Efforts to capture high amylose in rice

Screening of wild and cultivated rice in IRRI germplasm collection revealed that majority have intermediate apparent amylose content. It appears that ancient farmers selected rice based on texture of the lower amylose varieties, considering that the majority of rice consumers today prefer intermediate to soft-textured rice. Furthermore, 30% seems to be the natural upper natural limit of amylose levels in wild-type rice. If this is the case, the rich biodiversity of rice has been subjected to the bottleneck of domestication to select for grains that have superior cooking and eating but not nutritional or satiating qualities considering that the majority of rice consumers today eat rice three times a day. On the other hand, the amylose content of available rice mutants with deficient SBEIIb or an over-expressed GBSSI also revealed amylose levels of around 35% which is significantly lower by comparison with other high amylose cereals, whose amylose content ranges from 70–90%. Hence, to produce the high amylose phenotype in rice, one might need to target different sets of enzymes or regulatory pathways. Since increasing the amylose levels in rice might mean a concomitant increase in its resistant starch content and in its levels of satiety, and a decrease in its glycemic response, developing high amylose rice by biotechnology is imperative. This type of rice will be important not only in addressing the growing obesity epidemic which now also affects the developing countries but also as a basis of novel degradable biopolymers and for further elucidating the mechanisms of starch synthesis in the cereal endosperm. In this paper, we also present the status of our research project which aims to silence the expression of SBEIIa, SBEIIb and SSIIa singly or in combination using microRNA and RNAi silencing technologies with the aim of increasing the amylose levels in rice beyond its natural limits

Association between alleles of the waxy gene and traits of grain quality in Philippine Seed Board rice varieties

The association between alleles of the Waxy gene, defined by the number of CT repeats on exon 1, and traits of grain quality was carried out using 47 Philippine Seed Board rice varieties. The major alleles of the Waxy gene in the set of 47 were (CT), (CT) (CT)and (CT)20. Varieties were divided into four clusters based on the metric of each trait of grain quality. (CT) (27-32% amylose content (AC)) and (CT) (22-30% AC) were mainly in clusters 3 and 4 (hard texture), and (CT) (20-24% AC) and (CT) (18- 27% AC) were exclusively in grain quality cluster 1 (soft texture) and 2 (medium texture). (CT) associated negatively with AC in this set. Only six (CT) and one (CT) rices were in cluster 4 (high-AC low-GT) and had high RVA consistency (final viscosity - trough viscosity) > 200 RVU. (CT) seemed to be the preferred source of low-intermediate AC in the Philippine rice breeding program, followed by (CT)

Balancing the double‐edged sword effect of increased resistant starch content and its impact on rice texture: its genetics and molecular physiological mechanisms

Resistant starch (RS) is the portion of starch that escapes gastrointestinal digestion and acts as a substrate for fermentation of probiotic bacteria in the gut. Aside from enhancing gut health, RS contributes to a lower glycemic index. A genome‐wide association study coupled with targeted gene association studies was conducted utilizing a diverse panel of 281 resequenced Indica rice lines comprising of ~2.2 million single nucleotide polymorphisms. Low‐to‐intermediate RS phenotypic variations were identified in the rice diversity panel, resulting in novel associations of RS to several genes associated with amylopectin biosynthesis and degradation. Selected rice lines encoding superior alleles of SSIIa with medium RS and inferior alleles with low RS groups were subjected to detailed transcriptomic, metabolomic, non‐starch dietary fibre (DF), starch structural and textural attributes. The gene regulatory networks highlighted the importance of a protein phosphatase alongside multiple genes of starch metabolism. Metabolomics analyses resulted in the identification of several metabolite hubs (carboxylic acid, sugars and polyamines) in the medium RS group. Among DF, mannose and galactose from the water‐insoluble fraction were found to be highly associated with low and medium RS lines, respectively. Starch structural analyses revealed that a moderate increase in RS is also linked to an elevation of amylose 1 and amylose 2 fractions. Although rice lines with medium RS content negatively affected textural and viscosity properties in comparison to low RS, the textural property of medium RS lines was in the same acceptable range as IR64, a rice mega variety popular in Asia

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

Land, Livelihood, Poverty: Assessment of Selected Socioeconomic Factors Influencing Community Adaptive Capacity to Climate Change

Butardo-Toribio MZ, Tenefrancia ER. Land, Livelihood, Poverty: Assessment of Selected Socioeconomic Factors Influencing Community Adaptive Capacity to Climate Change. COMCAD Arbeitspapiere - Working Papers, 94. Bielefeld: COMCAD - Center on Migration, Citizenship and Development; 2011

Quantifying grain digestibility of starch fractions in milled rice

Rice is one of the staple foods which serves as the major source of carbohydrate in the human diet. A typical milled rice grain is mainly composed of starch of up to 80–90%, with an average of 6–8% proteins and some trace amounts of dietary fiber. Although cooked white rice can elicit variable glycemic response, a portion of rice starch may evade digestion in the human small intestine. The digested portion of rice can be estimated and characterized in vitro based on starch digestion extent and rate (kinetics). The indigestible portion of starch can also be quantified. This chapter will present micro-scale methods to quantify rice starch digestion rate and extent based on the sugar fractions released after treating the samples with digestive enzymes