Pullulanase and Starch Synthase III Are Associated with Formation of Vitreous Endosperm in Quality Protein Maize

The opaque-2 (o2) mutation of maize increases lysine content, but the low seed density and soft texture of this type of mutant are undesirable. Lines with modifiers of the soft kernel phenotype (mo2) called “Quality Protein Maize” (QPM) have high lysine and kernel phenotypes similar to normal maize. Prior research indicated that the formation of vitreous endosperm in QPM might involve changes in starch granule structure. In this study, we focused on analysis of two starch biosynthetic enzymes that may influence kernel vitreousness. Analysis of recombinant inbred lines derived from a cross of W64Ao2 and K0326Y revealed that pullulanase activity had significant positive correlation with kernel vitreousness. We also found that decreased Starch Synthase III abundance may decrease the pullulanase activity and average glucan chain length given the same Zpu1 genotype. Therefore, Starch Synthase III could indirectly influence the kernel vitreousness by affecting pullulanase activity and coordinating with pullulanase to alter the glucan chain length distribution of amylopectin, resulting in different starch structural properties. The glucan chain length distribution had strong positive correlation with the polydispersity index of glucan chains, which was positively associated with the kernel vitreousness based on nonlinear regression analysis. Therefore, we propose that pullulanase and Starch Synthase III are two important factors responsible for the formation of the vitreous phenotype of QPM endosperms.

玉米的opaque-2 (o2)突变可提升籽粒赖氨酸含量，但该突变体存在籽粒密度偏低、胚乳质地松软的不良性状。携带软粒表型修饰因子 (mo2) 的品系被称为“优质蛋白玉米 (Quality Protein Maize，QPM)”，其不仅赖氨酸含量较高，籽粒表型也与普通玉米相近。此前研究表明，QPM硬质胚乳的形成可能与淀粉颗粒结构的改变有关。本研究聚焦于两类可能影响籽粒硬质度的淀粉合成酶。对由W64Ao2与K0326Y杂交获得的重组自交系进行分析后发现，普鲁兰酶 (pullulanase) 活性与籽粒硬质度呈显著正相关。我们还发现，在Zpu1基因型一致的前提下，淀粉合酶III (Starch Synthase III) 的丰度降低可能会降低普鲁兰酶活性并缩短平均葡聚糖链长度。因此，淀粉合酶III可通过影响普鲁兰酶活性，并与普鲁兰酶协同改变支链淀粉的葡聚糖链长度分布，进而改变淀粉结构特性，间接影响籽粒硬质度。葡聚糖链长度分布与葡聚糖链多分散性指数呈强正相关；通过非线性回归分析可知，该多分散性指数与籽粒硬质度呈正相关。据此，我们提出：普鲁兰酶与淀粉合酶III是决定QPM胚乳硬质表型形成的两个关键因子。