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Open Access Publications from the University of California

Regulatory mechanism on sucrose metabolism in rice endosperm by a specific protein kinase, SPK

  • Author(s): Shimada, Hiroaki
  • Takeda, Haruka
  • Tsunoda, Akina
  • Sasaki, Tadamasa
  • et al.

Synthesis and accumulation of storage starch is a main action of endosperm organogenesis. The enzymes for starch synthesis should work in concert, but the regulatory mechanism is largely unknown. Rice SPK, a calcium-dependent protein kinase, is uniquely expressed in endosperm of immature seeds. Antisense-SPK transformants showed lacked accumulation of storage starch resulting in production of watery seeds with a large amount of sucrose. In vitro phosphorylation indicated that SPK specifically phosphorylated a serine residue in sucrose synthase involved in its activity, and thus SPK is a sucrose-synthase kinase. Sucrose synthase catalyzes the reversible conversion of sucrose in the presence of UDP to UDP-glucose and fructose. Recently, the reaction accompanied with ADP is also reported. Since sucrose synthase is regulated by reversible phosphorylation, we determined effect of the phosphorylation on its activity. The basal activity of sucrose synthase accompanied with ADP and ADP-glucose was quite low but apparently detected as well as that with UDP and UDP-glucose. The activity of sucrose cleavage accompanied with ADP was significantly elevated after sucrose synthase was phosphorylated, whereas little alteration was detected on other reactions. Thus, it is suggested that SPK enhances efficiency of sucrose cleavage reaction with ADP of sucrose synthase by its phosphorylation, and promotes the generation of ADP-glucose from sucrose in endosperm. Since storage starch is mainly produced from ADP-glucose, it appears that this reaction greatly contributes the supply of substrate for storage starch biosynthesis.

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