Control of plant carbohydrate partitioning by the Arabidopsis thaliana ATB2 bZIP transcription factor gene

Abstract

In this thesis the function of the ATB2 transcription factor in relation to carbohydrate partitioning was investigated. Transcription factors are targets of signal transduction cascades that serve to adapt transcriptional rates of specific downstream genes. Transcription factors do so by binding DNA in a sequence-specific manner. Such DNA elements are present in promoters of target genes which allows transcription factors to enhance or repress the activity of the general transcriptional apparatus. Plants express a plethora of different types of transcription factors which can be classified on the basis of overall sequence homology that likely reflects similar operational mechanisms. Completion of the Arabidopsis thaliana genome in 2000 allowed genome-wide comparison with other organisms. The Arabidopsis thaliana genome has a relatively high transcription factor content compared to organisms of other Kingdoms. ATB2 belongs to the group of bZIP transcription factors that is represented by 75 members which are involved in a broad variety of processes. Several aspects concerning the function of ATB2 and its signaling pathway have previously been presented. ATB2 expression is associated with the vasculature of sink tissues and is especially prominent in funiculi of fertilized ovules. Furthermore, ATB2 functions in a sucrose-signaling pathway. ATB2 expression is modulated by physiologically relevant concentrations of sucrose in a 5 untranslated region-dependent manner. Recently it was shown that ATB2 translation depends on the highly conserved sucrose-controlled upstream open reading frame. The studies described in this thesis show that ATB2 serves a function in carbohydrate partitioning, the process of prolonged and directed allocation of sugars that results in a specific distribution of carbohydrates over all plant organs. In Chapter 2 an overexpression study of ATB2 in tobacco shows its detrimental effects on plant growth and development. Carbohydrate partitioning was severely affected presumably via a combinatorial effect of cell-wall invertases and sucrose transporter misexpression and altered activity. In Chapter 3 it is described that ATB2 overexpression causes similar growth impairments in Arabidopsis thaliana. ATB2 was shown to regulate a specific subset of Arabidopsis thaliana cell-wall invertases genes which could be partially responsible for the observed growth impairment. Furthermore, ATB2 was shown to regulate transcription of two close homologs, bZIP2 and bZIP44. The sucrose-specific translational regulation and expression pattern of bZIP2 and bZIP44 using transgenic promoter-GUS plants are described in Chapter 4. The expression patterns of both bZIP2 and bZIP44 are associated with the vasculature and with metabolically demanding tissues. Expression of both bZIP2 and bZIP44 can be repressed by sucrose in a 5 UTR-dependent manner. The overlapping expression patterns and sucrose-repression strengthen the idea of in vivo concerted action. In conclusion, the data presented in this thesis support a model that explains the function of ATB2 in regulating sucrose supply to competing sinks in accordance to local sink demand. Furthermore, it appears that bZIP2 and bZIP44 could also play such a role as deduced from the similar and overlapping expression patterns. Also the presence of the sucrose-controlled upstream opern reading frame and the resulting translational dependency for sucrose suggests that all three bZIP transcription factors have similar roles in the control of carbohydrate partitioning.