Mobilization of seed storage lipid by Arabidopsis seedlings is retarded in the presence of exogenous sugars
© To et al 2002
Received: 9 October 2001
Accepted: 7 May 2002
Published: 7 May 2002
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© To et al 2002
Received: 9 October 2001
Accepted: 7 May 2002
Published: 7 May 2002
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.
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.
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.
Soluble sugars, such as glucose and sucrose, have been postulated to act as signaling molecules that help regulate a wide variety of plant developmental and physiological processes [1–13]. These processes include flowering [14–17], seed germination [18–20] and photosynthesis [21, 22]. One of the most thoroughly characterized sugar-regulated processes is the breakdown of starch by germinating cereal seeds. Starch metabolism by germinating seeds must be tightly controlled to ensure an adequate supply of sugar to the developing seedling [13, 23].
As alpha-amylases play a critical role in starch metabolism, the regulation of alpha-amylase activity has formed a major focus of studies on starch metabolism. Increasing sugar levels have been shown to repress expression of alpha-amylases in rice seedlings  and suspension culture cells . Studies using barley embryos provide evidence for the existence of independent glucose and disaccharide signaling mechanisms in the regulation of alpha-amylase expression . Sugar regulation of alpha-amylase expression has also been shown to occur at multiple levels. Sugar levels affect not only alpha-amylase transcript levels [24, 25], but also mRNA stability [27, 28] and protein transport and turnover . Sugar-signaling pathways may also interact with other signaling pathways in the control of alpha-amylase activity. For example, sugars repress a gibberellin-dependent pathway for alpha-amylase expression in barley embryos .
Little is known about the signal transduction pathways for sugar responses. Proteins postulated to act in control of at least some sugar-regulated processes include hexokinases [31–34] and SNF1-related protein kinases [35, 36]. Hexokinases have been suggested to play a role in sugar repression of alpha-amylase activity. This hypothesis is based on findings that only those glucose analogs that are believed to be substrates for phosphorylation by hexokinases are effective in repressing alpha-amylase expression . However, the role of hexokinases in sugar responses remains under debate [38–40].
Genetic approaches are being used to identify additional components of sugar-response pathways. Arabidopsis mutants that are defective in sugar-regulated expression of the genes encoding patatin , beta-amylase [42, 43], plastocyanin  and the ApL3 subunit of ADP-glucose pyrophosphorylase  have been isolated. Additional sugar-response mutants have been identified by taking advantage of the fact that high concentrations of exogenous sugars (glucose or sucrose) inhibit early seedling development of wild-type Arabidopsis. Several groups of Arabidopsis mutants that are resistant to the inhibitory effects of high sugar concentrations have been isolated [46–50]. Characterization of these mutants reveals that the sis5 , sun6 , gin6  and isi3  mutants are allelic to the abscisic acid insensitive mutant abi4  and the sis4, isi4  and gin1  mutants are allelic to the abscisic acid biosynthesis mutant aba2. In addition, the ethylene constitutive response mutant ctr1-1 , as well as the ethylene-overproduction mutant eto1 , display sugar-resistant phenotypes . Conversely, the sugar-response mutant sis1 exhibits an ethylene constitutive response phenotype and is allelic to ctr1-1 . The prl1 mutant, which exhibits increased sensitivity to exogenous sugars [55, 56], carries a mutation in a gene that encodes a WD protein that interacts with SNF1 protein kinases .
Although a large number of studies have been conducted on sugar regulation of starch breakdown by germinating cereal seeds, very little work has focused on determining whether lipid breakdown by germinating oilseeds is also sugar regulated. Findings that the expression of at least two genes involved in lipid metabolism is sugar regulated  and that sucrose can affect total lipid content in germinating seeds  suggest that lipid metabolism may also be sugar regulated. Work presented here indicates that storage lipid breakdown is significantly delayed in germinating Arabidopsis seeds exposed to exogenous glucose.
Exposure to exogenous sugars, such as glucose and sucrose, has long been known to retard rates of starch mobilization in germinating cereal seeds [13, 23]. Therefore, it was of interest to determine whether exogenous sugars also affect rates of lipid mobilization in germinating seeds of plants, such as Arabidopsis thaliana, that store their energy reserves in the form of lipids. Seed storage lipid levels were monitored by measuring eicosenoic (20:1) fatty acid levels. Eicosenoic fatty acid represents approximately 17% of seed lipid reserves, but is present in only very low amounts in membrane lipids. For example, in one study 20:1 fatty acid was found to constitute only 0.2% of the fatty acid present in Arabidopsis roots and was below the detection threshold in shoots . Therefore, the rate of decrease in 20:1 fatty acid levels reflects the rate of decrease in seed storage lipid levels.
Fatty acid accumulation by seedlings grown on glucose or sorbitol.
0.03 M Glc
0.3 M Glc
0.27 M Sorb + 0.03 M Glc
0.4 M Sorb + 0.03 M Glc
22.5 ± 1.9
3.6 ± 0.1
8.7 ± 0.7
1.4 ± 0.1
Although numerous studies have shown that high concentrations of exogenous sugars repress the ability of germinating cereal seeds to mobilize the starch they use as energy reserves [13, 23], little work of a similar nature had previously been conducted on oilseed plants, such as Arabidopsis. Among the few studies done previously is one showing that expression of two genes involved in lipid metabolism is negatively regulated by sugars . In addition, a study of glyoxylate cycle mutants showed a small inhibitory effect of sucrose on total lipid content in germinating seeds . Results presented here indicate that mobilization of seed storage lipid by Arabidopsis thaliana seedlings is significantly retarded in the presence of exogenous glucose. For example, seedlings grown for 5 days on 0.11 M glucose retain approximately five times as much seed storage lipid as seedlings grown in the absence of exogenous glucose. This effect is not due to osmotic stress, as equi-molar, or even twice equi-molar, concentrations of sorbitol have little effect on lipid breakdown.
Growth of seedlings on media containing higher (0.3 M) concentrations of glucose results in almost complete elimination of lipid breakdown. For example, seedlings grown in the presence of 0.3 M glucose retain approximately 80% of their seed storage lipid, even after 22 days of growth. The effects of high glucose concentrations on lipid breakdown could be relatively specific or could be due to a non-specific inhibition of developmental and/or metabolic processes. Interestingly, sorbitol can also inhibit seed storage lipid breakdown. However, the effect of sorbitol on seed storage lipid breakdown is only manifested at sorbitol concentrations (e.g. 0.4, M sorbitol + 0.03 M glucose) that also almost completely eliminate seedling growth. In addition, the effects of sorbitol on seedling growth appear less specific than the effects of glucose, as 0.4 M sorbitol + 0.03 M glucose exerts a similar effect on storage lipid breakdown as 0.3 M glucose, but has a significantly greater effect on accumulation of membrane lipids. In addition, although glucose and, to a lesser extent, sorbitol retard seed germination rates, both additives retard mobilization of storage lipids to a significantly greater extent than seed germination.
The effects of glucose analogs on lipid breakdown were also determined. Seedlings grown on media containing mannose, but not 3-O-methylglucose, exhibit decreased rates of seed storage lipid breakdown. In many organisms, mannose has been shown to be a substrate for hexokinases whereas 3-O-methylglucose is metabolized poorly, if at all, by hexokinases [62, 65]. As a result, processes that are affected by mannose but not by 3-O-methylglucose have been postulated to be regulated via a hexokinase-mediated pathway(s) [19, 31, 32]. By this logic, the fact that seed storage lipid breakdown is repressed by the presence of mannose but not by 3-O-methylglucose could also be interpreted to mean that inhibition of seed storage lipid breakdown occurs via a hexokinase-mediated pathway. However, alternative explanations cannot be ruled out at this time. First, the extent to which mannose and 3-O-methylglucose are metabolized by hexokinases and other enzymes has yet to be determined for Arabidopsis. In addition, a recent report indicates that mannose may affect some processes by sequestering phosphate, rather than by stimulating a hexokinase-mediated sugar-response pathway . Although the mechanism by which mannose acts is uncertain at this time, the fact that it retards lipid breakdown when present at concentrations of only 1.4 mM indicates that the effects of mannose cannot be attributed to osmotic stress.
A number of Arabidopsis sugar-response mutants have been isolated that, unlike wild-type plants, are able to develop substantial shoot systems in the presence of 0.27 to 0.33 M sucrose or glucose [46–50]. Analysis of a representative mutant of this type, the sis5-1/abi4-101 mutant , reveals that it is resistant to the inhibitory effects of exogenous glucose on seed storage lipid breakdown. This result indicates that mutations that alleviate the negative effects of exogenous sugars on early seedling development can also reduce glucose-mediated inhibition of seed storage lipid breakdown.
Media-shift experiments indicate that wild-type seedlings become insensitive to the inhibitory effects of exogenous glucose on seed storage lipid metabolism within 3 days of the start of imbibition. Interestingly, wild-type Arabidopsis seedlings were shown previously to become insensitive to sugar-mediated inhibition of early seedling development within 2 to 3 days of the start of imbibition . In addition, seedlings of Brassica napus, a close relative of Arabidopsis, undergo a dramatic metabolic shift during the same time period . Approximately 2 days after the start of imbibition, B. napus seedlings shift from being dependent on lipid breakdown to being dependent on photosynthesis for sugar formation. The similar timing with which these events occur suggests that completion of the metabolic transition from lipid breakdown to photosynthesis as the primary source of sugars results in loss of sensitivity to the negative effects of sugars on lipid breakdown and early seedling development. However, exogenous sugars can completely compensate for mutations that disrupt the glyoxylate cycle . This finding suggests that the negative effects of high concentrations of exogenous sugars on early seedling development and seed storage lipid breakdown are not the result of sugar-mediated inhibition of the glyoxylate cycle. Further experiments will be required to clarify the relationship between inhibition of early seedling development and seed storage lipid breakdown by exogenous sugars.
Although many studies have examined the effects of sugar levels on rate of starch breakdown by germinating cereal seeds, little effort has been devoted to characterizing the effects of sugar levels on lipid breakdown by germinating oilseed plants. Work presented here demonstrates that growth in the presence of exogenous glucose significantly retards mobilization of seed storage lipid in germinating seeds from wild-type Arabidopsis. In contrast, the sugar-insensitive mutant sis5-1/abi4-101 is resistant to the inhibitory effects of exogenous glucose on lipid breakdown. The effect of glucose on seed storage lipid breakdown 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. Mannose, but not 3-O-methylglucose, also inhibits lipid breakdown. This result suggests that phosphorylation by hexokinase may be required for inhibition of lipid breakdown. However, as metabolism of these glucose analogs remains to be characterized in Arabidopsis, alternative explanations cannot be ruled out at this time. The inhibitory effects of glucose on lipid breakdown are limited to a narrow developmental window. Within approximately three days of the start of imbibition, germinating Arabidopsis seeds become insensitive to the inhibitory effects of exogenous glucose on seed storage lipid mobilization. This result suggests that completion of some critical metabolic transition results in loss of sensitivity to the inhibitory effects of glucose on lipid breakdown.
Seeds of wild-type Arabidopsis thaliana var. Columbia were originally obtained from Dr. Chris Somerville (Carnegie Institution of Washington, Palo Alto, CA, USA). Isolation of the sis5-1/abi4-101 mutant of Arabidopsis thaliana var. Columbia has been described previously . Seeds/seedlings to be used in fatty acid assays were grown on sterilized 3 MW Gel Blot Paper (Midwest Scientific, Valley Park, MO, USA) placed on Petri plates containing solid Arabidopsis minimal media , supplemented with the indicated additives. The gel blot paper prevented growth of the roots into the media, allowing for easy and quantitative removal of seedling tissue for analysis. Unless otherwise noted, plants were grown under 60 to 80 μmol photons m-2 s-1 continuous fluorescent light, at a temperature of 21 to 25°C.
Lipids were extracted and fatty acids derivatized to form the corresponding methyl esters using an established procedure . Typically, 25 to 50 seeds/seedlings were collected from a sector of a Petri plate for each assay. The seeds/seedlings were incubated in 0.6 to 2.0 ml 1 N methanolic-HCl (Supelco, Bellefonte, PA, USA) at 80 to 83°C for 1 to 2 h. Equal volumes of 0.9% (w/v) NaCl and hexane were added to each sample. The samples were shaken by hand for 1 to 2 min and then spun at 2,000 g for 5 min at room temperature. The top (hexane) layers of each sample were transferred to vials and stored at -20°C prior to being analyzed by gas chromatography.
Fatty acid methyl ester levels were measured by gas chromatography using a previously described program . Two μl aliquots of each fatty acid methyl ester extract were injected onto a 5890 Series II gas chromatograph from Hewlett Packard containing a 30 m SP2330 column with an inner diameter of 0.75 mm (Supelco, Bellefonte, PA, USA). The column was exposed to the following temperature program: 100°C for 1 min, ramp to 160°C at 25°C per min, ramp to 220°C at 10°C per min, 220°C for 4.6 min. The pressure program for the column was: 30 kPa for 9 min, ramp to 70 kPa at 40 kPa per min, 70 kPa for 7 min. The temperatures of the injector and the flame ionization detector were 220°C and 250°C, respectively. Helium was used as the carrier gas.
Gas chromatograph peaks were identified by comparing their column retention times with the retention times of fatty acid methyl ester standards (Sigma, St. Louis, MO, USA). The quantities of all major fatty acids in each sample were determined by comparing the sizes of the gas chromatography peaks produced by each sample with the sizes of the peaks produced by fatty acid methyl ester standards of known concentration. This data was used to calculate the net amounts of membrane fatty acids produced by the seedlings in each sample. In oilseeds such as Arabidopsis, the bulk of the fatty acids present in seeds and very young seedlings are found in storage lipids. For example, storage lipids comprise 93% of the total lipid found in ungerminated seeds of Brassica napus, a plant species that is closely related to Arabidopsis. The remaining 7% of seed lipids consist of non-storage lipids, such as membrane lipids. To calculate the increase in the amount of membrane fatty acid in a particular group of seedlings, it is necessary to subtract the amount of seed fatty acid (both storage and non-storage) remaining in those seedlings from the total amount of fatty acid present in the seedlings.
The amount of seed storage fatty acid remaining in seedlings harvested at day "x" can be calculated using the following equation:
(μg seed-storage fatty acid at day "x") = (μg fatty acid in seeds)(0.93) [(μg 20:1 at day "x")/(μg 20:1 in seeds)]
As approximately 93% of seed lipids consist of storage lipids , the total amount of fatty acid found in ungerminated seeds is multiplied by 0.93 to determine the amount of fatty acid present in the storage lipid of ungerminated seeds. This number is then multiplied by the fraction of 20:1 (eicosenoic) fatty acid remaining at day "x", to account for the percentage of storage lipid that has been metabolized by day "x". Assuming that different seed-storage fatty acids are metabolized at similar rates, eicosenoic fatty acid levels will reflect seed storage lipid levels as eicosenoic fatty acid is present in significant amounts in seed storage lipids but is almost entirely lacking from non-storage lipids, such as membrane lipids .
The amount of seed non-storage fatty acid present in seedlings can be calculated using the following equation:
(μg seed non-storage fatty acid) = (μg fatty acid in seeds)(0.07)
As approximately 7% of seed lipids consist of non-storage lipids , the total amount of fatty acid found in ungerminated seeds is multiplied by 0.07 to determine the amount of fatty acid present in the non-storage lipid of ungerminated seeds.
Finally, the increase in the amount of fatty acid present in the membranes of seedlings harvested at day "x" after the start of imbibition can be calculated using the following equation:
(Increase in membrane fatty acid by day "x") = (total μg fatty acid at day "x") - (μg seed-storage fatty acid remaining at day "x") - (μg seed non-storage fatty acid)
Seeds were surface sterilized and sown on the indicated media. Seeds were scored for germination at regular intervals. Seed germination is defined as the emergence of any part of the seedling from the seed coat.
For media-shift experiments, 3 MW Gel Blot Paper was placed on top of solid media in Petri plates. Nytex mesh 3-300/46 screens (Tetko Incorp., Kansas City, MO, USA) were then placed on top of the blot paper and seeds sown on top of the nytex screens. At the indicated times, the nytex screens and seeds/seedlings were transferred to fresh Petri plates containing 3 MW Gel Blot Paper on solid media.
This work was supported by the U.S. Department of Energy, Energy Biosciences Program Grants DE-FG03-00ER15061 (S.I.G.) and DE-FG02-95ER20203 (W.-D.R.).
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