**4. Gel consistency**

Gel consistency (GC) is a measure of firmness of the rice after cooking and is performed to classify rice varieties of the same AC, particularly in the high AC class, into hard, medium, or soft texture (Cagampang et al., 1973; Kohlwey, 1994). GC is commonly measured by determining the length of a cooled gel made from flour previously cooked in 0.2 M KOH (Cagampang et al., 1973). Variations in the method exist and are used depending on the AC of the samples. For waxy varieties, which form pastes instead of gels (Lii et al., 1995; Tsai et al., 1997), the amount of flour used is higher (Bean et al., 1984) or neutral solutions are used instead of the alkaline solution (Perdon & Juliano, 1975). However, GC is not generally used in rice improvement programs focusing on rice varieties of intermediate and lower AC classes.

GC is a measure of the strength of the gel. The range of GC values to classify rice varieties according to this property is wide. Samples are grouped into arbitrarily set classes based on the length of the gel: hard (length of gel < 40 mm), medium (length of gel 41 – 60 mm), and soft (length of gel > 61 mm) (Graham, 2002). Weak and rigid gels depend on the association of starch polymers in the aqueous phase (Dea, 1989). Since amylose is the main polymer that leaches as starch granules are heated (Tsai & Lii, 2000) and amylose forms networks as the gel starts to cool (Gidley, 1989; Nguyen et al., 1998), GC could well be related to AC. Indeed, correlations between the two properties have been reported in many populations and landraces (Tan & Corke, 2002; Septiningsih et al., 2003; Zheng et al., 2007). In addition, the decrease in AC in irradiated rice was attributed to the softening of the rice gel (Yu & Wang, 2007).

The established correlations support the associations between GC and QTLs mapped to the *Wx* locus (He et al., 2006; Lanceras et al., 2000; Zheng et al., 2007). Studies have even reported that the gene coding for GC is located within the *Wx* locus (i.e., Tang et al., 1991; Tian et al., 2005). It was recently shown that the major gene for GC is in fact the *Wx* gene, and the mutation is a SNP on exon 10, a C T polymorphism, which groups high AC rice varieties into hard and soft GC classes only (Tran et al., 2011). In the same way that a major gene separates high and low GT, as reviewed above, the extremes of GC are explained by biallelic variability at a single locus, but the intermediate class was not accounted for by that locus (Tran et al., 2011).

The relationship between AC and GC in IRRI breeding materials, however, is not as clear as those previously reported (Figure 9). These results show that GC is spread across all the AC classes in the breeding materials. Varieties from waxy and very low AC tended to have soft

Other studies have investigated the role of chain termination on GT. Changes in functionality of BEIIb have been associated with variations in GT by affecting the chainlength distribution of amylopectin in *amylose-extender* mutants (Jiang et al., 2003; Nishi et al., 2001; Tanaka et al., 2004; Yamakawa et al., 2007). Comparisons of amylopectin chain-length distributions between IR36*ae*, an established *BEIIb* mutant (Asaoka et al., 1986; Juliano et al., 1990), and its wildtype IR36, and between Goami 2, a mutant variety with functional properties similar to those of *BEIIb* mutants (Kang et al., 2003), and its wildtype Ilpumbyeo, suggest that BEIIb functions in complex with SSIIa (Cuevas et al. 2010b). A number of gene interactions have been reported to affect GT in an inter-subspecific doubled haploid population (He et al., 2006). However, there is still genetic work to be done to discover the

Gel consistency (GC) is a measure of firmness of the rice after cooking and is performed to classify rice varieties of the same AC, particularly in the high AC class, into hard, medium, or soft texture (Cagampang et al., 1973; Kohlwey, 1994). GC is commonly measured by determining the length of a cooled gel made from flour previously cooked in 0.2 M KOH (Cagampang et al., 1973). Variations in the method exist and are used depending on the AC of the samples. For waxy varieties, which form pastes instead of gels (Lii et al., 1995; Tsai et al., 1997), the amount of flour used is higher (Bean et al., 1984) or neutral solutions are used instead of the alkaline solution (Perdon & Juliano, 1975). However, GC is not generally used in rice improvement programs focusing on rice varieties of intermediate and lower AC classes. GC is a measure of the strength of the gel. The range of GC values to classify rice varieties according to this property is wide. Samples are grouped into arbitrarily set classes based on the length of the gel: hard (length of gel < 40 mm), medium (length of gel 41 – 60 mm), and soft (length of gel > 61 mm) (Graham, 2002). Weak and rigid gels depend on the association of starch polymers in the aqueous phase (Dea, 1989). Since amylose is the main polymer that leaches as starch granules are heated (Tsai & Lii, 2000) and amylose forms networks as the gel starts to cool (Gidley, 1989; Nguyen et al., 1998), GC could well be related to AC. Indeed, correlations between the two properties have been reported in many populations and landraces (Tan & Corke, 2002; Septiningsih et al., 2003; Zheng et al., 2007). In addition, the decrease in AC in irradiated rice was attributed to the softening

The established correlations support the associations between GC and QTLs mapped to the *Wx* locus (He et al., 2006; Lanceras et al., 2000; Zheng et al., 2007). Studies have even reported that the gene coding for GC is located within the *Wx* locus (i.e., Tang et al., 1991; Tian et al., 2005). It was recently shown that the major gene for GC is in fact the *Wx* gene, and the mutation is a SNP on exon 10, a C T polymorphism, which groups high AC rice varieties into hard and soft GC classes only (Tran et al., 2011). In the same way that a major gene separates high and low GT, as reviewed above, the extremes of GC are explained by biallelic variability at a single locus, but the intermediate class was not accounted for by that

The relationship between AC and GC in IRRI breeding materials, however, is not as clear as those previously reported (Figure 9). These results show that GC is spread across all the AC classes in the breeding materials. Varieties from waxy and very low AC tended to have soft

basis of intermediate gelatinisation temperature.

**4. Gel consistency** 

of the rice gel (Yu & Wang, 2007).

locus (Tran et al., 2011).

GC (higher GC values) while varieties from the other AC classes had GC readings from hard to soft. Thus, other factors aside from amylose must be contributing to the strength of the cooling gel, and understanding these will assist breeding programs to select more accurately for traits of texture.

Fig. 9. Relationship of amylose content and gel consistency in IRRI breeding materials analysed in 2004 – 2007 (r = -0.169).

In the IRRI rice breeding programs, materials with soft GC appear to be preferred (Tran et al., 2011). Since the SNP in exon 10 of the *Wx* gene explains extreme variations in GC, it can be used as a tool in selecting breeding lines, particularly in programs working on varieties with high AC.

The diversity in GC in each amylose class indicates that GC may be controlled by several minor genes besides the *Wx* (He et al., 2006; Tang et al., 1991). In backcross populations derived from hard/medium and medium/soft parents, medium GC appeared to be recessive to hard GC while soft GC is recessive to medium GC, indicating three alleles for this trait (Tang et al., 1991). However, the exon 10 SNP in the *Wx* gene does not distinguish the medium type. The appearance of the medium GC trait might be contributed by other genes. For instance, various minor QTLs in chromosomes 1, 2, 6, and 7 have been associated with GC (He et al., 1999, 2006; Lanceras et al., 2000; Bao et al., 2002; Septiningsih et al., 2003; Zheng et al., 2007). Aside from minor genes, pleiotropic effects might be influencing GC as well. Interactions between *Wx* and *BEIII* and between *Wx* and *Pul* have been suggested to contribute to GC (He et al., 2006). With enzymes functioning in complexes, it may be possible that inactivity in one enzyme could affect the functionality of others within the complex, leading to medium GC, if complexes exist in rice.
