Amylose and amylopectin branch chain reactivity in a model derivatization system

Hydrothermal treatments of starch impact reaction patterns during subsequent chemical derivatization

Differences in derivatization patterns (using a fluorescent reagent, fluorescein isothiocyanate) of wheat, pea, and potato starches between native granular (NAT) starches and their respective annealed (ANN) and heat-moisture treated (HMT) starches were investigated to reveal structural changes associated with starch hydrothermal treatments. Size-exclusion chromatography with fluorescence and refractive index detection assessed the reactivity of amylose (AM), intermediate chains (IM1 and IM2), and amylopectin branch chains (AP1, AP2, and AP3) within the different starches. Shifts in X-ray diffraction patterns of HMT starches and in the gelatinization properties of both ANN and HMT starches confirmed molecular rearrangement. The reaction homogeneity (wheat and pea) and the overall extent of reaction (pea and potato) increased for HMT starches compared to other starches. The lower reactivities of IM2 chains (HMT starch) and AP3 chains (ANN starch) relative to NAT starches, indicated their involvement in molecular rearrangements and improved double helical order. IM2 and AP branch chains in ANN pea starch also were less reacted than NAT starch chains, suggesting their co-crystallization. Molecular rearrangements in ANN and HMT starches led to altered swelling and pasting viscosities. Thus, changes in the relative crystallinity of individual starch branch chains induced by hydrothermal processing impact the final physical properties.

Impact of static and dynamic modes of semi-dry heat reaction on the characteristics of starch citrates

The objective of this study was to demonstrate the validity of dynamic semi-dry heat reaction (SDHR) by investigating the effects of static and dynamic SDHR on the characteristics of starch citrates. The starch (normal and waxy corn)-citric acid (CA) mixture was heated in a convection oven (static mode) or a twin-screw extruder without a die (dynamic mode). The ester bonds of starch citrates were confirmed by FT-IR, and their molar degree of substitution did not differ between the reaction modes for the tested starch genotypes. Starch citrates by dynamic SDHR exhibited higher relative crystallinity, resistant starch content, solubility, swelling power, and gelatinization compared to those by static SDHR. Although static SDHR did not show a viscosity development during pasting, dynamic SDHR increased their pasting viscosities. Overall, these results suggest that dynamic SDHR could improve the defects (i.e., lack of solubility, swelling, and pasting attributes) of the traditional starch citrates.

Overexpression of ZmbZIP22 gene alters endosperm starch content and composition in maize and rice

Starch content and composition are major determinants of yield and quality in maize. In recent years, the major genes for starch metabolism have been cloned in this species. However, the role of transcription factors in regulating the starch metabolism pathway remains unclear. The ZmbZIP22 gene encodes a bZIP transcription factor. In our study, plants overexpressing ZmbZIP22 showed reductions in the size of starch granules, the size and weight of seeds, reduced amylose content, and alterations in the chemical structure of starch granules. Also, overexpression of ZmbZIP22 resulted in increases in the contents of soluble sugars and reducing sugars in transgenic rice and maize. ZmbZIP22 promotes the transcription of starch metabolism genes by binding to their promoters. Screening by yeast one-hybrid assays indicated a possible interaction between ZmbZIP22 and the promoters of eight key starch enzyme genes. Collectively, our results indicated that ZmbZIP22 functions as a negative regulator of starch synthesis, and suggest that this occurs through the regulation of key sugar and starch metabolism genes in maize.

The DOF-Domain Transcription Factor ZmDOF36 Positively Regulates Starch Synthesis in Transgenic Maize

Starch synthesis is a complex process that influences crop yield and grain quality in maize. Many key enzymes have been identified in starch biosynthesis; however, the regulatory mechanisms have not been fully elucidated. In this study, we identified a DOF family gene, ZmDOF36, through transcriptome sequencing analysis. Real-time PCR indicated that ZmDOF36 was highly expressed in maize endosperm, with lower expression in leaves and tassels. ZmDOF36 is a typical DOF transcription factor (TF) that is localized to the nucleus and possesses transcriptional activation activity, and its transactivation domain is located in the C-terminus (amino acids 227-351). Overexpression of ZmDOF36 can increase starch content and decrease the contents of soluble sugars and reducing sugars. In addition, abnormal starch structure in transgenic maize was also observed by scanning electron microscopy (SEM). Furthermore, the expression levels of starch synthesis-related genes were up-regulated in ZmDOF36-expressing transgenic maize. ZmDOF36 was also shown to bind directly to the promoters of six starch biosynthesis genes, ZmAGPS1a, ZmAGPL1, ZmGBSSI, ZmSSIIa, ZmISA1, and ZmISA3 in yeast one-hybrid assays. Transient expression assays showed that ZmDOF36 can activate the expression of ZmGBSSI and ZmISA1 in tobacco leaves. Collectively, the results presented here suggest that ZmDOF36 acts as an important regulatory factor in starch synthesis, and could be helpful in devising strategies for modulating starch production in maize endosperm.