Review: Crucial role of inorganic pyrophosphate in integrating carbon metabolism from sucrose breakdown to starch synthesis in rice endosperm

• Published in: Plant science (Limerick) 2020-09, Vol.298, p.110572-110572, Article 110572
• Main Authors: Lee, Sang-Kyu, Jeon, Jong-Seong
• Format: Article
• Language: English
• Subjects: Carbon - metabolism; Cereal; Endosperm; Endosperm - metabolism; Hypoxia; Inorganic pyrophosphate; Oryza - metabolism; Phosphates - metabolism; Rice; Starch; Starch - biosynthesis; Sucrose; Sucrose - metabolism
• ISSN: 0168-9452; 1873-2259
• DOI: 10.1016/j.plantsci.2020.110572

•In rice and other cereals, the endosperm, which stores starch, is commonly exposed to hypoxic conditions and thus requires an energy metabolism distinct from conventional reliance on ATP from mitochondrial respiration.•Endosperm cells generate a unique source of inorganic pyrophosphate (PPi) from the cytosolic reaction of ADP glucose pyrophosphorylase (AGP).•Endosperm mutants for PPi-utilizing enzymes reveal crucial roles of PPi as an alternative energy source in integrating carbon metabolism from sucrose breakdown to starch synthesis.•By using PPi, the endosperm operates the interlaced metabolic flux of both sucrose synthase/UDP-glucose pyrophosphorylase/AGP and glycolysis and a non-cyclic mode of the TCA cycle to sustain adequate levels of ATP. The endosperm is a primary constituent of mature seeds in rice as well as in other cereal crops, serving as the major storage reserve of starch. Observations indicate that the central part of the endosperm is subject to hypoxic conditions, which require a switch of energy metabolism owing to limited mitochondrial respiration. Uniquely, this endosperm generates a large source of inorganic pyrophosphate (PPi) as a byproduct of the reaction of ADP glucose pyrophosphorylase in the cytosol. Recent results derived from examination of the mutants of cereal crops, especially rice, for PPi-utilizing enzymes clearly suggest an important role of PPi as an alternative energy currency for integrating carbon metabolism from sucrose breakdown to starch synthesis in the endosperm. Thus, the present review provides an outline of the interlaced PPi-dependent metabolic pathways, which are critical for starch synthesis in the endosperm in terms of energy metabolism, along with its application to enhance yield potential.