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Because of growth and development, plant tissues are characterised by a permanent change in source-sink relations. Tissues with a net carbohydrate export (source) or import (sink) have to adopt their actual demand for assimilates according to the developmental status. Furthermore, plants, as sessile life forms, have developed regulatory mechanisms that enable a flexible response of assimilate partitioning to specific requirements of the habitat, like biotic and abiotic stress factors and changing light conditions. The distribution of assimilates involves specific enzyme functions including sugar transporters and sucrose cleaving enzymes and is regulated by a variety of stimuli. Extracellular invertases cover an essential function in apoplastic phloem unloading and play an important role in regulating source-sink relations. This property is reflected by the occurrence of different invertase isoenzymes with specific expression and regulation patterns that enable a co-ordination of the carbohydrate metabolism in diverse tissues, at different developmental stages, and under varying environmental conditions. Improved knowledge of extracellular invertase function might allow altering growth, development or pathogen resistance of crop plants in a specific way. The present study is aimed at elucidating the regulation patterns and functions of three members of the extracellular invertase gene family of tomato, Lin5, Lin6, and Lin7. Detailed promoter analysis revealed a tissue- and developmental-specific expression of isoenzymes and corresponding regulation patterns. Lin5 shows a developmental regulated expression in fruits. Lin6 is expressed in early developmental stages starting in germinating seeds; in grown up plants Lin6 is solely expressed in pollen and upon wound-stimulation. Lin7 is exclusively expressed in tapetum and pollen tissue. The hormonal regulation of all three isogenes was analysed in detail, whereby known GA- and JA-mediated flower phenotypes could be correlated with invertase functions. In addition, an important role of Lin7 invertase in pollen germination was demonstrated in a functional approach. This is the most profound analysis of extracellular invertases in the delicate process of floral organ development that includes three tomato isoenzymes. In particular, dissection of the individual roles of Lin5, Lin6, and Lin7 reveals novel insights in carbohydrate supply during flower and fruit development. The analysed tissue-specific promoters are profitable tools in plant biotechnology, which in particular applies to the pollen-specific Lin7 promoter. It has been demonstrated that the Lin6 promoter serves as target for hormonal-, sugar-, and wound-mediated signalling pathways. Moreover, a functional interaction of circadian oscillator elements of A. thaliana with the Lin6 promoter and a diurnal rhythm of Lin6 expression have been substantiated. This complex regulation pattern is reflected by the identification of many well-defined cis-acting elements within the Lin6 promoter. This feature supports an integration of various stimuli mediated via extracellular invertase expression resulting in a co-ordinated cellular response to changing internal and external conditions. As sugars on their part induce Lin6 expression, this could result in signal amplification via a positive feedback loop. Furthermore, the extensive appearance and constellation of cisacting elements within the Lin6 promoter provides the basis to answer questions in signal cross-talk and signal integration in plant gene expression. In addition, the Lin6 promoter was successfully used as an inducible expression system. In transgenic tobacco lines an invertase inhibitor was expressed under control of the cytokinin-inducible Lin6 promoter. Thereby, a causal relationship between cytokinin and extracellular invertase for the delay of senescence was demonstrated. This study emphasises the importance of inducible expression systems to address specific questions on a molecular basis. The above-mentioned promoter sequences were obtained via sequential genome walks. Hereby two interesting structural features appeared. First, Lin5 and Lin7 genes are arranged in a direct tandem repeat on the genome. Second, a CACTA-like transposon insertion in intron I of the Lin5 gene was revealed. A primer pair deduced from the transposase region of this transposon allowed the amplification of similar sequences of various Solanaceae species.