New insight into the sucrose biosynthesis pathway from genome-wide identification, gene expression analysis, and...

Sucrose, the main synthesized product and transported form of photoassimilates, moves from leaves to support plant growth and storage substance biosynthesis occurring in the heterotrophic sink organs. Enhancing sucrose biosynthesis efficiency is a top priority for crop yield breeding programs. However, the molecular mechanism of sucrose biosynthesis is still elusive, especially in wheat. We performed transcriptome sequencing, subcellular localization, and bioinformatics analysis to identify key isoforms and metabolic branches associated with sucrose biosynthesis in wheat. Our analysis identified 45 genes from 13 families that exhibited high expression in young leaves with an evident diurnal change. The carbon flux from photoassimilates to sucrose was divided into two pathways. In the cytoplasm, assimilates initiating at phosphotrioses (TPs) exported by TaTPT1 from chloroplasts flowed along the TP-Sucrose branch formed by TaALD6, TaFBP5, TacPGI, TacPGM, TaUGP1, TaSPS5, and TaSPP1. Intermediates either from the Calvin cycle or TP-Sucrose branch were converted into ADPGlc to synthesize the simple starch, which was transiently degraded by a series of enzymes, including TaBAM4 and TaSEX4 in the chloroplast. Similar to the starch-biosynthesis branch in endosperms, the TP-Sucrose branch is the most prominent in leaves because each reaction can be catalyzed by at least one highly expressed isoform with expected cytosolic localization. The key isoforms and major branches highlighted in the wheat sucrose biosynthesis pathway expand our molecular understanding of crop sucrose biosynthesis and provide clues to increase wheat yield by enhancing the sucrose synthesis efficiency of leaves.