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In recent years, several studies have focused on the factors and mechanisms that regulate plant growth and development, as well as the functioning of signaling pathways in plant cells, unraveling the involvement of sugars in the processes regulating such growth and development. Saccharides play an important role in the life of plants: they are structural and storage substances, respiratory substrates, and intermediate metabolites of many biochemical processes. Sucrose is the major transport form of assimilates in plants. Sugars can also play an important role in the defense reactions of plants. However, it has been shown that glucose, sucrose, or trehalose-6-phosphate (Tre6P) can regulate a number of growth and metabolic processes, acting independently of the basal functions; they can also act as signaling molecules. Changes in the concentration, qualitative composition, and transport of sugars occur continuously in plant tissues, during the day and night, as well as during subsequent developmental stages. Plants have developed an efficient system of perception and transmission of signals induced by lower or higher sugar availability. Changes in their concentration affect cell division, germination, vegetative growth, flowering, and aging processes, often independently of the metabolic functions. Currently, the mechanisms of growth regulation in plants, dependent on the access to sugars, are being increasingly recognized. The plant growth stimulating system includes hexokinase (as a glucose sensor), trehalose-6-phosphate, and TOR protein kinase; the lack of Tre6P or TOR kinase inhibits the growth of plants and their transition to the generative phase. It is believed that the plant growth inhibition system consists of SnRK1 protein kinases and C/S1 bZIP transcription factors. The signal transduction routes induced by sugars interact with other pathways in plant tissues (for example, hormonal pathways) creating a complex communication and signaling network in plants that precisely controls plant growth and development.
Badano wpływ Pi oraz cukrów na ekspresją genów kodujących pirofosforylazę UDP-glukozy (Ugp) oraz syntazę sacharozową (Sus1) w liściach rzodkiewnika (Arabidopsis thaliana L. HEYNH.). Niedobór fosforu zwiększał ekspresję Ugp lecz nie wpływał na poziom transkryptu Sus1. Egzogennie podawany do liści Pi (20-200 mmol-dm⁻³) stymulował Sus1 i nie zmieniał ekspresji Ugp. Podawanie D-mannozy z prądem transpiracyjnym do liści wzmagało ekspresję obu genów, jednak Ugp stymulowane było przez znacznie niższe stężenie D-mannozy niż Sus1. Ekspresja genów kodujących pirofosforylazę UDP-glukozy oraz syntazę sacharozową znacząco wzrastała po podaniu do liści sacharozy; ekspresja Sus1 wzrastała również po podaniu glukozy, mannitolu i PEG. Podkarmienie sacharozą liści mutanta pho1 (z deficytem fosforu w pędzie) zwiększało ekspresję Ugp i Sus1 w podobnym stopniu jak u roślin dzikiego typu. Podanie sacharozy do liści mutanta pho2 (akumulator fosforu) w niewielkim stopniu wpływało na ekspresję Ugp natomiast ekspresja Sus1 wzrastała silniej niż u roślin kontrolnych. Zależny od sacharozy wzrost ekspresji Ugp był hamowany po podaniu do liści kwasu okadaikowego, inhibitora fosfataz, natomiast ekspresja Sus1 była indukowana przez ten inhibitor. Uzyskane wyniki sugerują udział różnorodnych mechanizmów w regulacji ekspresji Ugp i Sus1, w zależności od czynnika indukującego zmiany.
The effects of inorganic phosphate (Pi) deficiency on expression of genes encoding ADP-glucose pyrophosphorylase small and large subunits (ApS and ApL1, ApL2, ApL3 genes), UDP-glucose pyrophosphorylase (Ugp gene), sucrose synthase (Sus1), soluble and insoluble acid invertases (Inv and Invcw) and hexokinase (Hxk1 gene), all involved in carbohydrate metabolism, were investigated in Arabidopsis thaliana (L.) Heynh. We used soil-grown pho mutants affected in Pi status, as well as wild-type (wt) plants grown under Pi deficiency conditions in liquid medium, and leaves of wt plants fed with D-mannose. Generally, ApS, ApL1, Ugp and Inv genes were upregulated, although to a varied degree, under conditions of Pi-stress. The applied conditions had differential effects on expression of other genes studied. For instance, Sus1 was down regulated in pho1 (Pi-deficient) mutant, but was unaffected in wt plants grown in liquid medium under P-defi- ciency. Mannose had distinct concentration-dependent effects on expression of genes under study, possibly reflecting a dual role of mannose as a sink for Pi and as glucose analog. Feeding Pi (at up to 200 mM) to the deiached leaves of wt plants strongly affected the expression of ApL1, ApL2, Sus1 and Inv genes, possibly due to an osmotic effect exerted by Pi. The data suggest that ADP-glucose and UDP-glucose pyrophosphorylases (enzymes indirectly involved in Pi recycling) as well as invertases (sucrose hydrolysis) are transcriptionally regulated by Pi-deficiency, which may play a role in homeostatic mechanisms that acclimate the plant to the Pi-stress conditions.
The effects of inorganic phosphate (Pi) deficiency and ABA/ethylene status on expression of UDP-glucose pyrophosphorylase (UGPase) genes (Ugp), involved in sucrose/ polysaccharide metabolism, were investigated. Both wild-type (wt), aba and abi mutants (ABA-deficient and -insensitive), etr, ein and eto (ethylene resistant and overproducing) grown on Pi-deficient and complete nutrient solution, as well as pho1 (Pi-deficient) mutants of Arabidopsis thaliana were used for experiments. Generally, Pi-deficiency conditions (including mannose feeding to decrease cytosolic Pi pool) resulted in an increase of Ugp expression in the leaves, under all experimental conditions. Mutant backgrounds reflecting differences in ABA or ethylene status/ sensitivity had no effect on the level of Ugp up-regulation by Pi-stress. Furthermore, feeding ABA to the leaves of wt and pho1 plants had no effect on Ugp expression, regardless of the sucrose status in the leaves. The data suggest that Pi deficiency leading to up-regulation of Ugp acts independently of ABA and ethylene status.
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