The analysis for alteration in starch biosynthesis metabolism in a japonica rice grain mutant which does not accumulate starch

Rice (Oryza sativa. L) is one of the most important staple foods worldwide and the development of rice grain is abundantly related to growth of the endosperm. The endosperm is initiated by the fusion between a sperm nucleus and two polar nuclei during the double fertilization process and undergoes a series of coordinated cellular and metabolic events, including starch accumulation, cell death and starch granule packaging during the storage phase.1 Starch generally accounts for about 70% of the total dry weight in cereal grains and serves as a primary source of food with a wide range of industrial applications.2 In previous reports, many efforts have been made to elucidate the metabolic mechanisms underlying the starch biosynthesis pathway in developing endosperms, and some key enzymes, involving ADP‒ glucose pyrophosphorylase (AGP), sucrose synthase (Susy), soluble starch synthase (SSS), granule‒bound starch synthase (GBSS), starch branching enzyme (SBE) and starch debranching enzyme (DBE) have been considered as necessities for the starch biosynthesis in cereal endosperm. In recent decades, a series of mutants related to deficient starch biosynthesis have been generated and provided new insights into the complex mechanism of starch biosynthesis in cereal endosperms. Several isoforms including SS I, SS II, SS III, SS IV and GBSS had specific functions in initiation, elongation, branching and debranching of transient or storage starch synthesis in source and sink tissues, respectively.3 Among all of these SS isoforms, SS I was highly and specifically expressed in rice endosperm and performs about 70% of total SS activities, and its activity was also higher than that in other isoforms.4 Two related forms of SBE (SBE I and SBE II) exist in cereal endosperm. Loss of SBE I isoform activity could limit the synthesis of starch in a way that cannot be compensated by SBE II isoform; thus, the activity of SBE I was essential for normal regular organization of the starch granule whether SBE II existed or not.5 Two DBE families occurred, such as ISA and PUL in plants.6 reported that sugary‒1 phenotype was caused by the loss of the activity of the PUL enzyme, suggesting that the DBE was also involved in starch biosynthesis apart from its function in starch degradation in conjunction with other hydrolytic activities Rice, barley and maize sugary mutants were generally caused by the lack of ISA genes and low DBE activity, accompanied with a decreased starch accumulation in the endosperm and alterations in the fine structure and numbers of starch granule.7 At least three ISA genes were present in plants. Among them, ISA 1 was highly expressed in the cereal endosperm and the reduced levels of ISA 1 could be found in the synthesis of phytoglycogen in the antisense‒expression of transgenic rice plants,8