淀粉是光合作用的终产物之一，也是许多农作物收获器官的主要贮藏性多糖之一。主食中淀粉的含量及特性会极大地影响消费者的口感和健康，因此改变淀粉含量或特性对于培育高品质主食产品意义重大。淀粉合成途径中的关键酶包括ADPG焦磷酸化酶（ADP Glucose Pyrophosphorylase, AGP），催化葡萄糖供体ADPG的合成；淀粉合成酶（Starch Synthases, SS），催化ADPG分子中的葡萄糖转移到α-1,4-葡聚糖引物的非还原性末端；淀粉分支酶（Starch Branching Enzymes, SBEs），催化α-1,6-糖苷键的形成；（Starch Debranching Enzyme, DBE），催化断裂淀粉中的α-1,6-葡萄糖苷键，将支链淀粉转化为直链淀粉。
最近，日本科学家在水稻中发现了淀粉分支酶IIb基因BEIIb的两个新型突变体age1和age2，利用它们培育的水稻品种可以得到蒸煮品质优良的大米。研究人员在粳稻品种日本晴中得到了两个淀粉糊化特性有变化的突变体，age1和age2（altered gelatinization）。利用传统遗传分析并结合最近流行的MutMapPlus基因克隆方法，测定出age1和age2的突变位点位于淀粉分支酶基因BEIIb中。其中age1中BEIIb蛋白的723位点由Met变为Lys，淀粉分支酶活性全无；age2中BEIIb基因中由于插入了转座子导致淀粉分支酶活性显著降低。利用这两个突变体，结合淀粉合成酶SSIIa突变体，研究人员得到了不同蒸煮品质的水稻材料，它们在直链淀粉含量、热性能、α-淀粉酶消化性、支链淀粉结构等指标上各不相同，而口感表现基本一致。
该研究不仅是日本科学家在水稻中对BSA（Bulked Segregation Analysis）技术的有一次成功应用，其鉴定出的突变体也将为培育高蒸煮品质水稻品种提供新的路径。
Plant Biotechnology Journal, 14 JUN 2017
MutMapPlus identified novel mutant alleles of a rice starch branching enzyme IIb gene for fine-tuning of cooked rice texture
Author
Masaru Nakata, Tomomi Miyashita, Rieko Kimura……Takeshi Yamaguchi, Takayuki Umemoto and Hiromoto Yamakawa
Division of Crop Development, Central Region Agricultural Research Center, National Agriculture and Food Research Organization (NARO), Japan
Summary
Physicochemical properties of storage starch largely determine rice grain quality and food characteristics. Therefore, modification of starch property is effective to fine-tune cooked rice textures. To obtain new resources with modified starch property as breeding materials, we screened a mutant population of a japonica cultivar Nipponbare and found two independent mutant lines, altered gelatinization (age)1 and age2, with moderate changes in starch gelatinization property. A combination of conventional genetic analyses and the latest mapping method, MutMapPlus, revealed that both of these lines harbour novel independent mutant alleles of starch branching enzyme IIb (BEIIb) gene. In age1, amino acid substitution of Met-723 to Lys completely abolished BEIIb enzyme activity without significant reduction in its protein level. A transposon insertion in an intron of BEIIb gene reduced BEIIb protein level and activity in age2. Production of a series of the mutant lines by combining age alleles and indica-type starch synthase IIa allele established stepwise alteration of the physicochemical properties of starch including apparent amylose content, thermal property, digestibility by α-amylase and branched structures of amylopectin. Consistent with the alteration of starch properties, the results of a sensory evaluation test demonstrated that warm cooked rice of the mutants showed a variety of textures without marked reduction in overall palatability. These results suggest that a series of the mutant lines are capable of manipulation of cooked rice textures.
