Analysis of Arabidopsis glucose insensitive mutants, gin5 and gin6, reveals a central role of the plant hormone ABA in the regulation of plant vegetative development by sugar

The roles of two transcription factors, ABI4 and CBFA, in ABA and plastid signalling and stress responses

Genetic and physiological studies have revealed evidences for multiple signaling pathways by which the plastid exerts retrograde control over photosynthesis-associated-nuclear-genes. In this study we have examined the mechanisms of control of transcription by plastid signals, focusing on transcription factors. We have also further addressed the physical nature of plastid signals and the physiological role, in stress acclimation of this regulatory pathway. ABI4, a master Apetala 2 (AP2)-type transcription factor (TF), is targeted by multiple signalling pathways in plant cells, such as abscisic acid (ABA) signals, sugar signals and plastid signals derived from reactive oxygen species (ROS) and chlorophyll intermediates. ABI4 binds the promoter of target genes to prevent their transcription by competing with other competitive TFs. However, we found that once ABI4 bound the element (CCACGT), it may not be bound by other TFs, therefore making the signalling long-lasting. Downstream of ABI4, CBFA (CCAAT binding factor A) is a subunit of the HAP2/HAP3/HAP5 (Heme activator protein) trimeric transcription complex. CBFA however is a redundant HAP3 subunit. When emergency occurs (such as herbicide treatments or environmental stresses followed by ABA and ROS accumulation), the master transcription factor ABI4 down-regulates some TFs, like CBFA, and then some other TF subunits enter the transcription complex and transcriptional efficiency of stress-responsive genes (including the transcription co-factor CBP) is improved instantaneously. abi4, cbfA and cbp mutants showed weaker drought-tolerance after a herbicide norflurazon treatment, which indicated the physiological role of these key transcription factors.

Overexpression of the vacuolar sugar carrier AtSWEET16 modifies germination, growth, and stress tolerance in Arabidopsis

Here, we report that SUGARS WILL EVENTUALLY BE EXPORTED TRANSPORTER (SWEET16) from Arabidopsis (Arabidopsis thaliana) is a vacuole-located carrier, transporting glucose (Glc), fructose (Fru), and sucrose (Suc) after heterologous expression in Xenopus laevis oocytes. The SWEET16 gene, similar to the homologs gene SWEET17, is mainly expressed in vascular parenchyma cells. Application of Glc, Fru, or Suc, as well as cold, osmotic stress, or low nitrogen, provoke the down-regulation of SWEET16 messenger RNA accumulation. SWEET16 overexpressors (35SPro:SWEET16) showed a number of peculiarities related to differences in sugar accumulation, such as less Glc, Fru, and Suc at the end of the night. Under cold stress, 35SPro:SWEET16 plants are unable to accumulate Fru, while under nitrogen starvation, both Glc and Fru, but not Suc, were less abundant. These changes of individual sugars indicate that the consequences of an increased SWEET16 activity are dependent upon the type of external stimulus. Remarkably, 35SPro:SWEET16 lines showed improved germination and increased freezing tolerance. The latter observation, in combination with the modified sugar levels, points to a superior function of Glc and Suc for frost tolerance. 35SPro:SWEET16 plants exhibited increased growth efficiency when cultivated on soil and showed improved nitrogen use efficiency when nitrate was sufficiently available, while under conditions of limiting nitrogen, wild-type biomasses were higher than those of 35SPro:SWEET16 plants. Our results identify SWEET16 as a vacuolar sugar facilitator, demonstrate the substantial impact of SWEET16 overexpression on various critical plant traits, and imply that SWEET16 activity must be tightly regulated to allow optimal Arabidopsis development under nonfavorable conditions.

Involvement of microRNA-related regulatory pathways in the glucose-mediated control of Arabidopsis early seedling development

In plants, sugars such as glucose act as signalling molecules that promote changes in gene expression programmes that impact on growth and development. Recent evidence has revealed the potential importance of controlling mRNA decay in some aspects of glucose-mediated regulatory responses suggesting a role of microRNAs (miRNAs) in these responses. In order to get a better understanding of glucose-mediated development modulation involving miRNA-related regulatory pathways, early seedling development of mutants impaired in miRNA biogenesis (hyl1-2 and dcl1-11) and miRNA activity (ago1-25) was evaluated. All mutants exhibited a glucose hyposensitive phenotype from germination up to seedling establishment, indicating that miRNA regulatory pathways are involved in the glucose-mediated delay of early seedling development. The expression profile of 200 miRNA primary transcripts (pri-miRs) was evaluated by large-scale quantitative real-time PCR profiling, which revealed that 38 pri-miRs were regulated by glucose. For several of them, the corresponding mature miRNAs are known to participate directly or indirectly in plant development, and their accumulation was shown to be co-regulated with the pri-miR by glucose. Furthermore, the expression of several miRNA target genes was found to be deregulated in response to glucose in the miRNA machinery mutants ago1-25, dcl1-11, and hyl1-2. Also, in these mutants, glucose promoted misexpression of genes for the three abscisic acid signalling elements ABI3, ABI4, and ABI5. Thus, miRNA regulatory pathways play a role in the adjustments of growth and development triggered by glucose signalling.

Starch metabolism and antiflorigenic signals modulate the juvenile-to-adult phase transition in Arabidopsis

The physiology and genetics underlying juvenility is poorly understood. Here, we exploit Arabidopsis as a system to understand the mechanisms that regulate floral incompetence during juvenility. Using an experimental assay that allows the length of juvenility to be estimated and mutants impaired in different pathways, we show that multiple inputs influence juvenility. Juvenile phase lengths of wild type (WT) accessions Col-0, Ler-0 and Ws-4 are shown to differ, with Col-0 having the shortest and Ws-4 the longest length. Plants defective in sugar signalling [gin1-1, gin2-1, gin6 (abi4)] and floral repressor mutants [hst1, tfl1, tfl2 (lhp1)] showed shortened juvenile phase lengths compared to their respective WTs. Mutants defective in starch anabolism (adg1-1, pgm1) and catabolism (sex1, sex4, bam3) showed prolonged juvenile phase lengths compared to Col-0. Examination of diurnal metabolite changes in adg1-1 and sex1 mutants indicates that their altered juvenile phase length may be due to lack of starch turnover, which influences carbohydrate availability. In this article, we propose a model in which a variety of signals including floral activators and repressors modulate the juvenile-to-adult phase transition. The role of carbohydrates may be in their capacity as nutrients, osmotic regulators, signalling molecules and/ or through their interaction with phytohormonal networks.

TIME FOR COFFEE is an essential component in the maintenance of metabolic homeostasis in Arabidopsis thaliana

Plants often respond to environmental changes by reprogramming metabolic and stress-associated pathways. Homeostatic integration of signaling is a central requirement for ensuring metabolic stability in living organisms. Under diurnal conditions, properly timed rhythmic metabolism provides fitness benefits to plants. TIME FOR COFFEE (TIC) is a circadian regulator known to be involved in clock resetting at dawn. Here we explored the mechanism of influence of TIC in plant growth and development, as initiated by a microarray analysis. This global profiling showed that a loss of TIC function causes a major reprogramming of gene expression that predicts numerous developmental, metabolic, and stress-related phenotypes. This led us to demonstrate that this mutant exhibits late flowering, a plastochron defect, and diverse anatomical phenotypes. We further observed a starch-excess phenotype and altered soluble carbohydrate levels. tic exhibited hypersensitivity to oxidative stress and abscisic acid, and this was associated with a striking resistance to drought. These phenotypes were connected to an increase in total glutathione levels that correlated with a readjustment of amino acids and polyamine pools. By comparatively analyzing our transcriptomic and metabolomic data, we concluded that TIC is a central element in plant homeostasis that integrates and coordinates developmental, metabolic, and environmental signals.

Hexokinase mediates stomatal closure

Stomata, composed of two guard cells, are the gates whose controlled movement allows the plant to balance the demand for CO2 for photosynthesis with the loss of water through transpiration. Increased guard-cell osmolarity leads to the opening of the stomata and decreased osmolarity causes the stomata to close. The role of sugars in the regulation of stomata is not yet clear. In this study, we examined the role of hexokinase (HXK), a sugar-phosphorylating enzyme involved in sugar-sensing, in guard cells and its effect on stomatal aperture. We show here that increased expression of HXK in guard cells accelerates stomatal closure. We further show that this closure is induced by sugar and is mediated by abscisic acid. These findings support the existence of a feedback-inhibition mechanism that is mediated by a product of photosynthesis, namely sucrose. When the rate of sucrose production exceeds the rate at which sucrose is loaded into the phloem, the surplus sucrose is carried toward the stomata by the transpiration stream and stimulates stomatal closure via HXK, thereby preventing the loss of precious water.

The glutamate carboxypeptidase AMP1 mediates abscisic acid and abiotic stress responses in Arabidopsis

ALTERED MERISTEM PROGRAM1 (AMP1) encodes a glutamate carboxypeptidase that plays an important role in shoot apical meristem development and phytohormone homeostasis. We isolated a new mutant allele of AMP1, amp1-20, from a screen for abscisic acid (ABA) hypersensitive mutants and characterized the function of AMP1 in plant stress responses. amp1 mutants displayed ABA hypersensitivity, while overexpression of AMP1 caused ABA insensitivity. Moreover, endogenous ABA concentration was increased in amp1-20- and decreased in AMP1-overexpressing plants under stress conditions. Application of ABA reduced the AMP1 protein level in plants. Interestingly, amp1 mutants accumulated excess superoxide and displayed hypersensitivity to oxidative stress. The hypersensitivity of amp1 to ABA and oxidative stress was partially rescued by reactive oxygen species (ROS) scavenging agent. Furthermore, amp1 was tolerant to freezing and drought stress. The ABA hypersensitivity and freezing tolerance of amp1 was dependent on ABA signaling. Moreover, amp1 had elevated soluble sugar content and showed hypersensitivity to high concentrations of sugar. By contrast, the contents of amino acids were changed in amp1 mutant compared to the wild-type. This study suggests that AMP1 modulates ABA, oxidative and abotic stress responses, and is involved in carbon and amino acid metabolism in Arabidopsis.

Towards the identification of new genes involved in ABA-dependent abiotic stresses using Arabidopsis suppressor mutants of abh1 hypersensitivity to ABA during seed germination

Abscisic acid plays a pivotal role in the abiotic stress response in plants. Although great progress has been achieved explaining the complexity of the stress and ABA signaling cascade, there are still many questions to answer. Mutants are a valuable tool in the identification of new genes or new alleles of already known genes and in elucidating their role in signaling pathways. We applied a suppressor mutation approach in order to find new components of ABA and abiotic stress signaling in Arabidopsis. Using the abh1 (ABA hypersensitive 1) insertional mutant as a parental line for EMS mutagenesis, we selected several mutants with suppressed hypersensitivity to ABA during seed germination. Here, we present the response to ABA and a wide range of abiotic stresses during the seed germination and young seedling development of two suppressor mutants—soa2 (suppressor of abh1 hypersensitivity to ABA 2) and soa3 (suppressor of abh1 hypersensitivity to ABA 3). Generally, both mutants displayed a suppression of the hypersensitivity of abh1 to ABA, NaCl and mannitol during germination. Both mutants showed a higher level of tolerance than Columbia-0 (Col-0—the parental line of abh1) in high concentrations of glucose. Additionally, soa2 exhibited better root growth than Col-0 in the presence of high ABA concentrations. soa2 and soa3 were drought tolerant and both had about 50% fewer stomata per mm² than the wild-type but the same number as their parental line—abh1. Taking into account that suppressor mutants had the same genetic background as their parental line—abh1, it was necessary to backcross abh1 with Landsberg erecta four times for the map-based cloning approach. Mapping populations, derived from the cross of abh1 in the Landsberg erecta background with each suppressor mutant, were created. Map based cloning in order to identify the suppressor genes is in progress.

ABI4 regulates primary seed dormancy by regulating the biogenesis of abscisic acid and gibberellins in arabidopsis

Seed dormancy is an important economic trait for agricultural production. Abscisic acid (ABA) and Gibberellins (GA) are the primary factors that regulate the transition from dormancy to germination, and they regulate this process antagonistically. The detailed regulatory mechanism involving crosstalk between ABA and GA, which underlies seed dormancy, requires further elucidation. Here, we report that ABI4 positively regulates primary seed dormancy, while negatively regulating cotyledon greening, by mediating the biogenesis of ABA and GA. Seeds of the Arabidopsis abi4 mutant that were subjected to short-term storage (one or two weeks) germinated significantly more quickly than Wild-Type (WT), and abi4 cotyledons greened markedly more quickly than WT, while the rates of germination and greening were comparable when the seeds were subjected to longer-term storage (six months). The ABA content of dry abi4 seeds was remarkably lower than that of WT, but the amounts were comparable after stratification. Consistently, the GA level of abi4 seeds was increased compared to WT. Further analysis showed that abi4 was resistant to treatment with paclobutrazol (PAC), a GA biosynthesis inhibitor, during germination, while OE-ABI4 was sensitive to PAC, and exogenous GA rescued the delayed germination phenotype of OE-ABI4. Analysis by qRT-PCR showed that the expression of genes involved in ABA and GA metabolism in dry and germinating seeds corresponded to hormonal measurements. Moreover, chromatin immunoprecipitation qPCR (ChIP-qPCR) and transient expression analysis showed that ABI4 repressed CYP707A1 and CYP707A2 expression by directly binding to those promoters, and the ABI4 binding elements are essential for this repression. Accordingly, further genetic analysis showed that abi4 recovered the delayed germination phenotype of cyp707a1 and cyp707a2 and further, rescued the non-germinating phenotype of ga1-t. Taken together, this study suggests that ABI4 is a key factor that regulates primary seed dormancy by mediating the balance between ABA and GA biogenesis.

Seed dormancy prevents or delays germination in maturated seeds. The optimal level of seed dormancy is a valuable trait for agricultural production and post-harvest management. High ABA and low GA content in seeds promote seed dormancy. However, the precise molecular mechanisms controlling seed dormancy and germination remain unclear. We found that ABI4, the key transcription factor in the ABA signaling pathway, indeed controls primary seed dormancy. This result contradicts the previous conclusion that ABI4 is not involved in the control of seed dormancy. Several lines of evidence support our conclusion. For example, detailed physiological analysis of the germination of abi4 seeds that were harvested immediately and stored for various periods of time and subjected to various treatments allowed us to conclude that ABI4 negatively regulates primary seed dormancy. The molecular mechanism responsible for this control is as follows: ABI4 directly or indirectly regulates the key genes of the ABA and GA biogenesis pathways, which then regulates the ABA and GA contents in seeds. Importantly, further genetic interactions between CYP707A1, CYP707A2, GA1, and ABI4 also support our conclusion.

Overexpression of Arachis hypogaea AREB1 gene enhances drought tolerance by modulating ROS scavenging and maintaining endogenous ABA content

AhAREB1 (Arachis hypogaea Abscisic-acid Response Element Binding Protein 1) is a member of the basic domain leucine zipper (bZIP)-type transcription factor in peanut. Previously, we found that expression of AhAREB1 was specifically induced by abscisic acid (ABA), dehydration and drought. To understand the drought defense mechanism regulated by AhAREB1, transgenic Arabidopsis overexpressing AhAREB1 was conducted in wild-type (WT), and a complementation experiment was employed to ABA non-sensitivity mutant abi5 (abscisic acid-insensitive 5). Constitutive expression of AhAREB1 confers water stress tolerance and is highly sensitive to exogenous ABA. Microarray and further real-time PCR analysis revealed that drought stress, reactive oxygen species (ROS) scavenging, ABA synthesis/metabolism-related genes and others were regulated in transgenic Arabidopsis overexpressing AhAREB1. Accordingly, low level of ROS, but higher ABA content was detected in the transgenic Arabidopsis plants’ overexpression of AhAREB1. Taken together, it was concluded that AhAREB1 modulates ROS accumulation and endogenous ABA level to improve drought tolerance in transgenic Arabidopsis.

Vitamin C and the abscisic acid-insensitive 4 transcription factor are important determinants of aphid resistance in Arabidopsis

AIMS

Aphids, like other insects, are probably unable to synthesize vitamin C (ascorbic acid), which is therefore an essential dietary nutrient that has to be obtained from the host plant. Plant responses to aphids involve hormones such as salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA), but hormone/redox interactions remain poorly characterized. We therefore investigated hormone/redox signaling in the response of Arabidopsis thaliana to infestation by the aphid Myzus persicae, focusing on the interactions between ascorbic acid and ABA, together with the influence of altered ascorbate and ABA signaling on the SA- and JA-dependent pathways.

RESULTS

Whole-genome microarray analysis revealed highly dynamic transcriptional responses to aphid infestation with extensive differences between transcript profiles of infested and systemic leaves, revealing aphid-dependent effects on the suites of transcripts involved in the redox, SA, and ABA responses. Central roles for ascorbate, ABA-insensitive 4 (ABI4), and oxidative signal-inducible 1 in plant resistance to aphids were demonstrated by altered fecundity on respective mutants. However, ABA had a negative effect on aphid resistance, as did ABI4 or redox-responsive transcription factor 1. The decrease in aphid fecundity observed in mutants defective in ascorbate accumulation (vtc2) was absent from abi4vtc2 double mutants that are also deficient in ABA signaling (abi4). Aphid-dependent transcriptome responses reveal a role for ascorbate-regulated receptor-like kinases in plant defenses against aphids.

INNOVATION

Vitamin C deficiency enhances plant resistance to aphids through redox signaling pathways rather than dietary requirements.

CONCLUSION

ABI4 is a linchpin of redox regulation of the innate immune response to aphids.

ASR1 mediates glucose-hormone cross talk by affecting sugar trafficking in tobacco plants

Asr (for ABA, stress, ripening) genes are exclusively found in the genomes of higher plants, and the encoded proteins have been found localized both to the nucleus and cytoplasm. However, before the mechanisms underlying the activity of ASR proteins can be determined, the role of these proteins in planta should be deciphered. Results from this study suggest that ASR is positioned within the signaling cascade of interactions among glucose, abscisic acid, and gibberellins. Tobacco (Nicotiana tabacum) transgenic lines with reduced levels of ASR protein showed impaired glucose metabolism and altered abscisic acid and gibberellin levels. These changes were associated with dwarfism, reduced carbon dioxide assimilation, and accelerated leaf senescence as a consequence of a fine regulation exerted by ASR to the glucose metabolism. This regulation resulted in an impact on glucose signaling mediated by Hexokinase1 and Snf1-related kinase, which would subsequently have been responsible for photosynthesis, leaf senescence, and hormone level alterations. It thus can be postulated that ASR is not only involved in the control of hexose uptake in heterotrophic organs, as we have previously reported, but also in the control of carbon fixation by the leaves mediated by a similar mechanism.

ABI4 downregulates expression of the sodium transporter HKT1;1 in Arabidopsis roots and affects salt tolerance

A plant's ability to cope with salt stress is highly correlated with their ability to reduce the accumulation of sodium ions in the shoot. Arabidopsis mutants affected in the ABSCISIC ACID INSENSITIVE (ABI) 4 gene display increased salt tolerance, whereas ABI4-overexpressors are hypersensitive to salinity from seed germination to late vegetative developmental stages. In this study we demonstrate that abi4 mutant plants accumulate lower levels of sodium ions and higher levels of proline than wild-type plants following salt stress. We show higher HKT1;1 expression in abi4 mutant plants and lower levels of expression in ABI4-overexpressing plants, resulting in reduced accumulation of sodium ions in the shoot of abi4 mutants. HKT1;1 encodes a sodium transporter which is known to unload sodium ions from the root xylem stream into the xylem parenchyma stele cells. We have shown recently that ABI4 is expressed in the root stele at various developmental stages and that it plays a key role in determining root architecture. Thus ABI4 and HKT1;1 are expressed in the same cells, which suggests the possibility of direct binding of ABI4 to the HKT1;1 promoter. In planta chromatin immunoprecipitation and in vitro electrophoresis mobility shift assays demonstrated that ABI4 binds two highly related sites within the HKT1;1 promoter. These sites, GC(C/G)GCTT(T), termed ABI4-binding element (ABE), have also been identified in other ABI4-repressed genes. We therefore suggest that ABI4 is a major modulator of root development and function.

ABI4: versatile activator and repressor

The ABSCISIC ACID INSENSITIVE4 (ABI4) gene was discovered to be an abscisic acid (ABA) signaling responsive transcription factor active during seed germination. The evolutionary history of the ABI4 gene supports its role as an ABA signaling intermediate in land plants. Investigating the ABI4 protein-cis element interaction supports the proposal that ABI4 binding to its known CE1 cis-element competes with transcription factor binding to the overlapping G-Box element. Recent publications report on ABI4 as a regulatory factor in diverse processes. In developing seedlings, ABI4 mediates sugar signaling, lipid breakdown, and plastid-to-nucleus signaling. Moreover, ABI4 is a regulator of rosette growth, redox signaling, cell wall metabolism and the effect of nitrate on lateral root development.

A novel role for Arabidopsis CBL1 in affecting plant responses to glucose and gibberellin during germination and seedling development

Glucose and phytohormones such as abscisic acid (ABA), ethylene, and gibberellin (GA) coordinately regulate germination and seedling development. However, there is still inadequate evidence to link their molecular roles in affecting plant responses. Calcium acts as a second messenger in a diverse range of signal transduction pathways. As calcium sensors unique to plants, calcineurin B-like (CBL) proteins are well known to modulate abiotic stress responses. In this study, it was found that CBL1 was induced by glucose in Arabidopsis. Loss-of-function mutant cbl1 exhibited hypersensitivity to glucose and paclobutrazol, a GA biosynthetic inhibitor. Several sugar-responsive and GA biosynthetic gene expressions were altered in the cbl1 mutant. CBL1 protein physically interacted with AKINβ1, the regulatory β subunit of the SnRK1 complex which has a central role in sugar signaling. Our results indicate a novel role for CBL1 in modulating responses to glucose and GA signals.

Arabidopsis suppressor mutant of abh1 shows a new face of the already known players: ABH1 (CBP80) and ABI4-in response to ABA and abiotic stresses during seed germination

Although the importance of abscisic acid (ABA) in plant development and response to abiotic and biotic stresses is well recognized, the molecular basis of the signaling pathway has not been fully elucidated. Mutants in genes related to ABA are widely used as a tool for gaining insight into the mechanisms of ABA signal transduction and ABA-dependent stress response. We used a genetic approach of a suppressor screening in order to decipher the interaction between ABH1 (CBP80) and other components of ABA signaling. ABH1 (CBP80) encodes a large subunit of CBC (CAP BINDING COMPLEX) and the abh1 mutant is drought-tolerant and hypersensitive to ABA during seed germination. The suppressor mutants of abh1 were generated after chemical mutagenesis. The mutant named soa1 (suppressor of abh1 hypersensitivity to ABA 1) displayed an ABA-insensitive phenotype during seed germination. The genetic analysis showed that the soa1 phenotype is dominant in relation to abh1 and segregates as a single locus. Based on soa1's response to a wide spectrum of physiological assays during different stages of development, we used the candidate-genes approach in order to identify a suppressor gene. The molecular analysis revealed that mutation causing the phenotype of soa1 occurred in the ABI4 (ABA insensitive 4) gene. Analysis of pre-miR159 expression, whose processing depends on CBC, as well as targets of miR159: MYB33 and MYB101, which are positive regulators of ABA signaling, revealed a possible link between CBP80 (ABH1) and ABI4 presented here.

Abscisic Acid synthesis and response

Abscisic acid (ABA) is one of the "classical" plant hormones, i.e. discovered at least 50 years ago, that regulates many aspects of plant growth and development. This chapter reviews our current understanding of ABA synthesis, metabolism, transport, and signal transduction, emphasizing knowledge gained from studies of Arabidopsis. A combination of genetic, molecular and biochemical studies has identified nearly all of the enzymes involved in ABA metabolism, almost 200 loci regulating ABA response, and thousands of genes regulated by ABA in various contexts. Some of these regulators are implicated in cross-talk with other developmental, environmental or hormonal signals. Specific details of the ABA signaling mechanisms vary among tissues or developmental stages; these are discussed in the context of ABA effects on seed maturation, germination, seedling growth, vegetative stress responses, stomatal regulation, pathogen response, flowering, and senescence.

EXO modifies sucrose and trehalose responses and connects the extracellular carbon status to growth

Plants have the capacity to adapt growth to changing environmental conditions. This implies the modulation of metabolism according to the availability of carbon (C). Particular interest in the response to the C availability is based on the increasing atmospheric levels of CO2. Several regulatory pathways that link the C status to growth have emerged. The extracellular EXO protein is essential for cell expansion and promotes shoot and root growth. Homologous proteins were identified in evolutionarily distant green plants. We show here that the EXO protein connects growth with C responses. The exo mutant displayed altered responses to exogenous sucrose supplemented to the growth medium. Impaired growth of the mutant in synthetic medium was associated with the accumulation of starch and anthocyanins, altered expression of sugar-responsive genes, and increased abscisic acid levels. Thus, EXO modulates several responses related to the C availability. Growth retardation on medium supplemented with 2-deoxy-glucose, mannose, and palatinose was similar to the wild type. Trehalose feeding stimulated root growth and shoot biomass production of exo plants whereas it inhibited growth of the wild type. The phenotypic features of the exo mutant suggest that apoplastic processes coordinate growth and C responses.

Asg1 is a stress-inducible gene which increases stomatal resistance in salt stressed potato

The identification of critical components in plant salt stress adaptation has greatly benefitted, in the last two decades, from fundamental discoveries in Arabidopsis and close model systems. Nevertheless, this approach has also highlighted a non-complete overlap between stress tolerance mechanisms in Arabidopsis and agricultural crops. Within a long-running research program aimed at identifying salt stress genetic determinants in potato by functional screening in Escherichia coli, we isolated Asg1, a stress-related gene with an unknown function. Asg1 is induced by salt stress in both potato and Arabidopsis and by abscisic acid in Arabidopsis. Asg1 is actively transcribed in all plant tissues. Furthermore, Asg1 promoter analysis confirmed its ubiquitous expression, which was remarkable in pollen, a plant tissue that undergoes drastic dehydration/hydration processes. Fusion of Asg1 with green fluorescent protein showed that the encoded protein is localized close to the plasma membrane with a non-continuous pattern of distribution. In addition, Arabidopsis knockout asg1 mutants were insensitive to both NaCl and sugar hyperosmotic environments during seed germination. Transgenic potato plants over-expressing the Asg1 gene revealed a stomatal hypersensitivity to NaCl stress which, however, did not result in a significantly improved tuber yield in stress conditions. Altogether, these data suggest that Asg1 might interfere with components of the stress signaling pathway by promoting stomatal closure and participating in stress adaptation.

Sugar-hormone cross-talk in anthocyanin biosynthesis

Anthocyanins, a class of flavonoids, are recognized for their diverse functions in plant development and beneficial effects on human health. Many of the genes encoding anthocyanin biosynthesis enzymes and the transcription factors that activate or repress them have been identified. Regulatory proteins that control anthocyanin biosynthesis by regulating the expression of different structural genes at the transcriptional and post-transcriptional levels are differentially modulated by environmental and biological factors such as light, temperature, sugar and hormones. This minireview summarizes the recent findings contributing to our understanding of the role of sugars and hormones in the modulation of the anthocyanin biosynthesis pathway with emphasis on the coordinated regulation of the critical transcriptional R2R3-MYB/bHLH/WD40 (MBW) complex in Arabidopsis.

A rice F-box gene, OsFbx352, is involved in glucose-delayed seed germination in rice

F-box proteins play diverse roles in regulating numerous physiological processes in plants. This study isolated a gene (OsFbx352) from rice encoding an F-box domain protein and characterized its role in seed germination. Expression of OsFbx352 was upregulated by abscisic acid (ABA). The transcripts of OsFbx352 were increased upon imbibition of rice seeds and the increase was markedly suppressed by glucose. Germination of seeds with overexpression of OsFbx352 was less suppressed by glucose than that of wild-type seeds, while glucose had greater inhibition for germination of seeds with knockdown of OsFbx352 by RNA interference (RNAi) than that of wild-type seeds. The differential response of germination of the transgenic and wild-type seeds to glucose may be accounted for by differences in ABA content among overexpressing, RNAi, and wild-type seeds such that overexpression of OsFbx352 and knockdown of OsFbx352 led to lower and higher ABA contents, respectively, than that of wild-type seeds in the presence of glucose. Overexpression of OsFbx352 led to a reduction in expression of genes responsible for ABA synthesis (OsNced2, OsNced3) and an increase in expression of genes encoding ABA catabolism (OsAba-ox2, OsAba-ox3) in the presence of glucose. These findings indicate that OsFbx352 plays a regulatory role in the regulation of glucose-induced suppression of seed germination by targeting ABA metabolism.

An autoregulatory feedback loop involving PAP1 and TAS4 in response to sugars in Arabidopsis

miR828 in Arabidopsis triggers the cleavage of Trans-Acting SiRNA Gene 4 (TAS4) transcripts and production of small interfering RNAs (ta-siRNAs). One siRNA, TAS4-siRNA81(-), targets a set of MYB transcription factors including PAP1, PAP2, and MYB113 which regulate the anthocyanin biosynthesis pathway. Interestingly, miR828 also targets MYB113, suggesting a close relationship between these MYBs, miR828, and TAS4, but their evolutionary origins are unknown. We found that PAP1, PAP2, and TAS4 expression is induced specifically by exogenous treatment with sucrose and glucose in seedlings. The induction is attenuated in abscisic acid (ABA) pathway mutants, especially in abi3-1 and abi5-1 for PAP1 or PAP2, while no such effect is observed for TAS4. PAP1 is under regulation by TAS4, demonstrated by the accumulation of PAP1 transcripts and anthocyanin in ta-siRNA biogenesis pathway mutants. TAS4-siR81(-) expression is induced by physiological concentrations of Suc and Glc and in pap1-D, an activation-tagged line, indicating a feedback regulatory loop exists between PAP1 and TAS4. Bioinformatic analysis revealed MIR828 homologues in dicots and gymnosperms, but only in one basal monocot, whereas TAS4 is only found in dicots. Consistent with this observation, PAP1, PAP2, and MYB113 dicot paralogs show peptide and nucleotide footprints for the TAS4-siR81(-) binding site, providing evidence for purifying selection in contrast to monocots. Extended sequence similarities between MIR828, MYBs, and TAS4 support an inverted duplication model for the evolution of MIR828 from an ancestral gymnosperm MYB gene and subsequent formation of TAS4 by duplication of the miR828* arm. We obtained evidence by modified 5'-RACE for a MYB mRNA cleavage product guided by miR828 in Pinus resinosa. Taken together, our results suggest that regulation of anthocyanin biosynthesis by TAS4 and miR828 in higher plants is evolutionarily significant and consistent with the evolution of TAS4 since the dicot-monocot divergence.

Arabidopsis sucrose transporter SUT4 interacts with cytochrome b5-2 to regulate seed germination in response to sucrose and glucose

It remains unknown whether a sucrose transporter mediates sugar signaling. Here, we report that the Arabidopsis (Arabidopsis thaliana) sucrose transporter SUT4 interacts with five members of the Arabidopsis cytochrome b5 (Cyb5) family, and sucrose represses the interaction between SUT4 and a Cyb5 member Cyb5-2/A. We observed that down-regulation of SUT4 and three cytochrome b5 members (Cyb5-2, Cyb5-4, and Cyb5-6) confers the sucrose- and glucose-insensitive phenotypes in the sucrose/glucose-induced inhibition of seed germination. The sut4 cyb5-2 double mutant displays slightly stronger sucrose/glucose-insensitive phenotypes than either the sut4 or cyb5-2 single mutant. We showed that the SUT4/Cyb5-2-mediated signaling in the sucrose/glucose-induced inhibition of seed germination does not require ABA or the currently known ABI2/ABI4/ABI5-mediated signaling pathway(s). These data provide evidence that the sucrose transporter SUT4 interacts with Cyb5 to positively mediate sucrose and glucose signaling in the sucrose/glucose-induced inhibition of seed germination.

Analysis of Arabidopsis glucose insensitive growth mutants reveals the involvement of the plastidial copper transporter PAA1 in glucose-induced intracellular signaling

Sugars play important roles in many aspects of plant growth and development, acting as both energy sources and signaling molecules. With the successful use of genetic approaches, the molecular components involved in sugar signaling have been identified and their regulatory roles in the pathways have been elucidated. Here, we describe novel mutants of Arabidopsis (Arabidopsis thaliana), named glucose insensitive growth (gig), identified by their insensitivity to high-glucose (Glc)-induced growth inhibition. The gig mutant displayed retarded growth under normal growth conditions and also showed alterations in the expression of Glc-responsive genes under high-Glc conditions. Our molecular identification reveals that GIG encodes the plastidial copper (Cu) transporter PAA1 (for P(1B)-type ATPase 1). Interestingly, double mutant analysis indicated that in high Glc, gig is epistatic to both hexokinase1 (hxk1) and aba insensitive4 (abi4), major regulators in sugar and retrograde signaling. Under high-Glc conditions, the addition of Cu had no effect on the recovery of gig/paa1 to the wild type, whereas exogenous Cu feeding could suppress its phenotype under normal growth conditions. The expression of GIG/PAA1 was also altered by mutations in the nuclear factors HXK1, ABI3, and ABI4 in high Glc. Furthermore, a transient expression assay revealed the interaction between ABI4 and the GIG/PAA1 promoter, suggesting that ABI4 actively regulates the transcription of GIG/PAA1, likely binding to the CCAC/ACGT core element of the GIG/PAA1 promoter. Our findings indicate that the plastidial Cu transporter PAA1, which is essential for plastid function and/or activity, plays an important role in bidirectional communication between the plastid and the nucleus in high Glc.

Killing two birds with one stone: transcriptional regulators coordinate development and stress responses in plants

Plants, being immobile, must adjust development in response to various stresses, external and internal. Although much has been learned about the mechanisms that regulate development and sugar signaling and response, how the two processes are coordinated is poorly understood. GRAS-family transcriptional regulators SHORT-ROOT (SHR) and SCARECROW (SCR) are crucial to radial patterning and stem-cell renewal in the Arabidopsis root. We found that they directly control genes involved not only in development but also in stress responses and that SCR is pivotal in modulating sugar homeostasis and response. Our data suggest that SHR and SCR promote root growth by suppressing the deleterious effects of stress and that ABI4 has a dual role in sugar response and root growth. Other transcriptional regulators have also been reported to play dual roles in plant growth and stress responses. I therefore propose that regulation of both development and stress responses by single transcriptional regulators is a general and efficient mechanism of adaptive response in plants.

Combinatorial signal integration by APETALA2/Ethylene Response Factor (ERF)-transcription factors and the involvement of AP2-2 in starvation response

Transcription factors of the APETALA 2/Ethylene Response Factor (AP2/ERF)- family have been implicated in diverse processes during development, stress acclimation and retrograde signaling. Fifty-three leaf-expressed AP2/ERFs were screened for their transcriptional response to abscisic acid (ABA), 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), methylviologen (MV), sucrose and high or low light, respectively, and revealed high reactivity to these effectors. Six of them (AP2-2, ARF14, CEJ1, ERF8, ERF11, RAP2.5) were selected for combinatorial response analysis to ABA, DCMU and high light. Additive, synergistic and antagonistic effects demonstrated that these transcription factors are components of multiple signaling pathways. AP2-2 (At1g79700) was subjected to an in depth study. AP2-2 transcripts were high under conditions linked to limited carbohydrate availability and stress and down-regulated in extended light phase, high light or in the presence of sugar. ap2-2 knock out plants had unchanged metabolite profiles and transcript levels of co-expressed genes in extended darkness. However, ap2-2 revealed more efficient germination and faster early growth under high sugar, osmotic or salinity stress, but the difference was abolished in the absence of sugar or during subsequent growth. It is suggested that AP2-2 is involved in mediating starvation-related and hormonal signals.

Sugars, signalling, and plant development

Like all organisms, plants require energy for growth. They achieve this by absorbing light and fixing it into a usable, chemical form via photosynthesis. The resulting carbohydrate (sugar) energy is then utilized as substrates for growth, or stored as reserves. It is therefore not surprising that modulation of carbohydrate metabolism can have profound effects on plant growth, particularly cell division and expansion. However, recent studies on mutants such as stimpy or ramosa3 have also suggested that sugars can act as signalling molecules that control distinct aspects of plant development. This review will focus on these more specific roles of sugars in development, and will concentrate on two major areas: (i) cross-talk between sugar and hormonal signalling; and (ii) potential direct developmental effects of sugars. In the latter, developmental mutant phenotypes that are modulated by sugars as well as a putative role for trehalose-6-phosphate in inflorescence development are discussed. Because plant growth and development are plastic, and are greatly affected by environmental and nutritional conditions, the distinction between purely metabolic and specific developmental effects is somewhat blurred, but the focus will be on clear examples where sugar-related processes or molecules have been linked to known developmental mechanisms.

SCARECROW has a SHORT-ROOT-independent role in modulating the sugar response

Sugar is essential for all cellular activities, but at high levels it inhibits growth and development. How plants balance the tradeoffs between the need for sugars and their growth inhibitory effects is poorly understood. SHORT-ROOT (SHR) and SCARECROW (SCR) are key regulators of stem cell renewal and radial patterning in the root of Arabidopsis (Arabidopsis thaliana). Recently, we identified direct targets of SHR at the genome scale. Intriguingly, among the top-ranked list, we found a number of genes that are involved in stress responses. By chromatin immunoprecipitation-polymerase chain reaction (PCR), we showed that SHR and SCR regulate a similar but not identical set of stress response genes. Consistent with this, scr and shr were found to be hypersensitive to abscisic acid (ABA). We further showed that both mutants were hypersensitive to high levels of glucose (Glc) but responded normally to high salinity and osmoticum. The endogenous levels of sucrose, Glc, and fructose were also elevated in shr and scr. Intriguingly, although shr had sugar content and developmental defects similar to those of scr, it was much less sensitive to Glc. Chromatin immunoprecipitation-PCR and reverse transcription-PCR assays as well as transgenic studies with an ABA-INSENSITIVE2 (ABI4)-β-glucuronidase reporter construct revealed that in root, SCR, but not SHR, repressed ABI4 and ABI5 directly and specifically in the apical meristem. When combined with abi4, scr became much more tolerant of high Glc. Finally, transgenic plants expressing ABI4 under the control of the SCR promoter manifested a short-root phenotype. These results together suggest that SCR has a SHR-independent role in mitigating the sugar response and that this role of SCR is important for root growth.

Involvement of the abscisic acid catabolic gene CYP707A2 in the glucose-induced delay in seed germination and post-germination growth of Arabidopsis

Earlier studies showed that sugars as signaling molecules play pivotal roles in the regulation of seed germination. ABA biosynthesis upregulation is suggested as one of the possible mechanisms mediating the glucose-induced delay in seed germination. In this study, the role of ABA catabolism in glucose-induced inhibition was investigated. Using Arabidopsis thaliana seeds, the results show that the repression of ABA catabolism by diniconazole aggravated the glucose-induced delay in seed germination. The transcript and protein profiles of CYP707A2, a key gene encoding ABA 8'-hydroxylase in ABA catabolism in Arabidopsis, were significantly decreased by exogenous glucose treatment. Transgenic experiments confirmed that the over-expression of the CYP707A2 gene alleviated the glucose-induced inhibition effect, whereas the cyp707a2 mutant seeds displayed a hypersensitivity to glucose during imbibition. Exogenous glucose also arrested the early seedling development of Arabidopsis. The CYP707A2 over-expression seedlings exhibited lower ABA levels and seemed less sensitive to exogenous glucose compared with wild type seedlings. In summary, the glucose-induced delay in seed germination and seedling development is directly related to the suppression of ABA catabolism through the repression of the CYP707A2 expression.

Over-expression of JcDREB, a putative AP2/EREBP domain-containing transcription factor gene in woody biodiesel plant Jatropha curcas, enhances salt and freezing tolerance in transgenic Arabidopsis thaliana

Jatropha curcas L. is an all-purpose biodiesel plant and is widely distributed in tropical and subtropical climates. It can grow well on poor quality soil which is not qualified for crop cultivation. This is very important for relieving land, food and energy crises. However, tropical and subtropical distribution limits the production of J. curcas seed. So it is valuable to know the molecular mechanism of J. curcas response to adverse abiotic environmental factors, especially freezing stress, in order to change the plant's characteristics. Until now there are just a few reports about J. curcas molecular biology. In this paper, we cloned and characterized a DNA binding protein from this plant, designated as JcDREB. Sequence analysis and yeast one-hybrid assays show that JcDREB can effectively function as a transcription factor of DREB protein family belonging to A-6 subgroup member. Expression patterns of JcDREB showed that it was induced by cold, salt and drought stresses, not by ABA. Over-expression of JcDREB in transgenic Arabidopsis exhibited enhanced salt and freezing stresses. Understanding the molecular mechanisms of J. curcas responses to environmental stresses, for example, high salinity, drought and low temperature, is crucial for improving their stress tolerance and productivity. This work provides more information about A-6 subgroup members of DREB subfamily.

ABA-mediated heterophylly is regulated by differential expression of 9-cis-epoxycarotenoid dioxygenase 3 in lilies

Although exogenous ABA-regulated heterophylly has been well documented in multiple plant species, the effect of endogenous ABA and its molecular mechanism remain uncharacterized. In the present study, the effects of endogenous ABA on heterophyllous switching were investigated in two different lily varieties, Lilium formosanum and Lilium oriental hybrid 'Casa Blanca'. Seedlings of L. formosanum, which have scale-leaf-type growth, displayed low levels of both 9-cis-epoxycarotenoid dioxygenase 3 (LfNCED3) transcripts and ABA, whereas seedlings of L. oriental hybrid 'Casa Blanca', which have scale-type growth, displayed high levels of both LoNCED3 transcripts and ABA. Sucrose induced endogenous ABA production in cultured lilies; low ABA induction shows scale-leaf-type growth, whereas scale-type growth becomes predominant when ABA levels are high. Heterologous expression of either LfNCED3 or LoNCED3 was found to complement the Arabidopsis Atnced3 mutant. Interestingly, the expression patterns of LfNCED3 and LoNCED3 in transgenic Arabidopsis plants are distinguishable. Further promoter analysis revealed that a putative E2F-like element in the LfNCED3 promoter, but not in the LoNCED3 promoter, plays a negative role in controlling its activity. Collectively, our results demonstrate that NCED3 plays a key role in ABA-mediated heterophylly in lilies.

The Arabidopsis bZIP gene AtbZIP63 is a sensitive integrator of transient abscisic acid and glucose signals

Glucose modulates plant metabolism, growth, and development. In Arabidopsis (Arabidopsis thaliana), Hexokinase1 (HXK1) is a glucose sensor that may trigger abscisic acid (ABA) synthesis and sensitivity to mediate glucose-induced inhibition of seedling development. Here, we show that the intensity of short-term responses to glucose can vary with ABA activity. We report that the transient (2 h/4 h) repression by 2% glucose of AtbZIP63, a gene encoding a basic-leucine zipper (bZIP) transcription factor partially involved in the Snf1-related kinase KIN10-induced responses to energy limitation, is independent of HXK1 and is not mediated by changes in ABA levels. However, high-concentration (6%) glucose-mediated repression appears to be modulated by ABA, since full repression of AtbZIP63 requires a functional ABA biosynthetic pathway. Furthermore, the combination of glucose and ABA was able to trigger a synergistic repression of AtbZIP63 and its homologue AtbZIP3, revealing a shared regulatory feature consisting of the modulation of glucose sensitivity by ABA. The synergistic regulation of AtbZIP63 was not reproduced by an AtbZIP63 promoter-5'-untranslated region::β-glucuronidase fusion, thus suggesting possible posttranscriptional control. A transcriptional inhibition assay with cordycepin provided further evidence for the regulation of mRNA decay in response to glucose plus ABA. Overall, these results indicate that AtbZIP63 is an important node of the glucose-ABA interaction network. The mechanisms by which AtbZIP63 may participate in the fine-tuning of ABA-mediated abiotic stress responses according to sugar availability (i.e., energy status) are discussed.

The transcription factor ABI4 Is required for the ascorbic acid-dependent regulation of growth and regulation of jasmonate-dependent defense signaling pathways in Arabidopsis

Cellular redox homeostasis is a hub for signal integration. Interactions between redox metabolism and the ABSCISIC ACID-INSENSITIVE-4 (ABI4) transcription factor were characterized in the Arabidopsis thaliana vitamin c defective1 (vtc1) and vtc2 mutants, which are defective in ascorbic acid synthesis and show a slow growth phenotype together with enhanced abscisic acid (ABA) levels relative to the wild type (Columbia-0). The 75% decrease in the leaf ascorbate pool in the vtc2 mutants was not sufficient to adversely affect GA metabolism. The transcriptome signatures of the abi4, vtc1, and vtc2 mutants showed significant overlap, with a large number of transcription factors or signaling components similarly repressed or induced. Moreover, lincomycin-dependent changes in LIGHT HARVESTING CHLOROPHYLL A/B BINDING PROTEIN 1.1 expression were comparable in these mutants, suggesting overlapping participation in chloroplast to nucleus signaling. The slow growth phenotype of vtc2 was absent in the abi4 vtc2 double mutant, as was the sugar-insensitive phenotype of the abi4 mutant. Octadecanoid derivative-responsive AP2/ERF-domain transcription factor 47 (ORA47) and AP3 (an ABI5 binding factor) transcripts were enhanced in vtc2 but repressed in abi4 vtc2, suggesting that ABI4 and ascorbate modulate growth and defense gene expression through jasmonate signaling. We conclude that low ascorbate triggers ABA- and jasmonate-dependent signaling pathways that together regulate growth through ABI4. Moreover, cellular redox homeostasis exerts a strong influence on sugar-dependent growth regulation.

AP2/ERF family transcription factors in plant abiotic stress responses

In terrestrial environments, temperature and water conditions are highly variable, and extreme temperatures and water conditions affect the survival, growth and reproduction of plants. To protect cells and sustain growth under such conditions of abiotic stress, plants respond to unfavourable changes in their environments in developmental, physiological and biochemical ways. These responses require expression of stress-responsive genes, which are regulated by a network of transcription factors. The AP2/ERF family is a large family of plant-specific transcription factors that share a well-conserved DNA-binding domain. This transcription factor family includes DRE-binding proteins (DREBs), which activate the expression of abiotic stress-responsive genes via specific binding to the dehydration-responsive element/C-repeat (DRE/CRT) cis-acting element in their promoters. In this review, we discuss the functions of the AP2/ERF-type transcription factors in plant abiotic stress responses, with special emphasis on the regulations and functions of two major types of DREBs, DREB1/CBF and DREB2. In addition, we summarise the involvement of other AP2/ERF-type transcription factors in abiotic stress responses, which has recently become clear. This article is part of a Special Issue entitled: Plant gene regulation in response to abiotic stress.

EXORDIUM-LIKE1 promotes growth during low carbon availability in Arabidopsis

Little is known about genes that control growth and development under low carbon (C) availability. The Arabidopsis (Arabidopsis thaliana) EXORDIUM-LIKE1 (EXL1) gene (At1g35140) was identified as a brassinosteroid-regulated gene in a previous study. We show here that the EXL1 protein is required for adaptation to C- and energy-limiting growth conditions. In-depth analysis of EXL1 transcript levels under various environmental conditions indicated that EXL1 expression is controlled by the C and energy status. Sugar starvation, extended night, and anoxia stress induced EXL1 gene expression. The C status also determined EXL1 protein levels. These results suggested that EXL1 is involved in the C-starvation response. Phenotypic changes of an exl1 loss-of-function mutant became evident only under corresponding experimental conditions. The mutant showed diminished biomass production in a short-day/low-light growth regime, impaired survival during extended night, and impaired survival of anoxia stress. Basic metabolic processes and signaling pathways are presumed to be barely impaired in exl1, because the mutant showed wild-type levels of major sugars, and transcript levels of only a few genes such as QUA-QUINE STARCH were altered. Our data suggest that EXL1 is part of a regulatory pathway that controls growth and development when C and energy supply is poor.

Accumulation of the transcription factor ABA-insensitive (ABI)4 is tightly regulated post-transcriptionally

ABA-INSENSITIVE (ABI)4 is a transcription factor implicated in response to ABA in maturing seeds, and seedling responses to ABA, salt, and sugar. Previous studies have shown that ABI4 transcripts are high in seeds and in seedlings exposed to high concentrations of glucose and, to a lesser extent, osmotic agents and ABA, but that transcript levels are very low through most of vegetative growth. This study examined ABI4 protein accumulation indirectly, using transgenic lines expressing fusions to GFP and GUS. The GFP fusions were active, but undetectable visually or immunologically. Comparison of transcript and activity levels for GUS expression showed that inclusion of the ABI4 coding sequence reduced the ratio of activity to transcript ∼40-fold when driven by the CaMV 35S promoter, and nearly 150-fold when controlled by the ABI4 promoter. At least part of this discrepancy is due to proteasomal degradation of ABI4, resulting in a half-life of 5-6 h for the ABI4-GUS fusion. Comparison of the spatial localization of transcripts and fusion proteins indicated that the protein preferentially accumulated in roots such that transcript and protein distribution had little similarity. The components mediating targeting to the proteasome or other mechanisms of spatial restriction have not yet been identified, but several domains of ABI4 appear to contribute to its instability.

The role of the pod in seed development: strategies for manipulating yield

Pods play a key role in encapsulating the developing seeds and protecting them from pests and pathogens. In addition to this protective function, it has been shown that the photosynthetically active pod wall contributes assimilates and nutrients to fuel seed growth. Recent work has revealed that signals originating from the pod may also act to coordinate grain filling and regulate the reallocation of reserves from damaged seeds to those that have retained viability. In this review we consider the evidence that pods can regulate seed growth and maturation, particularly in members of the Brassicaceae family, and explore how the timing and duration of pod development might be manipulated to enhance either the quantity of crop yield or its nutritional properties.

Control of abscisic acid catabolism and abscisic acid homeostasis is important for reproductive stage stress tolerance in cereals

Drought stress at the reproductive stage causes pollen sterility and grain loss in wheat (Triticum aestivum). Drought stress induces abscisic acid (ABA) biosynthesis genes in anthers and ABA accumulation in spikes of drought-sensitive wheat varieties. In contrast, drought-tolerant wheat accumulates lower ABA levels, which correlates with lower ABA biosynthesis and higher ABA catabolic gene expression (ABA 8'-hydroxylase). Wheat TaABA8'OH1 deletion lines accumulate higher spike ABA levels and are more drought sensitive. ABA treatment of the spike mimics the effect of drought, causing high levels of sterility. ABA treatment represses the anther cell wall invertase gene TaIVR1, and drought-tolerant lines appeared to be more sensitive to the effect of ABA. Drought-induced sterility shows similarity to cold-induced sterility in rice (Oryza sativa). In cold-stressed rice, the rate of ABA accumulation was similar in cold-sensitive and cold-tolerant lines during the first 8 h of cold treatment, but in the tolerant line, ABA catabolism reduced ABA levels between 8 and 16 h of cold treatment. The ABA biosynthesis gene encoding 9-cis-epoxycarotenoid dioxygenase in anthers is mainly expressed in parenchyma cells surrounding the vascular bundle of the anther. Transgenic rice lines expressing the wheat TaABA8'OH1 gene under the control of the OsG6B tapetum-specific promoter resulted in reduced anther ABA levels under cold conditions. The transgenic lines showed that anther sink strength (OsINV4) was maintained under cold conditions and that this correlated with improved cold stress tolerance. Our data indicate that ABA and ABA 8'-hydroxylase play an important role in controlling anther ABA homeostasis and reproductive stage abiotic stress tolerance in cereals.

Transcriptional control of aspartate kinase expression during darkness and sugar depletion in Arabidopsis: involvement of bZIP transcription factors

Initial steps of aspartate-derived biosynthesis pathway (Asp pathway) producing Lys, Thr, Met and Ile are catalyzed by bifunctional (AK/HSD) and monofunctional (AK-lys) aspartate kinase (AK) enzymes. Here, we show that transcription of all AK genes is negatively regulated under darkness and low sugar conditions. By using yeast one-hybrid assays and complementary chromatin immunoprecipitation analyses in Arabidopsis cells, the bZIP transcription factors ABI5 and DPBF4 were identified, capable of interacting with the G-box-containing enhancer of AK/HSD1 promoter. Elevated transcript levels of DPBF4 and ABI5 under darkness and low sugar conditions coincide with the repression of AK gene expression. Overexpression of ABI5, but not DPBF4, further increases this AK transcription suppression. Concomitantly, it also increases the expression of asparagines synthetase 1 (ASN1) that shifts aspartate utilization towards asparagine formation. However, in abi5 or dpbf4 mutant and abi5, dpbf4 double mutant the repression of AK expression is maintained, indicating a functional redundancy with other bZIP-TFs. A dominant-negative version of DPBF4 fused to the SRDX repressor domain of SUPERMAN could counteract the repression and stimulate AK expression under low sugar and darkness in planta. This effect was verified by showing that DPBF4-SRDX fails to recognize the AK/HSD1 enhancer sequence in yeast one-hybrid assays, but increases heterodimmer formation with DPBF4 and ABI5, as estimated by yeast two-hybrid assays. Hence it is likely that heterodimerization with DPBF4-SRDX inhibits the binding of redundantly functioning bZIP-TFs to the promoters of AK genes and thereby releases the repressing effect. These data highlight a novel transcription control of the chloroplast aspartate pathway that operates under energy limiting conditions.

Expression of ABSCISIC ACID INSENSITIVE 4 (ABI4) in developing Arabidopsis seedlings

We have recently demonstrated that the transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4) mediates abscisic acid and cytokinin inhibition of lateral root formation by reducing polar auxin transport in Arabidopsis thaliana. ( 1) In that study, we provided a direct demonstration of ABI4 expression in phloem companion cells and parenchyma of the vascular system in the mature regions of the roots. Although also studied in mature plants, ABI4 has been studied primarily in germinating seedlings, and its expression has been assumed by some researchers to be restricted to early germination stages. We thus constructed transgenic Arabidopsis plants expressing an ABI4:GUS construct, and followed ABI4 promoter activity during seedling development, focusing on the roots.

Water deficits uncouple growth from photosynthesis, increase C content, and modify the relationships between C and growth in sink organs

In plants, carbon (C) molecules provide building blocks for biomass production, fuel for energy, and exert signalling roles to shape development and metabolism. Accordingly, plant growth is well correlated with light interception and energy conversion through photosynthesis. Because water deficits close stomata and thus reduce C entry, it has been hypothesised that droughted plants are under C starvation and their growth under C limitation. In this review, these points are questioned by combining literature review with experimental and modelling illustrations in various plant organs and species. First, converging evidence is gathered from the literature that water deficit generally increases C concentration in plant organs. The hypothesis is raised that this could be due to organ expansion (as a major C sink) being affected earlier and more intensively than photosynthesis (C source) and metabolism. How such an increase is likely to interact with C signalling is not known. Hence, the literature is reviewed for possible links between C and stress signalling that could take part in this interaction. Finally, the possible impact of water deficit-induced C accumulation on growth is questioned for various sink organs of several species by combining published as well as new experimental data or data generated using a modelling approach. To this aim, robust correlations between C availability and sink organ growth are reported in the absence of water deficit. Under water deficit, relationships weaken or are modified suggesting release of the influence of C availability on sink organ growth. These results are interpreted as the signature of a transition from source to sink growth limitation under water deficit.

Direct targets of the transcription factors ABA-Insensitive(ABI)4 and ABI5 reveal synergistic action by ABI4 and several bZIP ABA response factors

The plant hormone abscisic acid (ABA) is a key regulator of seed development. In addition to promoting seed maturation, ABA inhibits seed germination and seedling growth. Many components involved in ABA response have been identified, including the transcription factors ABA insensitive (ABI)4 and ABI5. The genes encoding these factors are expressed predominantly in developing and mature seeds, and are positive regulators of ABA mediated inhibition of seed germination and growth. The direct effects of ABI4 and ABI5 in ABA response remain largely undefined. To address this question, plants over-expressing ABI4 or ABI5 were used to allow identification of direct transcriptional targets. Ectopically expressed ABI4 and ABI5 conferred ABA-dependent induction of slightly over 100 genes in 11 day old plants. In addition to effector genes involved in seed maturation and reserve storage, several signaling proteins and transcription factors were identified as targets of ABI4 and/or ABI5. Although only 12% of the ABA- and ABI-dependent transcriptional targets were induced by both ABI factors in 11 day old plants, 40% of those normally expressed in seeds had reduced transcript levels in both abi4 and abi5 mutants. Surprisingly, many of the ABI4 transcriptional targets do not contain the previously characterized ABI4 binding motifs, the CE1 or S box, in their promoters, but some of these interact with ABI4 in electrophoretic mobility shift assays, suggesting that sequence recognition by ABI4 may be more flexible than known canonical sequences. Yeast one-hybrid assays demonstrated synergistic action of ABI4 with ABI5 or related bZIP factors in regulating these promoters, and mutant analyses showed that ABI4 and these bZIPs share some functions in plants.

Fructose sensitivity is suppressed in Arabidopsis by the transcription factor ANAC089 lacking the membrane-bound domain

In living organisms sugars not only provide energy and carbon skeletons but also act as evolutionarily conserved signaling molecules. The three major soluble sugars in plants are sucrose, glucose, and fructose. Information on plant glucose and sucrose signaling is available, but to date no fructose-specific signaling pathway has been reported. In this study, sugar repression of seedling development was used to study fructose sensitivity in the Landsberg erecta (Ler)/Cape Verde Islands (Cvi) recombinant inbred line population, and eight fructose-sensing quantitative trait loci (QTLs) (FSQ1-8) were mapped. Among them, FSQ6 was confirmed to be a fructose-specific QTL by analyzing near-isogenic lines in which Cvi genomic fragments were introgressed in the Ler background. These results indicate the existence of a fructose-specific signaling pathway in Arabidopsis. Further analysis demonstrated that the FSQ6-associated fructose-signaling pathway functions independently of the hexokinase1 (HXK1) glucose sensor. Remarkably, fructose-specific FSQ6 downstream signaling interacts with abscisic acid (ABA)- and ethylene-signaling pathways, similar to HXK1-dependent glucose signaling. The Cvi allele of FSQ6 acts as a suppressor of fructose signaling. The FSQ6 gene was identified using map-based cloning approach, and FSQ6 was shown to encode the transcription factor gene Arabidopsis NAC (petunia No apical meristem and Arabidopsis transcription activation factor 1, 2 and Cup-shaped cotyledon 2) domain containing protein 89 (ANAC089). The Cvi allele of FSQ6/ANAC089 is a gain-of-function allele caused by a premature stop in the third exon of the gene. The truncated Cvi FSQ6/ANAC089 protein lacks a membrane association domain that is present in ANAC089 proteins from other Arabidopsis accessions. As a result, Cvi FSQ6/ANAC089 is constitutively active as a transcription factor in the nucleus.

Signaling role of fructose mediated by FINS1/FBP in Arabidopsis thaliana

Sugars are evolutionarily conserved signaling molecules that regulate the growth and development of both unicellular and multicellular organisms. As sugar-producing photosynthetic organisms, plants utilize glucose as one of their major signaling molecules. However, the details of other sugar signaling molecules and their regulatory factors have remained elusive, due to the complexity of the metabolite and hormone interactions that control physiological and developmental programs in plants. We combined information from a gain-of-function cell-based screen and a loss-of-function reverse-genetic analysis to demonstrate that fructose acts as a signaling molecule in Arabidopsis thaliana. Fructose signaling induced seedling developmental arrest and interacted with plant stress hormone signaling in a manner similar to that of glucose. For fructose signaling responses, the plant glucose sensor HEXOKINASE1 (HXK1) was dispensable, while FRUCTOSE INSENSITIVE1 (FINS1), a putative FRUCTOSE-1,6-BISPHOSPHATASE, played a crucial role. Interestingly, FINS1 function in fructose signaling appeared to be independent of its catalytic activity in sugar metabolism. Genetic analysis further indicated that FINS1-dependent fructose signaling may act downstream of the abscisic acid pathway, in spite of the fact that HXK1-dependent glucose signaling works upstream of hormone synthesis. Our findings revealed that multiple layers of controls by fructose, glucose, and abscisic acid finely tune the plant autotrophic transition and modulate early seedling establishment after seed germination.

Interactions between abscisic acid and plastidial glycolysis in Arabidopsis

The phytohormone abscisic acid (ABA) controls the development of plants and plays a crucial role in their response to adverse environmental conditions like salt and water stress. Complex interactions between ABA and sugar signal transduction pathways have been shown. However, the role played by glycolysis in these interactions is not known. In the associated study, we investigated the interactions between plastidial glycolytic glyceraldehyde-3-phosphate dehydrogenase (GAPCp) and ABA signal transduction in Arabidopsis. We followed physiological, genetic and genomic approaches to understand the processes and mechanisms underlying the ABA-glycolysis interactions. Our results indicated that GAPCp deficiency leads to ABA-insensitivity and impaired ABA signal transduction. The gene expression of the transcription factor ABI4, involved in both sugar and ABA signaling, was altered in gapcp double mutants (gapcp1gapcp2), suggesting that the ABA insensitivity of mutants is mediated, at least in part, through this transcriptional regulator. We also suggested that amino acid homeostasis and/or serine metabolism may also be important determinants in the connections of ABA with primary metabolism. These studies provide new insights into the links between plant primary metabolism and ABA signal transduction, and demonstrate the importance of plastidial glycolytic GAPCps in these interactions.

Isolation and functional characterization of CE1 binding proteins

BACKGROUND

Abscisic acid (ABA) is a plant hormone that controls seed germination, protective responses to various abiotic stresses and seed maturation. The ABA-dependent processes entail changes in gene expression. Numerous genes are regulated by ABA, and promoter analyses of the genes revealed that cis-elements sharing the ACGTGGC consensus sequence are ubiquitous among ABA-regulated gene promoters. The importance of the core sequence, which is generally known as ABA response element (ABRE), has been demonstrated by various experiments, and its cognate transcription factors known as ABFs/AREBs have been identified. Although necessary, ABRE alone is not sufficient, and another cis-element known as "coupling element (CE)" is required for full range ABA-regulation of gene expression. Several CEs are known. However, despite their importance, the cognate transcription factors mediating ABA response via CEs have not been reported to date. Here, we report the isolation of transcription factors that bind one of the coupling elements, CE1.

RESULTS

To isolate CE1 binding proteins, we carried out yeast one-hybrid screens. Reporter genes containing a trimer of the CE1 element were prepared and introduced into a yeast strain. The yeast was transformed with library DNA that represents RNA isolated from ABA-treated Arabidopsis seedlings. From the screen of 3.6 million yeast transformants, we isolated 78 positive clones. Analysis of the clones revealed that a group of AP2/ERF domain proteins binds the CE1 element. We investigated their expression patterns and analyzed their overexpression lines to investigate the in vivo functions of the CE element binding factors (CEBFs). Here, we show that one of the CEBFs, AtERF13, confers ABA hypersensitivity in Arabidopsis, whereas two other CEBFs enhance sugar sensitivity.

CONCLUSIONS

Our results indicate that a group of AP2/ERF superfamily proteins interacts with CE1. Several CEBFs are known to mediate defense or abiotic stress response, but the physiological functions of other CEBFs remain to be determined. Our in vivo functional analysis of several CEBFs suggests that they are likely to be involved in ABA and/or sugar response. Together with previous results reported by others, our current data raise an interesting possibility that the coupling element CE1 may function not only as an ABRE but also as an element mediating biotic and abiotic stress responses.

The plant-specific SR45 protein negatively regulates glucose and ABA signaling during early seedling development in Arabidopsis

The plant-specific SR45 belongs to the highly conserved family of serine/arginine-rich (SR) proteins, which play key roles in precursor-mRNA splicing and other aspects of RNA metabolism. An Arabidopsis (Arabidopsis thaliana) loss-of-function mutant, sr45-1, displays pleiotropic phenotypes, such as defects in flower and leaf morphology, root growth, and flowering time. Here, we show that the sr45-1 mutation confers hypersensitivity to glucose (Glc) during early seedling growth in Arabidopsis. Unlike wild-type plants, the sr45-1 mutant displays impaired cotyledon greening and expansion as well as reduced hypocotyl elongation of dark-grown seedlings when grown in the presence of low (3%) Glc concentrations. In addition, SR45 is involved in the control of Glc-responsive gene expression, as the mutant displays enhanced repression of photosynthetic and nitrogen metabolism genes and overinduction of starch and anthocyanin biosynthesis genes. Like many other sugar response mutants, sr45-1 also shows hypersensitivity to abscisic acid (ABA) but appears to be unaffected in ethylene signaling. Importantly, the sr45-1 mutant shows enhanced ability to accumulate ABA in response to Glc, and the ABA biosynthesis inhibitor fluridone partially rescues the sugar-mediated growth arrest. Moreover, three ABA biosynthesis genes and two key ABA signaling genes, ABI3 and ABI5, are markedly overinduced by Glc in sr45-1. These results provide evidence that the SR45 protein defines a novel player in plant sugar response that negatively regulates Glc signaling during early seedling development by down-regulating both Glc-specific ABA accumulation and ABA biosynthesis and signaling gene expression.