SummaryABI4 encodes an AP2 family transcription factor that is a central regulator in sugar responsive gene expression in plants. Sugar-induced ABI4 regulates plant genes essential for photosynthesis, and carbon, nitrogen and lipid metabolism. ABI4 activity is induced via the ABA-mediated sugar signalling pathway, which is initiated by the glucose sensing protein hexokinase. Natural variation in sugar sensitivity was used to identify new loci involved in sugar signalling. Five quantitative trait loci (QTLs) for glucose sensitivity (GSQ1-GSQ5) were identified in a Ler/Cvi recombinant inbred line (RIL) population. The GSQ3, GSQ4 and GSQ5 loci are positioned in regions not previously associated with known sugar-sensing genes. GSQ5 was fine mapped and cloned using a candidate-gene approach. The GSQ5 locus was shown to encode the DELAY OF GERMINATION 1 (DOG1) gene. DOG1 was previously identified as a major locus in seed dormancy control. Glucose addition induced the expression of the GSQ5/DOG1 Cvi allele, whereas the Ler and Col alleles did not respond to glucose. Positive feedback was observed between the ABA-mediated sugar signalling pathway and the GSQ5/ DOG1 Cvi allele. Expression of the GSQ5/DOG1 Cvi allele requires the ABA-mediated sugar signalling pathway, of which ABI4 is an important component. In addition, sugar induction of ABI4 was promoted by the GSQ5/ DOG1 Cvi allele.

Sugars affect a broad variety of processes, from growth and development to gene expression. Although it has already been shown that sugars act as signaling molecules, little is known about the mechanisms by which plants respond to them. Much progress has been made on understanding sugar sensing and signaling thanks to the analysis of mutants with abnormal sugar response. Some of the genetic strategies applied are based on the inhibitory effect of sugar on post-germinative development of Arabidopsis thaliana. High concentrations of exogenous sugars delay germination and arrest early growth, preventing seedlings from expanding cotyledons and developing true leaves and an extensive root system. The characterization of several Arabidopsis mutants identified for their altered sugar sensitivity has disclosed a network in which sugars and plant hormones cooperate to control seedling development. Remarkably, many mutations turned out to be novel alleles of hormone-related genes, mainly ABA and ethylene. The aspects described above, emphasizing the connections between sugar and plant hormones revealed by mutants derived in seedling-based screens, are reviewed in this paper.

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The paper deals with the effect of sugars on LeEXPA2 expression in tomato (Lycopersicon esculentum) hypocotyl segments, which is a wellstudied system for analysis of cell elongation. We tested the interactions between sugars and different plant hormones classically known to be involved in plant growth. We found the induction of LeEXPA2 transcript accumulation to be positively affected by the presence of sucrose and other metabolizable sugars. The effect mediated by sorbitol and by a non-metabolizable glucose analogue (3-O-methyl glucose) is lower, while the structural analogue of sucrose, turanose, leads to any auxin-induced increase in LeEXPA2 transcript abundance.