Fructose and glucose signaling in Arabidopsis thaliana

Abstract

All life on earth depends on photosynthesis, a process by which energy from sunlight is used to convert water and carbon dioxide into sugars and oxygen. In plants, sugars are used as building blocks for growth, energy and transport. Sugars have an important role as signaling molecule as well, and sugar signaling affects the speed of metabolism. For sugar signaling, the transcription factor ABI4 is important. This protein was originally discovered as a protein necessary for the response to the hormone abscisic acid in the plant Arabidopsis thaliana, but ABI4 also appears to play a role in the response of the plant to the presence of sugars. ABI4 inhibits the expression of genes that are involved in photosynthesis. The abi4 mutant, which does not contain ABI4, grows faster and this mutant also contains more chlorophyll than the wild-type plant. It is proposed that ABI4 inhibits photosynthesis by preventing DNA binding of transcription factors that stimulate photosynthesis genes. It has previously been shown that different sugars can each cause specific responses in the plant. Therefore, young seedlings were treated for 2 hours with glucose or fructose. The glucose treated seedlings show stronger mRNA transcript levels changes than fructose treated seedlings, and this is true for most of the transcripts. The reason is unclear but may have to do with the rate at which glucose and fructose is absorbed, glucose is possibly absorbed faster than fructose. In addition to sugar treatments, seedlings can be germinated on medium containing a high sugar concentration and the reaction can be measured. Seeds will germinate on a high sugar concentration, but young seedlings are inhibited in development and will have no functional chlorophyll production in the cotyledons. Some mutants can grow under these conditions, and develop green chlorophyll. Some of these mutants, including the abi4 mutant, will become green when they are germinated on medium with a high concentration of glucose, fructose or sucrose. There are, however, also mutants discovered that respond differently to only glucose or fructose. One of these mutants contains a protein which originates from the Arabidopsis ecotype found on the Cape Verde Islands (CVI). This protein is the transcription factor NAC089CVI and lacks one-third of the Columbia-0 wild-type amino acid sequence. This NAC089CVI mutant is insensitive to inhibition of growth and chlorophyll development that normally occurs on a high fructose concentration but reacts as wild-type to a high glucose concentration. The reason for this difference in sensitivity is still unclear, but it is possible that the fructose insensitivity is caused by repression of a fructose specific signal transduction chain. The genes that are overexpressed in the mutant NAC089CVI grown on high fructose levels encode for proteins that are necessary in photosynthesis. Besides the fructose insensitive mutant NAC089CVI, a specific glucose insensitive gin2-1 mutant exists. Both mutants have a similar abscisic acid concentration as wild-type when grown on high sugar containing medium and also respond similar to added abscisic acid. Abscisic acid (signaling) thus seems to play no role in the differential perception of fructose and glucose. It is likely that in the NAC089CVI and gin2-1 mutants other processes are incorrectly regulated. It seems that in the NAC089CVI mutant, the plastid-to-nucleus signaling cascade is incorrectly regulated. Plants respond differently to various sugars and in the future this will be further investigated. A better understanding of how sugars suppress photosynthesis may lead to crops that are less responsive to sugars and this hopefully leads to a higher yield.