The dynamics of carbohydrate and associated gene expression in the stems and roots of upland cotton (Gossypiumhirsutum L.) during carbon remobilization.

Carbohydrates remobilization in non-leaf organs has a potential association with the formation of cotton yield. However, our understanding of the physiological and molecular mechanisms regulating carbon remobilization during flowering is still limited. The objectives of the study were to: i) evaluate the potential of carbohydrate remobilization in stems and roots to yield formation; ii) unravel the carbon metabolism and transport associated gene expression patterns regulating carbon remobilization. Two cotton lines 4003-6 and 4003-10 were employed to examine leaf photosynthesis, reproductive biomass accumulation, and carbon dynamics in stems and roots during reproductive growth. The results showed that decreasing leaf photosynthetic capacity combined with rapidly increasing reproductive biomass and leaf area index is accompanied by the initiation of carbohydrate remobilization during first flowering to peak flowering. The proportion of sucrose to total nonstructural carbohydrate was also decreased at that period. The upper and lower of stem recorded higher soluble sugars and starch concentrations, respectively compared to the two others. The gross contribution rate of carbon remobilization to seed cotton yield ranged from 2.83% to 7.12%. Key genes and sugar transporters related to starch and sucrose metabolism in the lower stem exhibited significant up- or down-regulated expressions indicating their important roles in carbon reserves remobilization. Three pivotal sugar transporters SWEET1, TMT2, and ERLD5 presented higher transcript levels at peak flowering suggesting more active sugar movement occurring at that stage. The present study provides potential target genes for engineering carbohydrate metabolism and transport to improve the remobilization efficiency of nonstructural carbohydrates.