Characteristics of Grain Physicochemical Properties and the Starch Structure in Rice Carrying a Mutated ALK/SSIIa Gene

The gelatinization temperature (GT) of endosperm starch influences rice eating and the cooking quality (ECQ). ALK encoding soluble starch synthase IIa (SSIIa) is the major gene determining grain GT in rice. Herein, we identified a spontaneous ALK mutant named ALKd, which resulted from a G/T single-nucleotide polymorphism (SNP) in exon 1 of the ALKc allele from the high-GT indica rice cultivar. Compared with grains from the ALKc near-isogenic line (NIL), NIL(ALKd) grains exhibited a high GT (2.3 °C) and improved retrogradation properties. The NIL(ALKd) grain starch contained an increased proportion of amylopectin intermediate chains (DP 13–24) at the expense of short chains (DP < 12), resulting in enhancements in both the crystallinity and the lamellar peak intensity compared with low-GT rice grains. Moreover, both NIL(ALKd) and NIL(ALKc) grains also featured a significantly lower apparent amylose content (AAC), harder gel consistency (GC), higher pasting curve, and poorer taste values in comparison to Nip(ALKa) grains. Taken together, this work provides novel insights underlying the allelic variation of the ALK gene in rice.