PubMed · 13778422
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H M TSUCHIYA. 1960. Dextransucrase.. https://pubmed.ncbi.nlm.nih.gov/13778422/
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The role of sucrose in cyanobacteria is still not fully understood. It is generally considered a salt-response molecule, and particularly, in Synechocystis sp. strain PCC 6803, it is referred as a secondary osmolyte. We showed that sucrose accumulates transiently in Synechocystis cells at early stages of a salt shock, which could be ascribed to salt activation of sucrose-phosphate synthase (SPS, UDP-glucose: D-fructose-6-phosphate 2-alpha-D-glucosyltransferase; EC 2.4.1.14), the key enzyme in sucrose synthesis pathway, and to an increase of the expression of the SPS encoding gene. Experiments with a mutant strain impaired in sucrose biosynthesis showed that sucrose is essential in stationary phase cells to overcome a later salt stress. Taken together, these results led us to suggest a more intricate function for sucrose than to be an osmoprotectant compound.
Sucrose phosphate synthase (SPS) is responsible for sucrose synthesis in photosynthetic tissues. We have detected SPS expression in non-photosynthetic tissues. It was predominantly expressed in the basal region of developing endosperm, suggesting that sucrose is re-synthesized in this region, where sucrose is unloaded from the phloem and hydrolyzed into glucose and fructose. The SPS transcript in endosperm was approximately 300 nt smaller than in leaf. However, the size of the SPS protein was similar to that of leaf but had higher activity. SPS expression was also detected in developing and germinating embryos, indicating that sucrose resynthesis also occurs in embryos. Although the level of SPS mRNA and protein was lower in embryos than in leaf, enzymatic activity was higher. Similarly, the level of SPS transcript was 10-fold lower in endosperm than in leaf but the level of SPS protein was comparable, and activity was 2 fold higher. Thus, SPS expression was evident in maize kernels, and its expression and regulation were different from the SPS in leaf.
Sucrose cleavage is vital to multicellular plants, not only for the allocation of crucial carbon resources but also for the initiation of hexose-based sugar signals in importing structures. Only the invertase and reversible sucrose synthase reactions catalyze known paths of sucrose breakdown in vivo. The regulation of these reactions and its consequences has therefore become a central issue in plant carbon metabolism. Primary mechanisms for this regulation involve the capacity of invertases to alter sugar signals by producing glucose rather than UDPglucose, and thus also two-fold more hexoses than are produced by sucrose synthase. In addition, vacuolar sites of cleavage by invertases could allow temporal control via compartmentalization. In addition, members of the gene families encoding either invertases or sucrose synthases respond at transcriptional and posttranscriptional levels to diverse environmental signals, including endogenous changes that reflect their own action (e.g. hexoses and hexose-responsive hormone systems such as abscisic acid [ABA] signaling). At the enzyme level, sucrose synthases can be regulated by rapid changes in sub-cellular localization, phosphorylation, and carefully modulated protein turnover. In addition to transcriptional control, invertase action can also be regulated at the enzyme level by highly localized inhibitor proteins and by a system that has the potential to initiate and terminate invertase activity in vacuoles. The extent, path, and site of sucrose metabolism are thus highly responsive to both internal and external environmental signals and can, in turn, dramatically alter development and stress acclimation.