A Preliminary Study On The Dynamic Expression And Regulation Of Sucrose Metabolism Key Genes During Grain Filling Stage

Sucrose, the predominant product of photosynthesis, is the main form of long-distance energy transport and distribution in higher plants in vivo. After synthesis in autotrophic organs (source), sucrose is transported into heterotrophic organs (sink), primarily for synthesis of storage molecules or to provide energy for the plant itself. Sucrose is also an important signal molecule in plants that controls the transcription and translation of a number of genes. At the same time, sucrose can regulate osmotic pressure and participate in abiotic stress tolerance in plants. Three kinds of key enzymes and one kind of protein participate in sucrose synthesis, degradation and transport including invertase (Inv), sucrose synthase (SuSy), sucrose-phosphate synthase (SPS) and sucrose transporters/sucrose carriers (SUT). Invertase and sucrose synthase are primarily involved in sucrose degradation and sucrose-phosphate synthase is responsible for sucrose synthesis, while the transport of sucrose from source to sink organs is performed by sucrose transporters.Sucrose is connected closely with starch biosynthesis. Its degradation product is the source of precursor for starch biosynthesis. SuSy is the key enzyme that converts sucrose into UDPG. UDPG is then converted into ADPG, which is the direct substrate for starch biosynthesis, under the action of some enzymes. In addition, other enzymes related to sucrose metabolism also play an important role in starch synthesis. Therefore, sucrose metabolism has a close relationship with starch biosynthesis. Our previous study has found that the expression of most of genes involved in starch synthesis could be regulated by sugars and hormones in the maize endosperms. But little has been known for the transcriptional regulation of genes involved in sucrose metabolism. So, to investigate the relationship between sucrose metabolism and starch biosynthesis, and the genes expression differences between sucrose metabolism and starch biosynthesis under the treatment of sugers and hormones. We studied four maize inbred lines (two high-starch and two low-starch lines) in this study. The sucrose contents and expression levels of genes related to sucrose metabolism in the grains of four maize inbred lines were studied at different days after pollination (DAP). And the maize endosperms of inbred line B73 at 10 day after pollination.were treated with different sugars (sucrose, glucose and fructose) and hormones (GA, IAA and ABA), then gene expression involving sucrose metabolism (including synthesis, degradation and transport) were determined using the method of real-time quantitative RT-PCR (qRT-PCR). The results are as follows:1. The sucrose content trends in the four inbred lines presented a single peak that emerged at 15 DAP, and the contents in the high-starch lines were lower than in low-starch lines. The accumulation rate of sucrose reached a maximum at 10-15 DAP in the high-starch lines, and at 5-10 and 10-15 DAP in the two low-starch lines.2. Incw2 and Incw3 were not expressed in ear leaves, only expressed in kernels. The highest levels of Incw2 mRNA in the high-starch lines were found at 10 and 15 DAP, while its mRNA levels in the low-starch lines reached a maximum at 10 DAP. Incw3 transcripts were mostly detected at 5-15 DAP in the four inbred lines with a downward trend, and the transcript levels were lower in the high-starch lines than in the low-starch lines at 5 DAP, while no difference was observed in the other periods3. Ivrl and Incwl were expressed mainly in the late period of grain filling, and their expression levels were higher in high-starch lines than in low-starch lines at 25-35 DAP, but showed no obvious differences in other periods;4. The transcript levels of ZmSPS1 and Incw4 were higher in high-starch lines than in low-starch lines at 5-25 DAP and 10-35 DAP, respectively, and showed no difference in other periods5. The results show that genes tested were more or less affected by different sugars and hormones, and the majority of genes (13/14) were regulated by abscisic acid (ABA). Sucrose and ABA exhibited an additive effect on the expression of some genes. And the differences of transcriptional regulation of genes involved in sucrose metabolism and starch biosynthesis were found.