Increasing evidence shows that sugars can act as signals affecting plant metabolism and development. Some of the effects of sugars on plant growth and development suggest an interaction of sugar signals with hormonal regulation. We investigated the effects of sugars on the induction of [alpha]-amylase by gibberellic acid in barley embryos and aleurone layers. Our results show that sugar and hormonal signaling interact in the regulation of gibberellic acid-induced gene expression in barley grains. The induction of [alpha]-amylase by gibberellic acid in the aleurone layer is unaffected by the presence of sugars, but repression by carbohydrates is effective in the embryo. [alpha]-Amylase expression in the embryo is localized to the scutellar epithelium and is hormone and sugar modulated. The effects of glucose are independent from the effects of sugars on gibberellin biosynthesis. They are not due to an osmotic effect, they are independent of abscisic acid, and only hexokinase-phosphorylatable glucose analogs are able to trigger gene repression. Overall, the results suggest the existence of an interaction between the hormonal and metabolic regulation of [alpha]-amylase genes in barley grains.

Sugar Repression of a Gibberellin-Dependent Signaling Pathway in Barley Embryos.

The cereal aleurone is an excellent model system for hormonal signalling in plants. When treated with gibberellins (GAs), cereal aleurone layers and aleurone protoplasts initiate signalling cascades that result in the synthesis and secretion of hydrolytic enzymes, most notably α-amylases. Abscisic acid (ABA) antagonizes the effects of GA and stimulates the production of ABA-up-regulated proteins. Receptors for GA and ABA have been localized to the aleurone cell PM, and evidence suggests that another ABA receptor functions within the cytoplasm. Cytosolic and membrane-bound second messengers have been identified and signal transduction pathways are beginning to be understood. Transacting factors that regulate the transcription of hormonally induced genes have been cloned and bring us closer to linking cytosolic signals to changes in gene expression. In this review, recent data pertaining to hormonal signalling in cereal aleurone are summarized. Relationships between signalling and effector molecules are discussed, and models for hormone-induced signalling pathways are proposed.

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Hormonal signalling in cereal aleurone

Three homeologous genes encoding a sucrose (Suc) transporter (SUT) in hexaploid wheat (Triticum aestivum), TaSUT1A, 1B, and 1D, were expressed in germinating seeds, where their function is unknown All three TaSUT1 proteins were confirmed to be capable of transporting both Suc and maltose by complementation tests with the SUSY7/ura3 yeast (Saccharomyces cerevisiae) mutant strain The role of Suc transporters in germinating grain was examined by combining in situ hybridization, immunolocalization, fluorescent dye tracer movement, and metabolite assays TaSUT1 transcript and SUT protein were detected in cells of the aleurone layer, scutellar epidermis, scutellar ground cells, and sieve element-companion cell complexes located in the scutellum, shoot, and root Ester loading of the membrane-impermeable fluorescent dye carboxyfluorescein into the scutellum epidermal cells of germinating seeds showed that a symplasmic pathway connects the scutellum to the shoot and root via the phloem However, the scutellar epidermis provides an apoplasmic barrier to solute movement from endosperm tissue Measurements of sugars in the root, shoot, endosperm, and scutellum suggest that, following degradation of endosperm starch, the resulting hexoses are converted to Suc in the scutellum Suc was found to be the major sugar present in the endosperm early in germination, whereas maltose and glucose predominate during the later stage It is proposed that loading the scutellar phloem in germinating wheat seeds can proceed by symplasmic and apoplasmic pathways, the latter facilitated by SUT activity In addition, SUTs may function to transport Suc into the scutellum from the endosperm early in germination and later transport maltose

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Pathway of Sugar Transport in Germinating Wheat Seeds

Using expression analysis, the role of the sucrose transporter OsSUT1 during germination and early growth of rice seedlings has been examined in detail, over a time-course ranging from 1 d to 7 d post-imbibition. Unlike the wheat orthologue, TaSUT1, which is thought to be directly involved in sugar transfer across the scutellar epithelium, OsSUT1 is not expressed in the scutellar epithelial cell layer of germinating rice and is, therefore, not involved in transport of sugars across the symplastic discontinuity between the endosperm and the embryo. OsSUT1 expression was also absent from the aleurone cells, indicating it is not involved in the transport of sucrose in this cell layer during germination. However, by 3 d post-imbibition, OsSUT1 was present in the companion cells and sieve elements of the scutellar vascular bundle, where it may play a role in phloem loading of sucrose for transport to the developing shoot and roots. This sucrose is most likely sourced from hexoses imported from the endosperm. In addition, sucrose may be remobilized from starch granules which are present at a high density in the scutellar ground tissues surrounding the vasculature and at the base of the shoot. OsSUT1 was also present in the coleoptile and the first and second leaf blades, where it was localized to the phloem along the entire length of these tissues, and was also present within the phloem of the primary roots. OsSUT1 may be involved in retrieval of sugars from the apoplasm in these tissues.

The role of the sucrose transporter, OsSUT1, in germination and early seedling growth and development of rice plants

From storage compartment to lytic organelle: the metamorphosis of the aleurone protein storage vacuole

Aleurone layers of barley contain large amounts of a soluble oligosaccharide which was identified as sucrose (30–40 μg/mg fresh weight). Treatment of the layers with gibberellic acid (GA3) causes the release of sucrose from the cells. This release requires the participation of metabolic processes, including protein synthesis. When embryoless half-seeds are incubated sucrose accumulates in the aleurone layers, but when seeds are germinated the sucrose content of the aleurone layers declines. Labeling experiments with radioactive glucose and fructose show that aleurone layers continuously synthesize sucrose and that the release, but not the synthesis of sucrose is enhanced by GA3.

Andrea J. Tenner

Papers

Synthesis and release of sucrose by the aleurone layer of barley: Regulation by gibberellic acid