Molecular Plant Breeding 2016, Vol.7, No.30, 1
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(0–5%), very low (5–12%), low (12–20%), intermediate (20–25%), and high (25–33%) (Suwannaporn et al
.
Digestion of starch normally begins with salivary α-amylase followed by additional hydrolysis by pancreatic
amylase secreted to the small intestine. The ultimate blood glucose level depends on the amount and
physicochemical properties of starch in cooked rice, rate of hydrolysis of starch to glucose and rate of absorption
to blood. Rice varieties possessing slowly digestible starch (high amylose, low glycemic index) can be useful for
management of dreadful diabetic problems. On the other hand, lower amylose containing starches are readily
hydrated and highly gelatinized upon heating. This facilitates easy hydrolysis of starch by α-amylase and greater
tendency to raise blood sugar levels. In other words, starch with lower amylose content has higher Glycemic
Index(GI) (Rathinasabapathi et al
.,
2015). Therefore, biosynthesis pathway that allows more synthesis of amylose
than amylopectin is desirable. GI is the response of a 50 g carbohydrate portion of food expressed as a percentage
of that of a standard. This requires measurement of area under blood glucose curve (AUC) of fasting blood
samples followed by half-hourly samples for two hours after feeding cooked rice equivalent to 50g carbohydrate.
Thus, GI (%) = (AUC of test food)/(AUC of glucose as reference food) x 100. Any variety of rice with GI less
than or equal to 55 is considered diabetic-friendly. Different varieties of rice have variable GI (48-92%)
(Fitzgerald et al
.
2011) owing to varying levels of amylose content. Indian Institute of Rice Research, Hyderabad
has identified three rice varieties e.g. Lalat, BPT 5204 and Sampada with low GI values. Some Australian,
Indonesian, Indian and Bangladeshi varieties have been reported to have lower GI than other rices. Notable among
these are the amylose heavy basmati type rices and the mega rice variety “Swarna” showing low GI, while IR 65
is a waxy variety which contains no amylose (highest GI) (Fitzgerald et al
.
2011). Amylose structure affects the GI
and that the firm-textured varieties in general have a lower GI. Parboiled red raw rice has GI as low as 56 in case
of var. By 350 due to high non-starch polysaccharide (fibre) content. Low GI foods are reported to reduce the risk
of coronary heart disease (Frost et al
.,
1999), obesity (Slabber, 1994) and exhaustion during sports (Walton and
Rhodes 1997).
GI of rice is increased upon hydration and boiling for longer time as a result of gelatinization of starch. Similarly
puffed product of rice increases the GI by 15-20% (Montignac 2015).In contrast, cooling and drying of cooked
rice reduce the GI due to reorganization of amylose and amylopectin making the starchy product more complex.
On the other hand, rice flour being easy to hydrolyse by digestive enzymes, has higher GI than whole rice. In
some cases, protein and fibres are associated with starch making it more complex, slower rate of digestion and
lower GI score.
Several methods including iodine binding, near infrared spectroscopy, size-exclusion chromatography and most
recently, asymmetric field flow fractionation (Chiaramonte et al., 2012) are now available for determination of
amylose content. Among these, the iodine binding method has been validated for routine use (Fitzgerald et al.,
2009). Many often, phenotyping based on any of these methods becomes misleading owing to the effect of high
temperature during grain development, modifier genes and cytoplasmic factors (Kumar and Khush, 1987).
However, molecular markers associated with the key enzymes for starch biosynthesis could be a viable alternative
to assess amylose status in rice. Thus, the above informations clearly elucidate the genetic basis of amylose
content and GI status in rice.
2 Genetic Basis of Glycemic Index and Allele Mining
GI is a complex trait. Large variability in GI, ranging from low to high GI, was found using a set of 235 varieties
(Fitzgerald et al
.
2011). GI of rice is shown to have negative relationship with amylose content. Therefore, genetic
basis of amylose content can elucidate GI status in rice. In fact, at least 18 highly polymorphic starch
biosynthesis related genes contribute directly or indirectly to the GI by altering the amylose and amylopectin
content in rice (Kharabian-Masouleh et al
.,
2012). Waxy mutants containing amylose-free starch have been
isolated from many plant species. In fact, a multiallelic waxy gene (Wx) encoding Granule-Bound Starch
Synthase I (GBSS I :60kDa) determines amylose content in rice endosperm (Mikami et al.,
. It is located on
chromosome 6 and consists of 13 exons and 12 introns. Exploring sequence variation (allele mining) of this key
