Rice SPK, a calmodulin-like domain protein kinase, is required for storage product accumulation during seed development: Phosphorylation of sucrose synthase is a possible factor

Sue, an end product of photosynthesis, is metabolized by Sue synthase in sink organs as an initial step in the biosynthesis of storage products. Sue synthase activity is known to be regulated by reversible phosphorylation, but the details of this process are unclear at present. Rice SPK, a calcium-dependent protein kinase, is expressed uniquely in the endosperm of immature seed, and its involvement in the biosynthetic pathways of storage products was suggested. Antisense SPK transformants lacked the ability to accumulate storage products such as starch, but produced watery seed with a large amount of Sue instead, as the result of an inhibition of Sue degradation. Analysis of in vitro phosphorylation indicated that SPK phosphorylated specifically a Ser residue in Sue synthase that has been shown to be important for its activity in the degradation of Sue. This finding suggests that SPK is involved in the activation of Sue synthase. It appears that SPK is a Sue synthase kinase that may be important for supplying substrates for the biosynthesis of storage products.