Analysis of Physiological and Biochemical Characteristics of Six Mutants with Stable High Percentage of Chalkiness in Rice Grains

Abstract

For explaining the mechanism of chalkiness formation in rice ( Oryza sativa L.), the physiological and biochemical factors for chalkiness formation were analyzed using 6 mutants with chalky percentage higher than 80%, which were obtained from a T-DNA insertion mutant pool of cultivar Nipponbare. The appearance of high chalky percentages of the 6 mutants was proven to result from genetic factors because high temperature stress did not occur during grain-filling period. Based on the activities of key enzymes associated with starch synthesis, sucrose content, starch content, and ratio of amylose to starch in mature grains, the 6 mutants were classified into 3 groups. Compared with the wild type, mutants mu1 and mu2 had lower activities of sucrose synthase (SS) during the whole grain-filling period, higher activity of adenosine diphosphoglucose pyrophosphorylase (AGPase) at early grain-filling stage, and higher activities of SS I and total soluble starch synthase (SSS) at late grain-filling stage. Mutants mu3 and mu6 had lower activities of SS I, total SSS, pullulanase, and isoamylase at early filling stage and higher isoamylase activity at late filling stage than the wide type. Compared with the wild type, mutants mu4 and mu5 exhibited higher SS activity at early and middle stages of grain filling and lower activity of total SSS during the whole grain-filling period; the isoamylase activity increased from a lower level than the wild type at early filling stage to a higher level at late filling stage. All the 6 mutants had white belly grains with no visible differences to the wild type after iodine staining. The contents of fructose, sucrose, and total soluble sugar were higher in the 6 mutants than in the wild type, especially in mu2 and mu5. These results primarily revealed the physiological basis of chalkiness formation during rice grain development.