Information concerning the sugar status of plant cells is of great importance during a11 stages of the plant life cycle. The availability of or lack of sugars triggers many metabolic and developmental responses, and it is not surprising, therefore, that sugars profoundly affect the expression of a large number of genes (for review, see Koch, 1996;Graham, 1996). Sugar sensing occurs at the level of individual cells and the responses of such cells must be integrated at the tissue, organ, and plant level. Therefore, sugar-induced signals will interact with other sensing and signaling pathways. The mechanisms used by plant cells to sense sugars and to process this information are essentially unknown, and only recently are these questions being addressed experimentally. This lack of knowledge contrasts with the situation in yeast and bacteria, in which the molecular and physiological analysis of mutants have yielded extensive information about sugar perception (Trumbly, 1992;Ronne, 1995;Saier et al., 1995). SUGAR SENSING IN YEAST A N D ANIMALSYeast (Sacckauomyces ceuevisiae) serves as a model for investigating many basic biological questions about eukaryotes and is also an important paradigm for sugar sensing in plants. In yeast the availability of the preferred sugar substrate Glc signals the Glc repression phenomenon (for review, see Trumbly, 1992;Ronne, 1995;Thevelein and Hohmann, 1995). Glc repression dramatically alters yeast intermediary carbohydrate metabolism such that only Glc is being used as a carbon source, despite the presence of other readily accessible carbon sources. Glc is converted into Glu-6-P by HXK and is further metabolized via glycolysis. Genes involved in the metabolism of other carbon substrates are switched off, as are genes encoding key steps in gluconeogenic metabolism. A number of yeast mutants that are impaired in aspects of the Glc repression phenomenon have been isolated and their analysis has provided insight into the complexity of sugar sensing and signaling pathways. From these studies it was concluded that the Glc-phosphorylating enzyme HXK2 is a major Glc sensor responsible for sus- tained Glc repression. HXK2 activity initiates a signal transduction pathway that involves a number of different gene products (Fig. 1) and results in the repression of a large set of genes. Thus, the entry of Glc into glycolytic metabolism as mediated by HXK2 is a key step in Glc sensing.In the repression pathway the function of two protein complexes has been elucidated. These are the GLC7 type 1 protein phosphatase complex (Tu and Carlson, 1995, and refs. therein) and the SSN6/TUP1 complex, which functions as a general repressor of transcription through modulation of chromatin structure. Binding of the SSN6/TUP1 complex to specific sites is directed by the DNA-binding protein MIG1, and in this way genes that contain MIG1-binding sites are repressed. Exactly how the HXK2, GLC7, and SSNG/TUPl /MIG1 complexes are connected is unknown. For example, in the repression pathway no substrates for the REG1 ...

Sugar-induced anthocyanin accumulation has been observed in many plant species. We observed that sucrose (Suc) is the most effective inducer of anthocyanin biosynthesis in Arabidopsis (Arabidopsis thaliana) seedlings. Other sugars and osmotic controls are either less effective or ineffective. Analysis of Suc-induced anthocyanin accumulation in 43 Arabidopsis accessions shows that considerable natural variation exists for this trait. The Cape Verde Islands (Cvi) accession essentially does not respond to Suc, whereas Landsberg erecta is an intermediate responder. The existing Landsberg erecta/Cvi recombinant inbred line population was used in a quantitative trait loci analysis for Suc-induced anthocyanin accumulation (SIAA). A total of four quantitative trait loci for SIAA were identified in this way. The locus with the largest contribution to the trait, SIAA1, was fine mapped and using a candidate gene approach, it was shown that the MYB75/PAP1 gene encodes SIAA1. Genetic complementation studies and analysis of a laboratory-generated knockout mutation in this gene confirmed this conclusion. Suc, in a concentration-dependent way, induces MYB75/PAP1 mRNA accumulation. Moreover, MYB75/PAP1 is essential for the Suc-mediated expression of the dihydroflavonol reductase gene. The SIAA1 locus in Cvi probably is a weak or loss-of-function MYB75/PAP1 allele. The C24 accession similarly shows a very weak response to Suc-induced anthocyanin accumulation encoded by the same locus. Sequence analysis showed that the Cvi and C24 accessions harbor mutations both inside and downstream of the DNA-binding domain of the MYB75/PAP1 protein, which most likely result in loss of activity.

Genes for trehalose metabolism are widespread in higher plants. Insight into the physiological role of the trehalose pathway outside of resurrection plant species is lacking. To address this lack of insight, we express Escherichia coli genes for trehalose metabolism in Arabidopsis thaliana, which manipulates trehalose 6-phosphate (T6P) contents in the transgenic plants. Plants

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