Another player joins the complex field of sugar-regulated gene expression in plants

Definition of early transcriptional circuitry involved in light-induced reversal of PIF-imposed repression of photomorphogenesis in young Arabidopsis seedlings

Light signals perceived by the phytochromes induce the transition from skotomorphogenic to photomorphogenic development (deetiolation) in dark-germinated seedlings. Evidence that a quadruple mutant (pifq) lacking four phytochrome-interacting bHLH transcription factors (PIF1, 3, 4, and 5) is constitutively photomorphogenic in darkness establishes that these factors sustain the skotomorphogenic state. Moreover, photoactivated phytochromes bind to and induce rapid degradation of the PIFs, indicating that the photoreceptor reverses their constitutive activity upon light exposure, initiating photomorphogenesis. Here, to define the modes of transcriptional regulation and cellular development imposed by the PIFs, we performed expression profile and cytological analyses of pifq mutant and wild-type seedlings. Dark-grown mutant seedlings display cellular development that extensively phenocopies wild-type seedlings grown in light. Similarly, 80% of the gene expression changes elicited by the absence of the PIFs in dark-grown pifq seedlings are normally induced by prolonged light in wild-type seedlings. By comparing rapidly light-responsive genes in wild-type seedlings with those responding in darkness in the pifq mutant, we identified a subset, enriched in transcription factor-encoding genes, that are potential primary targets of PIF transcriptional regulation. Collectively, these data suggest that the transcriptional response elicited by light-induced PIF proteolysis is a major component of the mechanism by which the phytochromes pleiotropically regulate deetiolation and that at least some of the rapidly light-responsive genes may comprise a transcriptional network directly regulated by the PIF proteins.

Isolation and characterization of shs1, a sugar-hypersensitive and ABA-insensitive mutant with multiple stress responses

To identify salt tolerance determinants, we screened for double mutants from a T-DNA tagged sos3-1 mutant population in the Arabidopsis Col-0 gl1 background. The shs1-1 (sodium hypersensitive) sos3-1 mutant was isolated as more sensitive to NaCl than sos3-1 plants. TAIL-PCR revealed that the introduced T-DNA was located 62 bp upstream of the initiation codon of an adenylate translocator-like protein gene on chromosome IV. SHS1 mRNA did not accumulate in shs1-1 sos3-1 plants although it accumulated in shoots of both sos3-1 and the wild type plants, indicating that this gene is inactive in the mutant. Genetic co-linkage analysis revealed that the mutation causing the phenotype segregated as a recessive, single gene mutation. This mutant showed altered sensitive responses to salt as well as to cold stress. It also demonstrated sugar sensitive and ABA insensitive phenotypes including enhanced germination, reduced growth, altered leaf morphology, and necrosis on leaves at an early growth stage. Sensitivity of sos3-1 shs1-1 root growth to LiCl, KCl, and mannitol was not significantly different from growth of sos3-1 roots. Further, expression of 35S::SHS1 in sos3-1 shs1-1 plants complemented NaCl and sugar sensitivity and partially restored the leaf morphology.

The different growth responses of the Arabidopsis thaliana leaf blade and the petiole during shade avoidance are regulated by photoreceptors and sugar

During the shade-avoidance response, leaf blade expansion is inhibited and petiole elongation is enhanced. In this study, we examined the roles of photoreceptors and sugar on the differential growth of the leaf blade and petiole in shade conditions. Under the conditions examined, cell expansion, not cell division, played a major role in the differential leaf growth. The enhanced cell expansion in the leaf blade is associated with an increase in the ploidy level, whereas cell elongation was stimulated in the petiole in dark conditions without an increase in the ploidy level. Analysis of phytochrome, cryptochrome and phototropin mutants revealed that phytochromes and cryptochromes specifically regulate the contrasting growth patterns of the leaf blade and petiole in shade. Examination of the effects of photo-assimilated sucrose on the growth of the leaf blade and petiole revealed growth-promotional effects of sucrose that are highly dependent on the light conditions. The leaf blades of abscisic acid-deficient and sugar-insensitive mutants did not expand in blue light, but expanded normally in red light. These results suggest that both the regulation of light signals and the modulation of responses to sugar are important in the control of the differential photomorphogenesis of the leaf blade and petiole.

Arabidopsis mutants deficient in diacylglycerol acyltransferase display increased sensitivity to abscisic acid, sugars, and osmotic stress during germination and seedling development

Arabidopsis seeds store triacylglycerol (TAG) as the major carbon reserve, which is used to support postgerminative seedling growth. Diacylglycerol acyltransferase (DGAT) catalyzes the final step in TAG synthesis, and two isoforms of DGAT have previously been identified in Arabidopsis. It has been shown that DGAT1 plays an important role in seed development because Arabidopsis with mutations at the TAG1 locus accumulate less seed oil. There is also evidence showing that DGAT1 is active after seed germination. The aim of this study is to investigate the effect of mutations of DGAT1 on postembryonic development in Arabidopsis. We carried out detailed analyses of two tag1 mutants in different ecotypic backgrounds of Arabidopsis. Results show that during germination and seedling growth, seed storage TAG degradation was not affected in the tag1 mutants. However, sugar content of the mutant seedlings is altered, and activities of the hexokinases are significantly increased in the tag1 mutant seedlings. The tag1 mutants are also more sensitive to abscisic acid, glucose, and osmotic strength of the medium in germination and seedling growth.

Mobilization of seed storage lipid by Arabidopsis seedlings is retarded in the presence of exogenous sugars

BACKGROUND

Soluble sugar levels must be closely regulated in germinating seeds to ensure an adequate supply of energy and building materials for the developing seedling. Studies on germinating cereal seeds indicate that production of sugars from starch is inhibited by increasing sugar levels. Although numerous studies have focused on the regulation of starch metabolism, very few studies have addressed the control of storage lipid metabolism by germinating oilseeds.

RESULTS

Mobilization of storage lipid by germinating seeds of the model oilseed plant Arabidopsis thaliana (L.) Heynh. occurs at a greatly reduced rate in the presence of exogenous glucose or mannose, but not in the presence of equi-molar 3-O-methylglucose or sorbitol. The sugar-insensitive5-1/abscisic acid-insensitive4-101 (sis5-1/abi4-101) mutant is resistant to glucose inhibition of seed storage lipid mobilization. Wild-type seedlings become insensitive to glucose inhibition of storage lipid breakdown within 3 days of the start of imbibition.

CONCLUSIONS

Growth in the presence of exogenous glucose significantly retards mobilization of seed storage lipid in germinating seeds from wild-type Arabidopsis. This effect is not solely due to the osmotic potential of the media, as substantially higher concentrations of sorbitol than of glucose are required to exert significant effects on lipid breakdown. The inhibitory effect of glucose on lipid breakdown is limited to a narrow developmental window, suggesting that completion of some critical metabolic transition results in loss of sensitivity to the inhibitory effect of glucose on lipid breakdown.

Sucrose export defective1 encodes a novel protein implicated in chloroplast-to-nucleus signaling

The Sucrose export defective1 (Sxd1) gene of maize was cloned and shown to encode a novel protein conserved between plants and cyanobacteria. The structure of the Sxd1 locus was determined in wild-type plants and two independent sxd1 alleles. Expression analysis demonstrated that the gene was transcribed in all green tissues, with highest levels in maturing leaf blades. In situ hybridization studies revealed high levels of Sxd1 mRNA in bundle sheath cells, with lower levels within the mesophyll. The SXD1 protein was localized to chloroplasts, in both bundle sheath and mesophyll cells. Levels of sucrose, glucose, and fructose were compared between wild-type and sxd1 plants. Mutant plants were fully capable of producing sucrose and accumulated all three sugars at concentrations above those measured in wild-type plants. Despite these increased sugar concentrations, photosynthetic gene expression was not significantly downregulated in affected areas of sxd1 leaf blades. These results are consistent with photosynthate being trapped within anthocyanin-accumulating regions of sxd1 leaves due to plasmodesmal occlusion at the bundle sheath-vascular parenchyma boundary of the minor veins. A model for SXD1 function is proposed in which the protein is involved in a chloroplast-to-nucleus signaling pathway necessary for proper late-stage differentiation of maize bundle sheath cells, including the developmentally regulated modification of plasmodesmata.

The Arabidopsis sugar-insensitive mutants sis4 and sis5 are defective in abscisic acid synthesis and response

Although soluble sugar levels affect many aspects of plant development and physiology, little is known about the mechanisms by which plants respond to sugar. Here we report the isolation of 13 sugar-insensitive (sis) mutants of Arabidopsis that, unlike wild-type plants, are able to form expanded cotyledons and true leaves when germinated on media containing high concentrations of glucose or sucrose. The sis4 and sis5 mutants are allelic to the ABA-biosynthesis mutant aba2 and the ABA-insensitive mutant abi4, respectively. In addition to being insensitive to glucose and sucrose, the sis4/aba2 and sis5/abi4 mutants also display decreased sensitivity to the inhibitory effects of mannose on early seedling development. Mutations in the ABI5 gene, but not mutations in the ABI1, ABI2 or ABI3 genes, also lead to weak glucose- and mannose-insensitive phenotypes. Wild-type and mutant plants show similar responses to the effects of exogenous sugar on chlorophyll and anthocyanin accumulation, indicating that the mutants are not defective in all sugar responses. These results indicate that defects in ABA metabolism and some, but not all, defects in ABA response can also alter response to exogenous sugar.

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

Sugars have signaling roles in a wide variety of developmental processes in plants. To elucidate the regulatory components that constitute the glucose signaling network governing plant growth and development, we have isolated and characterized two Arabidopsisglucose insensitive mutants, gin5 and gin6, based on a glucose-induced developmental arrest during early seedling morphogenesis. The T-DNA-tagged gin6 mutant abrogates the glucose-induced expression of a putative transcription factor, ABI4, previously shown to be involved in seed-specific abscisic acid (ABA) responses. Thus, ABI4 might be a regulator involved in both glucose- and seed-specific ABA signaling. The characterization of thegin5 mutant, on the other hand, reveals that glucose-specific accumulation of ABA is essential for hexokinase-mediated glucose responses. Consistent with this result, we show that three ABA-deficient mutants (aba1-1, aba2-1, andaba3-2) are also glucose insensitive. Exogenous ABA can restore normal glucose responses in gin5 and aba mutants but not in gin6 plants. Surprisingly, only abi4 andabi5-1 but not other ABA-insensitive signaling mutants (abi1-1, abi2-1, and abi3-1) exhibit glucose insensitivity, indicating the involvement of a distinct ABA signaling pathway in glucose responses. These results provide the first direct evidence to support a novel and central role of ABA in plant glucose responses mediated through glucose regulation of both ABA levels by GIN5 and ABA signaling by GIN6/ABI4.

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

Sugars have signaling roles in a wide variety of developmental processes in plants. To elucidate the regulatory components that constitute the glucose signaling network governing plant growth and development, we have isolated and characterized two Arabidopsis glucose insensitive mutants, gin5 and gin6, based on a glucose-induced developmental arrest during early seedling morphogenesis. The T-DNA-tagged gin6 mutant abrogates the glucose-induced expression of a putative transcription factor, ABI4, previously shown to be involved in seed-specific abscisic acid (ABA) responses. Thus, ABI4 might be a regulator involved in both glucose- and seed-specific ABA signaling. The characterization of the gin5 mutant, on the other hand, reveals that glucose-specific accumulation of ABA is essential for hexokinase-mediated glucose responses. Consistent with this result, we show that three ABA-deficient mutants (aba1-1, aba2-1, and aba3-2) are also glucose insensitive. Exogenous ABA can restore normal glucose responses in gin5 and aba mutants but not in gin6 plants. Surprisingly, only abi4 and abi5-1 but not other ABA-insensitive signaling mutants (abi1-1, abi2-1, and abi3-1) exhibit glucose insensitivity, indicating the involvement of a distinct ABA signaling pathway in glucose responses. These results provide the first direct evidence to support a novel and central role of ABA in plant glucose responses mediated through glucose regulation of both ABA levels by GIN5 and ABA signaling by GIN6/ABI4.

SUGAR-INDUCED SIGNAL TRANSDUCTION IN PLANTS

Sugars have important signaling functions throughout all stages of the plant's life cycle. This review presents our current understanding of the different mechanisms of sugar sensing and sugar-induced signal transduction, including the experimental approaches used. In plants separate sensing systems are present for hexose and sucrose. Hexokinase-dependent and -independent hexose sensing systems can further be distinguished. There has been progress in understanding the signal transduction cascade by analyzing the function of the SNF1 kinase complex and the regulatory PRL1 protein. The role of sugar signaling in seed development and in seed germination is discussed, especially with respect to the various mechanisms by which sugar signaling controls gene expression. Finally, recent literature on interacting signal transduction cascades is discussed, with particular emphasis on the ethylene and ABA signal transduction pathways.

Sugars as signaling molecules

Recent studies indicate that, in a manner similar to classical plant hormones, sugars can act as signaling molecules that control gene expression and developmental processes in plants. Crucial evidence includes uncoupling glucose signaling from its metabolism, identification of glucose sensors, and isolation and characterization of mutants and other regulatory components in plant sugar signal transduction pathways. The emerging scenario points to the existence of a complex signaling network that interconnects transduction pathways from sugars and other hormone and nutrient signals.