| Carbohydrate accumulation is a common phenomenon in frost-resistant plants, and many enzymes participate in this process. The Triticum aestivum cultivar Dongnongdongmai 1 has an 85% winter survival rate and is the only wheat variety grown in Heilongjiang. The T. Aestivum Jimai 22 cultivar is planted over a wide area in northern China, but cannot survive the cold winter in Heilongjiang. In this study, we examined the changes in sugar content and gene expression levels of key enzymes involved in sugar metabolism in two cultivars of Triticum aestivum as temperatures dropped. The gene expression levels of the ribulose bisphosphate carboxylase small subunit (RBC, EC 4.1.1.39), the fructose-1, 6-bisphosphatase chloroplast precursor (FBP, EC 3.1.3.11), the phosphoribulokinase chloroplast precursor (PRK, EC 2.7.1.19), UTP-glucose-1-phosphate uridylyltransferase (UGP, EC 2.7.7.9), sucrose synthase (SS, EC 2.4.1.13), sucrose phosphate synthase (SPS, EC 2.4.1.14), the triose phosphate translocator (TPT), and pyruvate kinase (PK, EC 2.7.1.40) were also investigated. The results are as follows:(1)The soluble sugar content in two cultivars increased at first and then decreased with the peak reached at -1℃. However, this decrease was more modest below -10℃in Dongnongdongmai 1, which had higher sugar content. The sugar accumulation may benefit the cold resistance of Dongnongdongmai 1.(2)Sucrose and fructose were the main components of the soluble sugars, indicating important roles in freezing tolerance. Their content increased at first and then decreased, being similar to soluble sugar content. Dongnongdongmai 1 also had a higher sugar content than Jimai 22. The glucose and starch content in two varieties were very low with no significant accumulation when temperature decreased.(3)RBC, FBP, and PRK are key enzymes of the reductive pentose phosphate pathway and of the Calvin cycle of photosynthetic carbon dioxide assimilation. The expression levels of the genes encoding RBC, FBP, and PRK had a similar tendency in the tillering nodes of Dongnongdongmai 1, where the highest value was found at -1℃(increases of 4.9-, 8.1-, and 12.6-fold respectively), then gradually decreased. In contrast, the expression level in the tillering nodes of Jimai 22 peaked at -12℃, with a smaller increment (increases of 3.2-, 4.1-, and 6.1-fold respectively). In the leaves, the gene expression levels decreased in Dongnongdongmai 1 when the temperature fell, and they had a peak at -1℃in Jimai 22.(4)In the two cultivars, the expression levels of UGP in two organs and SS in tillering nodes decreased when temperature below -1℃. In the leaves, the expression level of SS doubled at -1℃. For the gene TaSPS, the expression levels increased significantly at lower temperature (-19℃), which contrasts with the expression patterns of the other genes.(5)TaTPT expression increased in the tillering nodes but decreased in the leaves. However, the highest points of TaTPT expression in tillering nodes of Dongnongdongmai 1 appeared at -1℃, but they appeared at -12℃in Jimai 22. In two tissue types of the two cultivars, TaPK reached a maximum expression level at -1℃. Moreover, TaPK in Dongnongdongmai 1 showed a greater increase, about 157% in the leaves and 80% in the tillering nodes, than in Jimai 22. The increase of TaPK indicated that winter wheat at low temperatures may increase the rate of glycolysis.(6)The correlation analysis between gene expression and sugar content showed that there are high correlation between TaRBC, TaFBP, TaPRK and TaTPT. In the leaves, TaUGP and TaPK have certain relevance with the content of sucrose and total soluble sugar. In the tillering nodes, there are more high correlations between genes and sugars.(7)According to the results above, the expression levels of the genes involved in photosynthesis and sugar metabolism in winter wheat increased significently at low temperature adove freezing. This phenomenon enabled plant to accumulate soluble sugars and to accelerate sugar consumption at the same time. The accumulation of sugar helps to improve cold tolerance in plants. The consumption of sugar will provide more energy sources for other metabolism pathways, which may improve plant cold tolerance indirectly. As the temperature decreases, gene expression reduced and sugar content was also in a decreasing trend. The cultivar with stronger cold resistance had a greater capacity of sugar accumulation and retention. |
