and micro-density were recorded by anatomical analyses and X-ray measurements. Our structural equation model explained 80% of micro-density variation within the tree-ring with direct effects of wall thickness (σ = 0.61), cell diameter (σ = -0.51), and photoperiod (σ = -0.26). Wood formation dynamics had an indirect effect on micro-density. Micro-density increased under longer periods of cell-wall deposition and shorter durations of enlargement. Our results fill a critical gap in understanding the relationships underlying micro-density variation in conifers. We demonstrated that short-term responses to environmental variations could be overridden by plastic responses that modulate cell differentiation. Our results point to wood formation dynamics as a reliable predictor of carbon allocation in trees.

Genetic diversity and population structure of advanced clones selected over forty years by a potato breeding program in the USA

Knowledge regarding genetic diversity and population structure of breeding materials is essential for crop improvement. The Texas A&M University Potato Breeding Program has a collection of advanced clones selected and maintained in-vitro over a 40-year period. Little is known about its genetic makeup and usefulness for the current breeding program. In this study, 214 potato clones were genotyped with the Infinium Illumina 22 K V3 Potato Array. After filtering, a total of 10,106 single nucleotide polymorphic (SNP) markers were used for analysis. Heterozygosity varied by SNP, with an overall average of 0.59. Three groups of tetraploid clones primarily based on potato market classes, were detected using STRUCTURE software and confirmed by discriminant analysis of principal components. The highest coefficient of differentiation observed between the groups was 0.14. Signatures of selection were uncovered in genes controlling potato flesh and skin color, length of plant cycle and tuberization, and carbohydrate metabolism. A core set of 43 clones was obtained using Core Hunter 3 to develop a sub-collection that retains similar genetic diversity as the whole population, minimize redundancies, and facilitates long-term conservation of genetic resources. The comprehensive molecular characterization of our breeding clone bank collection contributes to understanding the genetic diversity of existing potato resources. This analysis could be applied to other breeding programs and assist in the selection of parents, fingerprinting, protection, and management of the breeding collections.

PHOSPHATIDYLETHANOLAMINE-BINDING PROTEINS: the conductors of dual reproduction in plants with vegetative storage organs

Geophytes, the plants that form vegetative storage organs, are characterized by a dual reproduction system, in which vegetative and sexual propagation are tightly regulated to ensure fitness in harsh climatic conditions. Recent findings highlight the role of the PEBP (PHOSPHATIDYLETHANOLAMINE-BINDING PROTEIN) gene family in geophytes as major players in the molecular cascades underlying both types of reproduction. In this review, we briefly explain the life cycle and reproduction strategies of different geophytes and what is known about the physiological aspects related to these processes. Subsequently, an in-depth overview is provided of the molecular and genetic pathways driving these processes. In the evolution of plants, the PEBP gene family has expanded, followed by neo- and subfunctionalization. Careful characterization revealed that differential expression and differential protein complex formation provide the members of this gene family with unique functions, enabling them to mediate the crosstalk between the two reproductive events in geophytes in response to environmental and endogenous cues. Taking all these studies into account, we propose to regard the PEBPs as conductors of geophyte reproductive development.

Sugar and Nitrate Sensing: A Multi-Billion-Year Story

Sugars and nitrate play a major role in providing carbon and nitrogen in plants. Understanding how plants sense these nutrients is crucial, most notably for crop improvement. The mechanisms underlying sugar and nitrate sensing are complex and involve moonlighting proteins such as the nitrate transporter NRT1.1/NFP6.3 or the glycolytic enzyme HXK1. Major components of nutrient signaling, such as SnRK1, TOR, and HXK1, are relatively well conserved across eukaryotes, and the diversification of components such as the NRT1 family and the SWEET sugar transporters correlates with plant terrestrialization. In plants, Tre6P plays a hormone-like role in plant development. In addition, nutrient signaling has evolved to interact with the more recent hormone signaling, allowing fine-tuning of physiological and developmental responses.

Expression Level of Mature miR172 in Wild Type and StSUT4-Silenced Plants of Solanum tuberosum Is Sucrose-Dependent

In potato plants, the phloem-mobile miR172 is involved in the sugar-dependent transmission of flower and tuber inducing signal transduction pathways and a clear link between solute transport and the induction of flowering and tuberization was demonstrated. The sucrose transporter StSUT4 seems to play an important role in the photoperiod-dependent triggering of both developmental processes, flowering and tuberization, and the phenotype of StSUT4-inhibited potato plants is reminiscent to miR172 overexpressing plants. The first aim of this study was the determination of the level of miR172 in sink and source leaves of StSUT4-silenced as well as StSUT4-overexpressing plants in comparison to Solanum tuberosum ssp. Andigena wild type plants. The second aim was to investigate the effect of sugars on the level of miRNA172 in whole cut leaves, as well as in whole in vitro plantlets that were supplemented with exogenous sugars. Experiments clearly show a sucrose-dependent induction of the level of mature miR172 in short time as well as long time experiments. A sucrose-dependent accumulation of miR172 was also measured in mature leaves of StSUT4-silenced plants where sucrose export is delayed and sucrose accumulates at the end of the light period.

Hormonal and transcriptional analyses provides new insights into the molecular mechanisms underlying root thickening and isoflavonoid biosynthesis in Callerya speciosa (Champ. ex Benth.) Schot

Callerya speciosa (Champ. ex Benth.) Schot is a traditional Chinese medicine characterized by tuberous roots as the main organ of isoflavonoid accumulation. Root thickening and isoflavonoid accumulation are two major factors for yield and quality of C. speciosa. However, the underlying mechanisms of root thickening and isoflavonoid biosynthesis have not yet been elucidated. Here, integrated morphological, hormonal and transcriptomic analyses of C. speciosa tuberous roots at four different ages (6, 12, 18, 30 months after germination) were performed. The growth cycle of C. speciosa could be divided into three stages: initiation, rapid-thickening and stable-thickening stage, which cued by the activity of vascular cambia. Endogenous changes in phytohormones were associated with developmental changes during root thickening. Jasmonic acid might be linked to the initial development of tuberous roots. Abscisic acid seemed to be essential for tuber maturation, whereas IAA, cis-zeatin and gibberellin 3 were considered essential for rapid thickening of tuberous roots. A total of 4337 differentially expressed genes (DEGs) were identified during root thickening, including 15 DEGs participated in isoflavonoid biosynthesis, and 153 DEGs involved in starch/sucrose metabolism, hormonal signaling, transcriptional regulation and cell wall metabolism. A hypothetical model of genetic regulation associated with root thickening and isoflavonoid biosynthesis in C. speciosa is proposed, which will help in understanding the underlying mechanisms of tuberous root formation and isoflavonoid biosynthesis.

GeSUT4 mediates sucrose import at the symbiotic interface for carbon allocation of heterotrophic Gastrodia elata (Orchidaceae)

Gastrodia elata, a fully mycoheterotrophic orchid without photosynthetic ability, only grows symbiotically with the fungus Armillaria. The mechanism of carbon distribution in this mycoheterotrophy is unknown. We detected high sucrose concentrations in all stages of Gastrodia tubers, suggesting sucrose may be the major sugar transported between fungus and orchid. Thick symplasm-isolated wall interfaces in colonized and adjacent large cells implied involvement of sucrose importers. Two sucrose transporter (SUT)-like genes, GeSUT4 and GeSUT3, were identified that were highly expressed in young Armillaria-colonized tubers. Yeast complementation and isotope tracer experiments confirmed that GeSUT4 functioned as a high-affinity sucrose-specific proton-dependent importer. Plasma-membrane/tonoplast localization of GeSUT4-GFP fusions and high RNA expression of GeSUT4 in symbiotic and large cells indicated that GeSUT4 likely functions in active sucrose transport for intercellular allocation and intracellular homeostasis. Transgenic Arabidopsis overexpressing GeSUT4 had larger leaves but were sensitive to excess sucrose and roots were colonized with fewer mutualistic Bacillus, supporting the role of GeSUT4 in regulating sugar allocation. This is not only the first documented carbon import system in a mycoheterotrophic interaction but also highlights the evolutionary importance of sucrose transporters for regulation of carbon flow in all types of plant-microbe interactions.

Sugar and Hormone Dynamics and the Expression Profiles of SUT/SUC and SWEET Sweet Sugar Transporters during Flower Development in Petunia axillaris

Flowering is the first committed step of plant sexual reproduction. While the developing flower is a strong sink requiring large quantity of sugars from photosynthetic source tissues, this process is under-temper-spatially controlled via hormone signaling pathway and nutrient availability. Sugar transporters SUT/SUC and SWEET mediate sugars movement across membranes and play a significant role in various physiological processes, including reproductive organ development. In Petunia axillaris, a model ornamental plant, 5 SUT/SUC and 36 SWEET genes are identified in the current version of the genome. Analysis of their gene structure and chromosomal locations reveal that SWEET family is moderately expanded. Most of the transporter genes are abundantly expressed in the flower than in other organs. During the five flower developmental stages, transcript levels of PaSUT1, PaSUT3, PaSWEET13c, PaSWEET9a, PaSWEET1d, PaSWEET5a and PaSWEET14a increase with the maturation of the flower and reach their maximum in the fully open flowers. PaSWEET9c, the nectar-specific PhNEC1 orthologous, is expressed in matured and fully opened flowers. Moreover, determination of sugar concentrations and phytohormone dynamics in flowers at the five developmental stages shows that glucose is the predominant form of sugar in young flowers at the early stage but depletes at the later stage, whereas sucrose accumulates only in maturated flowers prior to the corolla opening. On the other hand, GA₃ content and to a less extent IAA and zeatin decreases with the flower development; however, JA, SA and ABA display a remarkable peak at mid- or later flower developmental stage.

What determines the composition of the phloem sap? Is there any selectivity filter for macromolecules entering the phloem sieve elements?

In view of recent findings, it is still a matter of debate whether the composition of the phloem sap of higher plants is specific and based on a plasmodesmal selectivity filter for macromolecular transport, or whether simply related to size, abundance and half-life of the macromolecules within the phloem sap. A range of reports indicates specific function of phloem-mobile signaling molecules such as the florigen making it indispensable to discriminate specific macromolecules entering the phloem from others which cannot cross this selectivity filter. Nevertheless, several findings have discussed for a non-selective transport via plasmodesmata, or contamination of the phloem sap by degradation products coming from immature still developing young sieve elements undergoing differentiation. Here, we discuss several possibilities, and raise the question how selectivity of the phloem sap composition could be achieved thereby focusing on mobility and dynamics of sucrose transporter mRNA and proteins.

The sucrose transporter MdSUT4.1 participates in the regulation of fruit sugar accumulation in apple

BACKGROUND

Sugar content is an important determinant of fruit sweetness, but details on the complex molecular mechanism underlying fruit sugar accumulation remain scarce. Here, we report the role of sucrose transporter (SUT) family in regulating fruit sugar accumulation in apple.

RESULTS

Gene-tagged markers were developed to conduct candidate gene-based association study, and an SUT4 member MdSUT4.1 was found to be significantly associated with fruit sugar accumulation. MdSUT4.1 encodes a tonoplast localized protein and its expression level had a negative correlation with fruit sugar content. Overexpression of MdSUT4.1 in strawberry and apple callus had an overall negative impact on sugar accumulation, suggesting that it functions to remobilize sugar out of the vacuole. In addition, MdSUT4.1 is located on chromosomal region harboring a previously reported QTL for sugar content, suggesting that it is a candidate gene for fruit sugar accumulation in apple.

CONCLUSIONS

MdSUT4.1 is involved in the regulation of fruit sugar accumulation in apple. This study is not only helpful for understanding the complex mechanism of fruit sugar accumulation, but it also provides molecular tools for genetic improvement of fruit quality in breeding programs of apple.

Transcriptome Analysis of Carbohydrate Metabolism Genes and Molecular Regulation of Sucrose Transport Gene LoSUT on the Flowering Process of Developing Oriental Hybrid Lily 'Sorbonne' Bulb

The quality of Lily cut flower was determined by the quality of bulbs. During the process of vernalization and flower bud differentiation, sugar massively accumulated in the bulb, which influenced the bulb development. However, the details of sugar genes’ regulation mechanism for these processes were not fully understood. Here, morphological physiology, transcriptomes and gene engineering technology were used to explore this physiological change. Seventy-two genes of 25 kinds of sugar metabolism-related genes were annotated after re-analyzing transcriptome data of Oriental hybrid lily ‘Sorbonne’ bulbs, which were generated on Hiseq Illumina 2000. The results showed that these genes were closely related to lily bulb vernalization and development. Combining gene expression pattern with gene co-expression network, five genes (Contig5669, Contig13319, Contig7715, Contig1420 and Contig87292) were considered to be the most potential signals, and the sucrose transporter gene (SUT) was the focus of this study. Carbohydrate transport pathway and genes’ regulation mechanism were inferred through a physiological and molecular test. SUT seemed to be the sugar sensor that could sense and regulate sugar concentration, which might have effects on other genes, such as FT, LFY and so on. LoSUT2 and LoSUT4 genes were cloned from Oriental hybrid lily ‘Sorbonne’ by RACE, which was the first time for these genes in Oriental hybrid lily ‘Sorbonne’. The physiological properties of these proteins were analyzed such as hydrophobicity and phosphorylation. In addition, secondary and tertiary structures of proteins were predicted, which indicated the two proteins were membrane proteins. Their cellular locations were verified through positioning the experiment of the fluorescent vector. They were highly expressed in cells around phloem, which illustrated the key role of these genes in sugar transport. Furthermore, transient expression assays showed that overexpressed LoSUT2 and LoSUT4 in Arabidopsis thaliana bloomed significantly earlier than the wild type and the expression of FT, SOC1 and LFY were also affected by LoSUT2 and LoSUT4, which indicated that LoSUT2 and LoSUT4 may regulate plants flowering time.

A novel sucrose transporter gene IbSUT4 involves in plant growth and response to abiotic stress through the ABF-dependent ABA signaling pathway in Sweetpotato

BACKGROUND

To maintain sweetpotato (Ipomoea batatas (L.) Lam) growth and yield, sucrose must be transported from the leaves to the roots. Sucrose transporters or carriers (SUTs or SUCs) transport sucrose and are involved in plant growth and response to abiotic stress. However, the mechanisms of SUTs in sweetpotato abiotic stress resistance remains to be determined.

RESULTS

In the present study, we cloned a novel IbSUT4 gene; the protein encoded by this gene is localized in the tonoplast and plasma membrane. The plant growth was promoted in the IbSUT4 transgenic Arabidopsis thaliana lines, with increased expression of AtFT, a regulator of flowering time in plants. Over-expression of IbSUT4 in Arabidopsis thaliana resulted in higher sucrose content in the roots and lower sucrose content in the leaves, as compared to the wild-type (WT) plants, leading to improved stress tolerance during seedling growth. Moreover, we systematically analyzed the mechanisms of IbSUT4 in response to abiotic stress. The results suggest that the ABRE-motif was localized in the IbSUT4 promoter region, and the expression of the ABA signaling pathway genes (i.e., ABF2, ABF4, SnRK2.2, SnRK2.3, and PYL8/RCAR3) were induced, and the expression of ABI1 was inhibited.

CONCLUSIONS

Our dates provide evidence that IbSUT4 is not only involved in plant growth but also is an important positive regulator in plant stress tolerance through the ABF-dependent ABA signaling pathway.

Expression of Sucrose Transporters from Vitis vinifera Confer High Yield and Enhances Drought Resistance in Arabidopsis

Sucrose is the predominant form of sugar transported from photosynthetic (source) to non-photosynthetic (sink) organs in higher plants relying on the transporting function of sucrose transporters (SUTs or SUCs). Many SUTs have been identified and characterized in both monocots and dicots. However, the function of sucrose transporters (SUTs or SUCs) from Vitis is not clear. As the world’s most planted grape species, Vitis vinifera owns three sucrose transport activity verified SUTs. In this study, we constructed three kinds of VvSUC (Vitis vinifera SUC)-overexpressing transgenic Arabidopsis. VvSUC-overexpressing transgenic Arabidopsis was cultured on sucrose-supplemented medium. VvSUC11- and VvSUC12-overexpressing lines had similar thrived growth phenotypes, whereas the size and number of leaves and roots from VvSUC27-overexpressing lines were reduced compared with that of WT. When plants were cultured in soil, all SUT transgenic seedlings produced more number of leaves and siliques, resulting in higher yield (38.6% for VvSUC12-transformants) than that of WT. Besides, VvSUC27-transformants and VvSUC11-transformants enhanced drought resistance in Arabidopsis, providing a promising target for crop improvement

Subcellular Targeting of Plant Sucrose Transporters Is Affected by Their Oligomeric State

Post-translational regulation of sucrose transporters represents one possibility to adapt transporter activity in a very short time frame. This can occur either via phosphorylation/dephosphorylation, oligomerization, protein-protein interactions, endocytosis/exocytosis, or degradation. It is also known that StSUT1 can change its compartmentalization at the plasma membrane and concentrate in membrane microdomains in response to changing redox conditions. A systematic screen for protein-protein-interactions of plant sucrose transporters revealed that the interactome of all three known sucrose transporters from the Solanaceous species Solanum tuberosum and Solanum lycopersicum represents a specific subset of interaction partners, suggesting different functions for the three different sucrose transporters. Here, we focus on factors that affect the subcellular distribution of the transporters. It was already known that sucrose transporters are able to form homo- as well as heterodimers. Here, we reveal the consequences of homo- and heterodimer formation and the fact that the responses of individual sucrose transporters will respond differently. Sucrose transporter SlSUT2 is mainly found in intracellular vesicles and several of its interaction partners are involved in vesicle traffic and subcellular targeting. The impact of interaction partners such as SNARE/VAMP proteins on the localization of SlSUT2 protein will be investigated, as well as the impact of inhibitors, excess of substrate, or divalent cations which are known to inhibit SUT1-mediated sucrose transport in yeast cells. Thereby we are able to identify factors regulating sucrose transporter activity via a change of their subcellular distribution.

Genome-wide identification of MST, SUT and SWEET family sugar transporters in root parasitic angiosperms and analysis of their expression during host parasitism

BACKGROUND

Root parasitic weeds are a major constraint to crop production worldwide causing significant yearly losses in yield and economic value. These parasites cause their destruction by attaching to their hosts with a unique organ, the haustorium, that allows them to obtain the nutrients (sugars, amino acids, etc.) needed to complete their lifecycle. Parasitic weeds differ in their nutritional requirements and degree of host dependency and the differential expression of sugar transporters is likely to be a critical component in the parasite's post-attachment survival.

RESULTS

We identified gene families encoding monosaccharide transporters (MSTs), sucrose transporters (SUTs), and SWEETs (Sugars Will Eventually be Exported Transporters) in three root-parasitic weeds differing in host dependency: Triphysaria versicolor (facultative hemiparasite), Phelipanche aegyptiaca (holoparasite), and Striga hermonthica (obligate hemiparasite). The phylogenetic relationship and differential expression profiles of these genes throughout parasite development were examined to uncover differences existing among parasites with different levels of host dependence. Differences in estimated gene numbers are found among the three parasites, and orthologs within the different sugar transporter gene families are found to be either conserved among the parasites in their expression profiles throughout development, or to display parasite-specific differences in developmentally-timed expression. For example, MST genes in the pGLT clade express most highly before host connection in Striga and Triphysaria but not Phelipanche, whereas genes in the MST ERD6-like clade are highly expressed in the post-connection growth stages of Phelipanche but highest in the germination and reproduction stages in Striga. Whether such differences reflect changes resulting from differential host dependence levels is not known.

CONCLUSIONS

While it is tempting to speculate that differences in estimated gene numbers and expression profiles among members of MST, SUT and SWEET gene families in Phelipanche, Striga and Triphysaria reflect the parasites' levels of host dependence, additional evidence that altered transporter gene expression is causative versus consequential is needed. Our findings identify potential targets for directed manipulation that will allow for a better understanding of the nutrient transport process and perhaps a means for controlling the devastating effects of these parasites on crop productivity.

Seasonal changes of sucrose transporter expression and sugar partitioning in common European tree species

In temperate woody species, carbon transport from source to sink tissues is a striking physiological process, particularly considering seasonal changes. The functions of different tissues can also alternate across the seasons. In this regard, phloem loading and sugar distribution are important aspects of carbon partitioning, and sucrose uptake transporters (SUTs) play a key role in these processes. Therefore, the influence of seasons and different light-dark conditions on the expression of SUTs from 3-year-old Fagus sylvatica L., Quercus robur L. and Picea abies (L.) Karst. trees were analyzed. In addition, tissue-specific sugar and starch contents under these different environmental conditions were determined. Putative SUTs were identified in the gymnosperms (Picea abies, Ginkgo biloba L.), here for the first time, and also in the angiosperms (Q. robur, F. sylvatica). The identified SUT sequences of the different tree species cluster into three types, similar to other SUTs from herbaceous and tree species. Furthermore, the sequences from angiosperm and those from gymnosperm species form distinct clusters within the three types of SUTs. In F. sylvatica, Q. robur and P. abies, the expression levels of the different SUTs during seasons showed marked variations. Because of the high expression levels of type I SUTs in bark, wood and leaves during active growing phases in spring and summer, it can be assumed that they are involved in phloem loading, sucrose retrieval and possibly in further physiological processes. The expression patterns also indicate a flexible expression in all tissues depending on physiological requirements and environmental conditions. Compared with type I SUTs, the seasonal variations of type II SUT expression were less pronounced, whereas the seasonal variations of the type III SUT expression patterns were partly reverse. In addition to the seasonal regulation, the expressions of the different SUTs were also regulated by light in a diurnal manner.

New insights into the evolution and functional divergence of the SWEET family in Saccharum based on comparative genomics

BACKGROUND

The SWEET (Sugars Will Eventually be Exported Transporters) gene family is a recently identified group of sugar transporters that play an indispensable role in sugar efflux, phloem loading, plant-pathogen interaction, nectar secretion, and reproductive tissue development. However, little information on Saccharum SWEET is available for this crop with a complex genetic background.

RESULTS

In this study, 22 SWEET genes were identified from Saccharum spontaneum Bacterial Artificial Chromosome libraries sequences. Phylogenetic analyses of SWEETs from 11 representative plant species showed that gene expansions of the SWEET family were mainly caused by the recent gene duplication in dicot plants, while these gene expansions were attributed to the ancient whole genome duplication (WGD) in monocot plant species. Gene expression profiles were obtained from RNA-seq analysis. SWEET1a and SWEET2s had higher expression levels in the transitional zone and maturing zone than in the other analyzed zones. SWEET1b was mainly expressed in the leaf tissues and the mature zone of the leaf of both S. spontaneum and S. officinarum, and displayed a peak in the morning and was undetectable in both sclerenchyma and parenchyma cells from the mature stalks of S. officinarum. SsSWEET4a\4b had higher expression levels than SWEET4c and were mainly expressed in the stems of seedlings and mature plants. SWEET13s are recently duplicated genes, and the expression of SWEET13s dramatically increased from the maturing to mature zones. SWEET16b's expression was not detected in S. officinarum, but displayed a rhythmic diurnal expression pattern.

CONCLUSIONS

Our study revealed the gene evolutionary history of SWEETs in Saccharum and SWEET1b was found to be a sucrose starvation-induced gene involved in the sugar transportation in the high photosynthetic zones. SWEET13c was identified as the key player in the efflux of sugar transportation in mature photosynthetic tissues. SWEET4a\4b were found to be mainly involved in sugar transportation in the stalk. SWEET1a\2a\4a\4b\13a\16b were suggested to be the genes contributing to the differences in sugar contents between S. spontaneum and S. officinarum. Our results are valuable for further functional analysis of SWEET genes and utilization of the SWEET genes for genetic improvement of Saccharum for biofuel production.

Molecular Evolution and Stress and Phytohormone Responsiveness of SUT Genes in Gossypium hirsutum

Sucrose transporters (SUTs) play key roles in allocating the translocation of assimilates from source to sink tissues. Although the characteristics and biological roles of SUTs have been intensively investigated in higher plants, this gene family has not been functionally characterized in cotton. In this study, we performed a comprehensive analysis of SUT genes in the tetraploid cotton Gossypium hirsutum. A total of 18 G. hirsutum SUT genes were identified and classified into three groups based on their evolutionary relationships. Up to eight SUT genes in G. hirsutum were placed in the dicot-specific SUT1 group, while four and six SUT genes were, respectively, clustered into SUT4 and SUT2 groups together with members from both dicot and monocot species. The G. hirsutum SUT genes within the same group displayed similar exon/intron characteristics, and homologous genes in G. hirsutum At and Dt subgenomes, G. arboreum, and G. raimondii exhibited one-to-one relationships. Additionally, the duplicated genes in the diploid and polyploid cotton species have evolved through purifying selection, suggesting the strong conservation of SUT loci in these species. Expression analysis in different tissues indicated that SUT genes might play significant roles in cotton fiber elongation. Moreover, analyses of cis-acting regulatory elements in promoter regions and expression profiling under different abiotic stress and exogenous phytohormone treatments implied that SUT genes, especially GhSUT6A/D, might participate in plant responses to diverse abiotic stresses and phytohormones. Our findings provide valuable information for future studies on the evolution and function of SUT genes in cotton.

Plant Desiccation Tolerance and its Regulation in the Foliage of Resurrection “Flowering-Plant” Species

The majority of flowering-plant species can survive complete air-dryness in their seed and/or pollen. Relatively few species (‘resurrection plants’) express this desiccation tolerance in their foliage. Knowledge of the regulation of desiccation tolerance in resurrection plant foliage is reviewed. Elucidation of the regulatory mechanism in resurrection grasses may lead to identification of genes that can improve stress tolerance and yield of major crop species. Well-hydrated leaves of resurrection plants are desiccation-sensitive and the leaves become desiccation tolerant as they are drying. Such drought-induction of desiccation tolerance involves changes in gene-expression causing extensive changes in the complement of proteins and the transition to a highly-stable quiescent state lasting months to years. These changes in gene-expression are regulated by several interacting phytohormones, of which drought-induced abscisic acid (ABA) is particularly important in some species. Treatment with only ABA induces desiccation tolerance in vegetative tissue of Borya constricta Churchill. and Craterostigma plantagineum Hochstetter. but not in the resurrection grass Sporobolus stapfianus Gandoger. Suppression of drought-induced senescence is also important for survival of drying. Further research is needed on the triggering of the induction of desiccation tolerance, on the transition between phases of protein synthesis and on the role of the phytohormone, strigolactone and other potential xylem-messengers during drying and rehydration.

The FBH family of bHLH transcription factors controls ACC synthase expression in sugarcane

Ethylene is a phytohormone involved in the regulation of several aspects of plant development and in responses to biotic and abiotic stress. The effects of exogenous application of ethylene to sugarcane plants are well characterized as growth inhibition of immature internodes and stimulation of sucrose accumulation. However, the molecular network underlying the control of ethylene biosynthesis in sugarcane remains largely unknown. The chemical reaction catalyzed by 1-aminocyclopropane-1-carboxylic acid synthase (ACS) is an important rate-limiting step that regulates ethylene production in plants. In this work, using a yeast one-hybrid approach, we identified three basic helix-loop-helix (bHLH) transcription factors, homologs of Arabidopsis FBH (FLOWERING BHLH), that bind to the promoter of ScACS2 (Sugarcane ACS2), a sugarcane type 3 ACS isozyme gene. Protein-protein interaction assays showed that sugarcane FBH1 (ScFBH1), ScFBH2, and ScFBH3 form homo- and heterodimers in the nucleus. Gene expression analysis revealed that ScFBHs and ScACS2 transcripts are more abundant in maturing internodes during afternoon and night. In addition, Arabidopsis functional analysis demonstrated that FBH controls ethylene production by regulating transcript levels of ACS7, a homolog of ScACS2. These results indicate that ScFBHs transcriptionally regulate ethylene biosynthesis in maturing internodes of sugarcane.

The Pivotal Role of Ethylene in Plant Growth

Being continuously exposed to variable environmental conditions, plants produce phytohormones to react quickly and specifically to these changes. The phytohormone ethylene is produced in response to multiple stresses. While the role of ethylene in defense responses to pathogens is widely recognized, recent studies in arabidopsis and crop species highlight an emerging key role for ethylene in the regulation of organ growth and yield under abiotic stress. Molecular connections between ethylene and growth-regulatory pathways have been uncovered, and altering the expression of ethylene response factors (ERFs) provides a new strategy for targeted ethylene-response engineering. Crops with optimized ethylene responses show improved growth in the field, opening new windows for future crop improvement. This review focuses on how ethylene regulates shoot growth, with an emphasis on leaves.

The Pivotal Role of Ethylene in Plant Growth

Being continuously exposed to variable environmental conditions, plants produce phytohormones to react quickly and specifically to these changes. The phytohormone ethylene is produced in response to multiple stresses. While the role of ethylene in defense responses to pathogens is widely recognized, recent studies in arabidopsis and crop species highlight an emerging key role for ethylene in the regulation of organ growth and yield under abiotic stress. Molecular connections between ethylene and growth-regulatory pathways have been uncovered, and altering the expression of ethylene response factors (ERFs) provides a new strategy for targeted ethylene-response engineering. Crops with optimized ethylene responses show improved growth in the field, opening new windows for future crop improvement. This review focuses on how ethylene regulates shoot growth, with an emphasis on leaves.

Genome Reduction in Tetraploid Potato Reveals Genetic Load, Haplotype Variation, and Loci Associated With Agronomic Traits

The cultivated potato (Solanum tuberosum) has a complex genetic structure due to its autotetraploidy and vegetative propagation which leads to accumulation of mutations and a highly heterozygous genome. A high degree of heterozygosity has been considered to be the main driver of fitness and agronomic trait performance in potato improvement efforts, which is negatively impacted by genetic load. To understand the genetic landscape of cultivated potato, we constructed a gynogenic dihaploid (2n = 2x = 24) population from cv. Superior, prior to development of a high-density genetic map containing 12,753 single nucleotide polymorphisms (SNPs). Common quantitative trait loci (QTL) were identified for tuber traits, vigor and height on chromosomes 2, 4, 7, and 10, while specific QTL for number of inflorescences per plant, and tuber shape were present on chromosomes 4, 6, 10, and 11. Simplex rather than duplex loci were mainly associated with traits. In general, the Q allele (main effect) detected in one or two homologous chromosomes was associated with lower mean trait values suggesting the importance of dosage allelic effects, and the presence of up to two undesired alleles in the QTL region. Loss of heterozygosity has been associated with a lower rate of fitness, yet no correlation between the percent heterozygosity and increased fitness or agronomic performance was observed. Based upon linkage phase, we reconstructed the four homologous chromosome haplotypes of cv. Superior. revealing heterogeneity throughout the genome yet nearly duplicate haplotypes occurring among the homologs of particular chromosomes. These results suggest that the potentially deleterious mutations associated with genetic load in tetraploid potato could be mitigated by multiple loci which is consistent with the theory that epistasis complicates the identification of associations between markers and phenotypic performance.

Regulation of Sucrose Transporters and Phloem Loading in Response to Environmental Cues

Suc transporters (SUTs) play a key role in the allocation and partitioning of photosynthetically fixed carbon in plants. While a function could be assigned to many members of the SUT family, almost no information is available on their regulation. Here, the transcriptional regulation of SUTs in response to various environmental stimuli in the leaves of five dicots (Arabidopsis [Arabidopsis thaliana], soybean [Glycine max], potato [Solanum tuberosum], tomato [Solanumlycopersicum], and poplar [Populus spp.]) and four monocots (maize [Zeamays], rice [Oryza sativa], wheat [Triticum aestivum], and barley [Hordeum vulgare]) was investigated. Extensive data on expression of SUTs in relation to changes of environmental conditions were obtained through a global analysis of 168 transcriptomics data sets. Results were validated by quantitative PCR measurements and extended by the measurement of photosynthesis rate and phloem sugar content to draw insight on the correlation of SUT expression and sugar export from leaves. For the apoplasmic phloem loaders, a clear difference in transcriptional regulation in response to different environmental stimuli was observed. The consistent patterns of SUT expression under abiotic stress indicates which types of SUTs are involved in the regulation of leaf sugar status and in stress signaling. Furthermore, it is shown that down-regulation of phloem loading is likely to be caused by transcriptional regulation of SUTs, while up-regulation depends on post-transcriptional regulation. In poplar, expression of PtaSUT4 was found to consistently respond to environmental stimuli, suggesting a significant role in the regulation of sugar export from leaves in this passive symplasmic phloem loader.

Differential Signaling and Sugar Exchanges in Response to Avirulent Pathogen- and Symbiont-Derived Molecules in Tobacco Cells

Plants interact with microbes whose ultimate aim is to exploit plant carbohydrates for their reproduction. Plant–microbe interactions (PMIs) are classified according to the nature of their trophic exchanges: while mutualistic microbes trade nutrients with plants, pathogens unilaterally divert carbohydrates. The early responses following microbe recognition and the subsequent control of plant sugar distribution are still poorly understood. To further decipher PMI functionality, we used tobacco cells treated with microbial molecules mimicking pathogenic or mutualistic PMIs, namely cryptogein, a defense elicitor, and chitotetrasaccharide (CO4), which is secreted by mycorrhizal fungi. CO4 was perceived by tobacco cells and triggered widespread transient signaling components such as a sharp cytosolic Ca²⁺ elevation, NtrbohD-dependent H₂O₂ production, and MAP kinase activation. These CO4-induced events differed from those induced by cryptogein, i.e., sustained events leading to cell death. Furthermore, cryptogein treatment inhibited glucose and sucrose uptake but not fructose uptake, and promoted the expression of NtSUT and NtSWEET sugar transporters, whereas CO4 had no effect on sugar uptake and only a slight effect on NtSWEET2B expression. Our results suggest that microbial molecules induce different signaling responses that reflect microbial lifestyle and the subsequent outcome of the interaction.

Key players associated with tuberization in potato: potential candidates for genetic engineering

Tuberization in potato (Solanum tuberosum L.) is a complex biological phenomenon which is affected by several environmental cues, genetic factors and plant nutrition. Understanding the regulation of tuber induction is essential to devise strategies to improve tuber yield and quality. It is well established that short-day photoperiods promote tuberization, whereas long days and high-temperatures inhibit or delay tuberization. Worldwide research on this complex biological process has yielded information on the important bio-molecules (proteins, RNAs, plant growth regulators) associated with the tuberization process in potato. Key proteins involved in the regulation of tuberization include StSP6A, POTH1, StBEL5, StPHYB, StCONSTANS, Sucrose transporter StSUT4, StSP5G, etc. Biomolecules that become transported from "source to sink" have also been suggested to be important signaling candidates regulating the tuberization process in potatos. Four molecules, namely StSP6A protein, StBEL5 RNA, miR172 and GAs, have been found to be the main candidates acting as mobile signals for tuberization. These biomolecules can be manipulated (overexpressed/inhibited) for improving the tuberization in commercial varieties/cultivars of potato. In this review, information about the genes/proteins and their mechanism of action associated with the tuberization process is discussed.

Metabolite transport and associated sugar signalling systems underpinning source/sink interactions

Metabolite transport between organelles, cells and source and sink tissues not only enables pathway co-ordination but it also facilitates whole plant communication, particularly in the transmission of information concerning resource availability. Carbon assimilation is co-ordinated with nitrogen assimilation to ensure that the building blocks of biomass production, amino acids and carbon skeletons, are available at the required amounts and stoichiometry, with associated transport processes making certain that these essential resources are transported from their sites of synthesis to those of utilisation. Of the many possible posttranslational mechanisms that might participate in efficient co-ordination of metabolism and transport only reversible thiol-disulphide exchange mechanisms have been described in detail. Sucrose and trehalose metabolism are intertwined in the signalling hub that ensures appropriate resource allocation to drive growth and development under optimal and stress conditions, with trehalose-6-phosphate acting as an important signal for sucrose availability. The formidable suite of plant metabolite transporters provides enormous flexibility and adaptability in inter-pathway coordination and source-sink interactions. Focussing on the carbon metabolism network, we highlight the functions of different transporter families, and the important of thioredoxins in the metabolic dialogue between source and sink tissues. In addition, we address how these systems can be tailored for crop improvement.

Structure, phylogeny, allelic haplotypes and expression of sucrose transporter gene families in Saccharum

BACKGROUND

Sugarcane is an economically important crop contributing to about 80% of the world sugar production. Increasing efforts in molecular biological studies have been performed for improving the sugar yield and other relevant important agronomic traits. However, due to sugarcane's complicated genomes, it is still challenging to study the genetic basis of traits, such as sucrose accumulation. Sucrose transporters (SUTs) are critical for both phloem loading in source tissue and sucrose uptaking in sink tissue, and are considered to be the control points for regulating sucrose storage. However, no genomic study for sugarcane sucrose transporter (SsSUT) families has been reported up to date.

RESULTS

By using comparative genomics and bacterial artificial chromosomes (BACs), six SUT genes were identified and characterized in S. spontaenum. Phylogenetic analyses revealed that the two pairs SsSUTs (SsSUT1/SsSUT3 and SsSUT5/SsSUT6) could be clustered together into two separate monocot specific SUT groups, while SsSUT2 and SsSUT4 were separated into the other two groups, with members from both dicot and monocot species. Gene structure comparison demonstrated that the number and position of exons/introns in SUTs were highly conserved among the close orthologs; in contrast, there were variations among the paralogous SUTs in Sacchuarm. Though with the high polyploidy level, gene allelic haplotype comparative analysis showed that the examined four SsSUT members exhibited conservations of gene structures and amino acid sequences among the allelic haplotypes accompanied by variations of intron sizes. Gene expression analyses were performed for tissues from seedlings under drought stress and mature plants of three Saccharum species (S.officinarnum, S.spotaneum and S.robustum). Both SUT1 and SUT4 expressed abundantly at different conditions. SUT2 had similar expression level in all of the examined tissues, but SUT3 was undetectable. Both of SUT5 and SUT6 had lower expression level than other gene member, and expressed stronger in source leaves and are likely to play roles in phloem loading. In the seeding plant leave under water stress, four genes SUT1, SUT2, SUT4 and SUT5 were detectable. In these detectable genes, SUT1 and SUT4 were down regulated, while, SUT2 and SUT5 were up regulated.

CONCLUSIONS

In this study, we presented the first comprehensive genomic study for a whole gene family, the SUT family, in Saccharum. We speculated that there were six SUT members in the S. spotaneum genome. Out of the six members, SsSUTs, SsSUT5 and SsSUT6 were recent duplication genes accompanied by rapid evolution, while, SsSUT2 and SsSUT4 were the ancient members in the families. Despite the high polypoidy genome, functional redundancy may not exist among the SUTs allelic haplotypes supported by the evidence of strong purifying selection of the gene allele. SUT3 could be a low active member in the family because it is undetectable in our study, but it might not be a pseudogene because it harbored integrated gene structure. SUT1 and SUT4 were the main members for the sucrose transporter, while, these SUTs had sub-functional divergence in response to sucrose accumulation and plant development in Saccharum.

Expression of peach sucrose transporters in heterologous systems points out their different physiological role

Sucrose is the major phloem-translocated component in a number of economically important plant species. The comprehension of the mechanisms involved in sucrose transport in peach fruit appears particularly relevant, since the accumulation of this sugar, during ripening, is crucial for the growth and quality of the fruit. Here, we report the functional characterisation and subcellular localisation of three sucrose transporters (PpSUT1, PpSUT2, PpSUT4) in peach, and we formulate novel hypotheses about their role in accumulation of sugar. We provide evidence, about the capability of both PpSUT1 and PpSUT4, expressed in mutant yeast strains to transport sucrose. The functionality of PpSUT1 at the plasma membrane, and of PpSUT4 at the tonoplast, has been demonstrated. On the other hand, the functionality of PpSUT2 was not confirmed: this protein is unable to complement two sucrose uptake-deficient mutant yeast strains. Our results corroborate the hypotheses that PpSUT1 partakes in phloem loading in leaves, and PpSUT4 sustains cell metabolism by regulating sucrose efflux from the vacuole.

A re-assessment of sucrose signaling involved in cluster-root formation and function in phosphate-deficient white lupin (Lupinus albus)

Apart from substrate functions, a signaling role of sucrose in root growth regulation is well established. This raised the question whether sucrose signals might also be involved in formation of cluster-roots (CRs) under phosphate (Pi) limitation, mediating exudation of phosphorus (P)-mobilizing root exudates, e.g. in Lupinus albus and members of the Proteaceae. Earlier studies demonstrated that CR formation in L. albus was mimicked to some extent by external application of high sucrose concentrations (25 mM) in the presence of extremely high P supply (1-10 mM), usually suppressing CR formation. In this study, we re-addressed this question using an axenic hydroponic culture system with normal P supply (0.1 mM) and a range of sucrose applications (0.25-25 mM). The 2.5 mM sucrose concentration was comparable with internal sucrose levels in the zone of CR initiation in first-order laterals of P-deficient plants (3.4 mM) and induced the same CR morphology. Similar to earlier studies, high sucrose concentrations (25 mM) resulted in root thickening and inhibition of root elongation, associated with a 10-fold increase of the internal sucrose level. The sucrose analog palatinose and a combination of glucose/fructose failed to stimulate CR formation under P-sufficient conditions, demonstrating a signal function of sucrose and excluding osmotic or carbon source effects. In contrast to earlier findings, sucrose was able to induce CR formation but had no effect on CR functioning with respect to citrate exudation, in vitro activity and expression of genes encoding phosphoenolpyruvate carboxylase, secretory acid phosphatase and MATE transporters, mediating P-mobilizing functions of CRs.

Conserved function of FLOWERING LOCUS T (FT) homologues as signals for storage organ differentiation

Due to their high carbohydrate content and relative low farming demands, tuber-bearing species are an important contribution to human dietary needs in many climatic zones, and interest in these staple crops for processed food and other industrial uses is increasing. Over the past years we have seen remarkable advances in our understanding of the signalling mechanisms involved in the differentiation of these organs, partly aided by their conservation with the well-characterized photoperiodic control of flowering in Arabidopsis. Recent studies have led to the identification of members of the FT gene family as major component of the tuber-inducing signal and the characterization of circadian and photoperiodic components involved in the regulation of these genes. A relevant role of microRNAs in the control of storage organ formation has been established, and hormonal balance requirements similar to those controlling shoot branching were shown to be implicated in the activation of stolon meristem cells. Hence, the recent finding that FT controls branching through direct interaction with the TCP factors holds great promise for the identification of genes acting as FT signal integrators in the stolon.

Circadian regulation of abiotic stress tolerance in plants

Extremes of temperatures, drought and salinity cause widespread crop losses throughout the world and impose severe limitations on the amount of land that can be used for agricultural purposes. Hence, there is an urgent need to develop crops that perform better under such abiotic stress conditions. Here, we discuss intriguing, recent evidence that circadian clock contributes to plants' ability to tolerate different types of environmental stress, and to acclimate to them. The clock controls expression of a large fraction of abiotic stress-responsive genes, as well as biosynthesis and signaling downstream of stress response hormones. Conversely, abiotic stress results in altered expression and differential splicing of the clock genes, leading to altered oscillations of downstream stress-response pathways. We propose a range of mechanisms by which this intimate coupling between the circadian clock and environmental stress-response pathways may contribute to plant growth and survival under abiotic stress.

Bayesian phylogeny of sucrose transporters: ancient origins, differential expansion and convergent evolution in monocots and dicots

Sucrose transporters (SUTs) are essential for the export and efficient movement of sucrose from source leaves to sink organs in plants. The angiosperm SUT family was previously classified into three or four distinct groups, Types I, II (subgroup IIB), and III, with dicot-specific Type I and monocot-specific Type IIB functioning in phloem loading. To shed light on the underlying drivers of SUT evolution, Bayesian phylogenetic inference was undertaken using 41 sequenced plant genomes, including seven basal lineages at key evolutionary junctures. Our analysis supports four phylogenetically and structurally distinct SUT subfamilies, originating from two ancient groups (AG1 and AG2) that diverged early during terrestrial colonization. In both AG1 and AG2, multiple intron acquisition events in the progenitor vascular plant established the gene structures of modern SUTs. Tonoplastic Type III and plasmalemmal Type II represent evolutionarily conserved descendants of AG1 and AG2, respectively. Type I and Type IIB were previously thought to evolve after the dicot-monocot split. We show, however, that divergence of Type I from Type III SUT predated basal angiosperms, likely associated with evolution of vascular cambium and phloem transport. Type I SUT was subsequently lost in monocots along with vascular cambium, and independent evolution of Type IIB coincided with modified monocot vasculature. Both Type I and Type IIB underwent lineage-specific expansion. In multiple unrelated taxa, the newly-derived SUTs exhibit biased expression in reproductive tissues, suggesting a functional link between phloem loading and reproductive fitness. Convergent evolution of Type I and Type IIB for SUT function in phloem loading and reproductive organs supports the idea that differential vascular development in dicots and monocots is a strong driver for SUT family evolution in angiosperms.