Abstract
SummaryResistant 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.
Highlights
Rice varieties differ in their grain starch composition, with variable proportion of amylose and amylopectin contributing to differential textural and digestibility properties (Butardo et al, 2017; Li et al, 2016)
Starch can be divided into three categories: (i) rapidly digestible starch (RDS) with most of its being digested in 20 min treated as high caloric, (ii) slowly digestible starch (SDS) as partially caloric with digestion being completed within 120 min and a (iii) non-digestible resistant starch (RS) fraction resistant to digestion beyond 120 min and thereby categorized as non caloric (BeMiller, 2019; Guzman et al, 2017)
Phenotypic variability of resistant starch content in indica diversity panel We employed a diverse panel of 310 resequenced indica (Oryza sativa L. indica) landraces and pre-breeding lines in order to assess phenotypic plasticity of RS and to delineate its relationship with the in vitro glycemic index (GI), amylose content (AC) and gelatinization temperature (GT)
Summary
Rice grain represents a staple food composed of up to 90% starch, which provides up to 40% of the calorific intake of humans. The amylose and amylopectin are arranged in alternating amorphous and semi-crystalline lamellae to form starch granules. Processing including milling, cooking and cooling affects RS. RS2 represents starch that is in the form of B and C native crystalline structures arranged in a compact radial pattern, found in high amylose cereals, remains as ungelatinized resistant granules (Sajilata et al, 2006). RS3 is retrograde starch formed when cooked rice is cooled (Alhambra et al, 2019). During this process, the starch structure becomes re-associated to form a semi-crystalline double
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