Mechanisms of Glucose Signaling during Germination of Arabidopsis

Abstract

Glucose (Glc) signaling, along with abscisic acid (ABA) signaling, has been implicated in regulating early plant development in Arabidopsis. It is generally believed that high levels of exogenous Glc cause ABA accumulation, which results in a delay of germination and an inhibition of seedling development-a typical stress response. To test this hypothesis and decipher the complex interactions that occur in the signaling pathways, we determined the effects of sugar and ABA on one developmental event, germination. We show that levels of exogenous Glc lower than previously cited could delay the rate of seed germination in wild-ecotype seeds. Remarkably, this effect could not be mimicked by an osmotic effect, and ABA was still involved. With higher concentrations of Glc, previously known Glc-insensitive mutants gin2 and abi4 exhibited germination kinetics similar to wild type, indicating that Glc-insensitive phenotypes are not the same for all developmental stages of growth and that the signaling properties of Glc vary with concentration. Higher concentrations of Glc were more potent in delaying seed germination. However, Glc-delayed seed germination was not caused by increased cellular ABA concentration, rather Glc appeared to slow down the decline of endogenous ABA. Except for the ABA-insensitive mutants, all tested genotypes appeared to have similar ABA perception during germination, where germination was correlated with the timing of ABA drop to a threshold level. In addition, Glc was found to modulate the transcription of genes involved in ABA biosynthesis and perception only after germination, suggesting a critical role of the developmental program in sugar sensing. On the basis of an extensive phenotypic, biochemical, and molecular analysis, we suggest that exogenous Glc application creates specific signals that vary with concentration and the developmental stage of the plant and that Glc-induced fluctuations in endogenous ABA level generate a different set of signals than those generated by external ABA application.Because metabolic and structural functions require the proper amount of carbon source, different organisms have developed the ability to sense internal levels of sugar and accordingly adjust their cellular and metabolic activities. These regulatory mechanisms are particularly important for plants, because sugar production, consumption, and storage occur in the same organism. On one hand, plants have developed sophisticated programs in managing sugar production in source tissue and sugar storage in sink tissue; on the other hand, a complex regulatory circuit controlling gene expression has evolved to accommodate constant changes of sugar-dependent cellular activities (Smeekens, 2000;Coruzzi and Bush, 2001;Coruzzi and Zhou, 2001).Because sugars affect the expression of a diverse array of genes involved in different cellular processes, it is proposed that distinct signaling pathways are employed for the control of these genes. At least three types of Glc signal transduction mechanisms have been found ...

Effects of Sucrose on Germination and Seedling Development of Brassica Napus

“…As degraded product of sucrose, glucose caused ABA accumulation, which resulted in a delay of germination in wide levels of concentration (John Price, 2003). As our results showed, exogenous sucrose also caused a delay of germination like glucose (Figure1).…”

“…For the seedlings of Brassica napus, exogenous 167mM sucrose only inhibited the hypocotyls growth but promoted the roots, while exogenous 167mM glucose caused ABA accumulation and inhibited seedling development in Arabidopsis (John Price, 2003). Our results implied that Sucrose promoted the roots growth probably because the sucrose was absorbed and used as carbon source by the roots, which improved growth of roots by counteracting negative regulating of sucrose signaling.…”

“…It has been suggested that availability of abundant glucose can exert a profound influence, resulting in seedling developmental arrest (Jang JC, 1997).Similar to glucose, High levels of exogenous sucrose inhibited seedlings development seriously, and this effect could not be mimicked by an osmotic effect (John Price, 2003). Our results showed that in Brassica napus, the development of hypocotyls was inhibited (figure 3) while roots were promoted (figure 4) in the medium containing 167mM sucrose, which was different from the pattern in Arabidopsis.…”

“…In this study, expression profiles of genes involved in energy metabolism in Brassica napus were analyzed ( Figure 5). Hexokinase (HXK) and pyruvate dehydrogenase (PDH) are rate limited enzymes in glycolysis, HXK also plays a central role as a conserved glucose sensor in sugar signaling pathway (John Price, 2003). Citrate synthase (CS) is the first enzyme of the tricarboxylic-acid cycle.…”

Arabidopsis extra‐large G proteins (XLGs) regulate root morphogenesis

“…Because mutants in sugar sensing often exhibit seed germination phenotypes Gibson, 2005;Price et al, 2003;Rognoni et al, 2007), we also evaluated the sugar and ABA sensitivity of germination in the xlg triple mutant (Figure 7b). At low sugar concentrations (1% or 3%), germination of the xlg triple mutant was statistically identical to wild type with all sugars tested, including sucrose.…”