Umeå universitets logga

Background: Sugars modulate expression of hundreds of genes in plants. Previous studies on sugar signaling, using intact plants or plant tissues, were hampered by tissue heterogeneity, uneven sugar transport and/or inter-conversions of the applied sugars. This, in turn, could obscure the identity of a specific sugar that acts as a signal affecting expression of given gene in a given tissue or cell-type. Methodology/Principal Findings: To bypass those biases, we have developed a novel biological system, based on stem-cell-like Arabidopsis suspension culture. The cells were grown in a hormone-free medium and were sustained on xylose as the only carbon source. Using functional genomics we have identified 290 sugar responsive genes, responding rapidly (within 1 h) and specifically to low concentration (1 mM) of glucose, fructose and/or sucrose. For selected genes, the true nature of the signaling sugar molecules and sites of sugar perception were further clarified using non-metabolizable sugar analogues. Using both transgenic and wild-type A. thaliana seedlings, it was shown that the expression of selected sugar-responsive genes was not restricted to a specific tissue or cell type and responded to photoperiod-related changes in sugar availability. This suggested that sugar-responsiveness of genes identified in the cell culture system was not biased toward heterotrophic background and resembled that in whole plants. Conclusions: Altogether, our research strategy, using a combination of cell culture and whole plants, has provided an unequivocal evidence for the identity of sugar-responsive genes and the identity of the sugar signaling molecules, independently from their inter-conversions or use for energy metabolism.

Simple sugars, like glucose (Glc) and sucrose (Suc), act as signals to modulate the expression of hundreds of genes in plants. Frequently, however, it remains unclear whether this regulation is induced by the sugars themselves or by their derivatives generated in the course of carbohydrate (CH) metabolism. In the present study, we tested the relevance of different CH metabolism and allocation pathways affecting expression patterns of five selected sugar-responsive genes (bZIP63, At5g22920, BT2, MGD2, and TPS9) in Arabidopsis thaliana. In general, the expression followed diurnal changes in the overall sugar availability. However, under steady growth conditions, this response was hardly impaired in the mutants for CH metabolizing/transporting proteins (adg1, sex1, sus1-4, sus5/6, and tpt2), including also hexokinase1 (HXK1) loss- and gain-of-function plants—gin2.1 and oe3.2, respectively. In addition, transgenic plants carrying pbZIP63::GUS showed no changes in reporter-gene-expression when grown on sugar under steady-state conditions. In contrast, short-term treatments of agar-grown seedlings with 1% Glc or Suc induced pbZIP63::GUS repression, which became even more apparent in seedlings grown in liquid media. Subsequent analyses of liquid-grown gin2.1 and oe3.2 seedlings revealed that Glc -dependent regulation of the five selected genes was not affected in gin2.1, whereas it was enhanced in oe3.2 plants for bZIP63, At5g22920, and BT2. The sugar treatments had no effect on ATP/ADP ratio, suggesting that changes in gene expression were not linked to cellular energy status. Overall, the data suggest that HXK1 does not act as Glc sensor controlling bZIP63, At5g22920, and BT2 expression, but it is nevertheless required for the production of a downstream metabolic signal regulating their expression.

Within the last decades, research on sugar-dependent plant growth provided evidence for a directregulation of cell division by sugars. Recently we showed, in A. thaliana cell suspension cultures, thatdistinct sugars differentially regulate a rapid transcriptional response of genes, some of which functionduring the cell cycle progression. In order to assess the relationship between sugar species and celldivision, we developed a new methodology for long-term real-time live-cell imaging on dividing A.thaliana cell suspension cultures. This technique, using cells grown in hormone-free media, allowed toestimate the cell cycle synchronicity and efficiency of an entire cell population and to monitor thedynamics and geometry of cell division in single cells in response to a given sugars. A marker cell lineconstitutively expressing TUA::GFP, a protein that labels microtubule-based structures hallmarkingprogression of the mitotic division, was used to measure the sugar-dependent efficiency andsynchronicity of the cell cycle progression. Altogether, we were able to confirm the distinct relationshipsof specific sugar molecules on the cell cycle progression at a single cell level. Cell division efficiencyand synchronicity were altered when grown on the different sugars sucrose, glucose and xylose.Interestingly, the progression of the mitotic division appeared unaffected by the sugar species supplied,indicating that length of the interphase is likely to control the cell division rate. In contrast, treatment ofA. thaliana cell cultures and seedlings with the Glc-analogue 2-deoxy-glucose (2dog) led to growtharrest and to cell death during long exposure. The growth resulting from 2dog removal in cell culturesand seedlings showed the unique feature of plants to induce new active zones of cell division.