A Dual-Promoter Gene Orchestrates the Sucrose-Coordinated Synthesis of Starch and Fructan in Barley

Abstract

Sequential carbohydrate synthesis is important for plant survival because it guarantees energy supplies for growth and development during plant ontogeny and reproduction. Starch and fructan are two important carbohydrates in many flowering plants and in human diets. Understanding this coordinated starch and fructan synthesis and unraveling how plants allocate photosynthates and prioritize different carbohydrate synthesis for survival could lead to improvements to cereals in agriculture for the purposes of greater food security and production quality. Here, we report a system from a single gene in barley employing two alternative promoters, one intronic/exonic, to generate two sequence-overlapping but functionally opposing transcription factors, in sensing sucrose, potentially via sucrose/glucose/fructose/trehalose 6-phosphate signaling. The system employs an autoregulatory mechanism in perceiving a sucrose-controlled trans activity on one promoter and orchestrating the coordinated starch and fructan synthesis by competitive transcription factor binding on the other promoter. As a case in point for the physiological roles of the system, we have demonstrated that this multitasking system can be exploited in breeding barley with tailored amounts of fructan to produce healthy food ingredients. The identification of an intron/exon-spanning promoter in a hosting gene, resulting in proteins with distinct functions, adds to the complexity of plant genomes.