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Review
. 2002;14 Suppl(Suppl):S185-205.
doi: 10.1105/tpc.010455.

Sugar sensing and signaling in plants

Filip Rolland  1 , Brandon Moore, Jen Sheen
Affiliations
Review

Sugar sensing and signaling in plants

Filip Rolland et al. Plant Cell. 2002.
No abstract available

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Figures

Figure 1.
Figure 1.
Differential Effects of Sugars on Plant Source and Sink Activities. Suc is transported from photosynthesizing source leaves to sink organs such as roots, meristems, young leaves, flowers, fruit, and developing seed. Lowered (L) sugar levels can increase source activities, including photosynthesis, nutrient mobilization, and export. In contrast, higher (H) sugar levels in sink tissues stimulate growth and storage. Accumulation of higher (H) sugar levels in source tissues, however, is believed to downregulate photosynthesis, ensuring the maintenance of sugar homeostasis. The differential source-sink effects allow the adaptation of carbon metabolism to changing environmental conditions and to the availability of other nutrients.
Figure 2.
Figure 2.
Possible Sugar Signals and Sensing Sites in Plant Cells. Glc (and Fru) can be transported into the cell by hexose transporters or mobilized from cytosolic and vacuolar Suc and plastid starch. Glc then enters metabolism after HXK-catalyzed phosphorylation. The HXK sugar sensor, as a cytosolic protein or associated with mitochondria or other organelles (see text), then could activate a signaling cascade through HXK-interacting proteins (HIPs) or affect transcription directly after nuclear translocation. Possibly, different HXK (and fructokinase [FRK]) isoforms and HXK-like proteins have distinct metabolic and signaling functions. Metabolic intermediates could trigger signal transduction by activating metabolite sensors (S). Negative regulation of SnRK activity by Glc-6-phosphate, for example, suggests that SnRKs might act as sensors of metabolic activity. Finally, sugars, including Suc and hexoses and nonmetabolizable sugars and sugar analogs, also could be sensed at the plasma membrane by sugar transporters or transporter-like proteins or by specific sugar receptors (R). Solid lines represent transport and enzymatic reactions involved in sugar sensing and signaling, and dashed lines represent putative interactions and translocations. ER, endoplasmic reticulum.
Figure 3.
Figure 3.
gin2 Mutant Phenotype and Complementation by 35S:: AtHXK1. Plants were grown on 6% Glc Murashige and Skoog (1962) medium for 5 days under light. WT, wild type.
Figure 4.
Figure 4.
Arabidopsis Seedling Phenotypes on High-Glc Medium. Wild-type (WT), Glc-insensitive (gin), and Glc-oversensitive (glo) plants were grown on 6% Glc Murashige and Skoog (1962) medium for 5 days under light.
Figure 5.
Figure 5.
Genetic Model of Interactions between Sugar and Hormone Signaling in Arabidopsis. The gin phenotype (shown in Figure 4) is mimicked by ethylene precursor treatment of wild-type plants and is displayed in constitutive ethylene biosynthesis (eto1) and constitutive ethylene signaling (ctr1) mutants, whereas the ethylene-insensitive mutants etr1-1 and ein2 exhibit the glo phenotype (shown in Figure 4). Epistasis analysis with the gin1 etr1 and gin1 ein2 double mutants puts GIN1 downstream of the ETR1 receptor and EIN2 (Zhou et al., 1998; W.-H. Cheng and J. Sheen, unpublished data). Thus, Glc and ethylene signaling pathways antagonize each other (Zhou et al., 1998; W.-H. Cheng and J. Sheen, unpublished data). However, the triple response is not affected by Glc. The gin1, sis4, and isi4 mutants are allelic to aba2 (Laby et al., 2000; Rook et al., 2001; W.-H. Cheng and J. Sheen, unpublished data). ABA2 encodes a short-chain dehydrogenase/reductase (SDR1) that is involved in the second to last step of ABA biosynthesis (Rook et al., 2001; W.-H. Cheng and J. Sheen, unpublished data; P.L. Rodríguez, personal communication; Seo and Koshiba, 2002) and is controlled directly by Glc (W.-H. Cheng and J. Sheen, unpublished data). Other ABA-deficient mutants (aba1-1, aba2-1, and aba3-2) also are Glc insensitive (Arenas-Huertero et al., 2000; Huijser et al., 2000; Laby et al., 2000). ABA1 and ABA3 are important for ABA biosynthesis and are regulated directly by Glc (W.-H. Cheng and J. Sheen, unpublished data). Characterization of the gin5 mutant shows the requirement of Glc-specific ABA accumulation for HXK-mediated Glc signaling (Arenas-Huertero et al., 2000). In addition, gin6, sun6, sis5, and isi3 are allelic to abi4, an ABA-insensitive mutant (Arenas-Huertero et al., 2000; Huijser et al., 2000; Laby et al., 2000; Rook et al., 2001). Glc activation of ABI4, which encodes an AP2 domain transcription factor (Finkelstein et al., 1998), requires ABA, although ABI4 is not induced by ABA directly (Arenas-Huertero et al., 2000; Soderman et al., 2000; W.-H. Cheng and J. Sheen, unpublished data). The abi5 mutant also is Glc insensitive. Glc activates ABI5 directly (W.-H. Cheng and J. Sheen, unpublished data), encoding a basic Leu zipper transcription factor (Finkelstein and Lynch, 2000b). However, other ABA-insensitive signaling mutants (abi1-1, abi2-1, and abi3-1) do not exhibit the gin phenotype, as do abi4 and abi5 mutants (Arenas-Huertero et al., 2000; Huijser et al., 2000; Laby et al., 2000), suggesting that a distinct ABA signaling pathway is involved in Glc signaling. In summary, Glc activates ABA biosynthesis and ABA signaling, and both antagonize ethylene signaling (W.-H. Cheng and J. Sheen, unpublished data). It remains possible that Glc also inhibits ethylene signaling directly. The AtHXK1 mutant (gin2) affects ABA and ethylene signaling but also displays reduced sensitivity to auxin and increased sensitivity to cytokinin (L. Zhou and J. Sheen, unpublished data).
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