Impact of down-regulation of starch branching enzyme IIb in rice by artificial microRNA- and hairpin RNA-mediated RNA silencing

The inactivation of starch branching IIb (SBEIIb) in rice is traditionally associated with elevated apparent amylose content, increased peak gelatinization temperature, and a decreased proportion of short amylopectin branches. To elucidate further the structural and functional role of this enzyme, the phenotypic effects of down-regulating SBEIIb expression in rice endosperm were characterized by artificial microRNA (amiRNA) and hairpin RNA (hp-RNA) gene silencing. The results showed that RNA silencing of SBEIIb expression in rice grains did not affect the expression of other major isoforms of starch branching enzymes or starch synthases. Structural analyses of debranched starch showed that the doubling of apparent amylose content was not due to an increase in the relative proportion of amylose chains but instead was due to significantly elevated levels of long amylopectin and intermediate chains. Rices altered by the amiRNA technique produced a more extreme starch phenotype than those modified using the hp-RNA technique, with a greater increase in the proportion of long amylopectin and intermediate chains. The more pronounced starch structural modifications produced in the amiRNA lines led to more severe alterations in starch granule morphology and crystallinity as well as digestibility of freshly cooked grains. The potential role of attenuating SBEIIb expression in generating starch with elevated levels of resistant starch and lower glycaemic index is discussed

Maintaining the yield of edible rice in a warming world

High temperature increases the amount of chalk in rice (Oryza sativa L.) grains, which causes grains to break during polishing, lowering the amount of rice for consumption. Here, we examined the effect of elevated temperature on substrate supply to the panicle, the capacity of the panicle to produce edible grains, and underlying factors affecting yield of edible grain in two varieties. During grain-filling, substrate supply followed a bell shaped curve, and high temperature significantly shortened supply time. The rate of grain-filling did not change and paddy yield fell in both varieties. In high temperature, yield loss in IR8 was due to lighter grains relative to those grown in cool temperature, but in IR60, it was due to the early sacrifice of 30% of the spikelets. The yield of edible rice was zero for IR8 and similar to 60% for IR60 for the high temperature treatments, and 100% for IR60 and 70% for IR8 in the cool temperature. IR60 differs from IR8 in regulation of substrate supply, architecture of the panicles and the capacity of the panicles to alter sink size in response to the stress and these factors may be responsible for the difference in edible rice in the two varieties

Advances in understanding the role of rice in nutrition

Malnutrition includes both under- and over-nutrition. The development of rice varieties to improve dietary quality can play a role in addressing both forms of malnutrition. This chapter begins by considering under-nutrition, with an emphasis on the problem of micronutrient deficiency. It reviews how a staple crop such as rice can be biofortified to deliver key nutrients such as iron, zinc and vitamin A. Over-nutrition and its negative health consequences are then discussed, and we explore the possibility of biofortifying rice to carry nutritional properties of benefit to those at risk of or suffering from chronic disease

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Biomolecular analyses of starch and starch granule proteins in the high-amylose rice mutant goami 2

Elevated proportions of amylose in cereals are commonly associated with either the loss of starch branching or starch synthase activity. Goami 2 is a high-amylose mutant of the temperate japonica rice variety Ilpumbyeo. Genotyping revealed that Goami 2 and Ilpumbyeo carry the same alleles for starch synthase IIa and granule-bound starch synthase I genes. Analyses of granule-bound proteins revealed that SSI and SSIIa accumulate inside the mature starch granules of Goami 2, which is similar to the amylose extender mutant IR36ae. However, unlike the amylose extender mutants, SBEIIb was still detectable inside the starch granules of Goami 2. Detection of SBEIIb after protein fractionation revealed that most of the SBEIIb in Goami 2 accumulates inside the starch granules, whereas most of it accumulates at the granule surface in Ilpumbyeo. Exhaustive mass spectrometric characterisations of granule-bound proteins failed to detect any peptide sequence mutation or major post-translational modifications in Goami 2. Moreover, the signal peptide was found to be cleaved normally from the precursor protein, and there is no apparent N-linked glycosylation. Finally, no difference was found in the SBEIIb structural gene sequence of Goami 2 compared with Ilpumbyeo. In contrast, a G-to-A mutation was detected in the SBEIIb gene of IR36ae located at the splice site between exon and intron 11, which could potentially introduce a premature stop codon and produce a truncated form of SBEIIb. It is suggested that the mutation responsible for producing high amylose in Goami 2 is not due to a defect in SBEIIb gene as was observed in IR36ae, even though it produces a phenotype analogous to the amylose extender mutation. Understanding the molecular genetic basis of this mutation will be important in identifying novel targets for increasing amylose and resistant starch contents in rice and other cereals