28. Genetic variation of granule-bound starch synthase (GBSS) level in Myanmar local rice cultivars

Plant Genetics Laboratory, Faculty of Agriculture, Kyushu University, Hakozaki, Fukuoka, 812-8581 Japan.

Amylose content is one of the most important factors affecting the cooking and processing behavior of rice. Amylose is synthesized by granule-bound starch synthase (GBSS); the expression level seems to control the amylose content (Sano 1984). This paper deals with the genetic variation of GBSS level in Myanmar local rice cultivars.

A total of 350 local rice cultivars were obtained from the Seed Bank of Myanmar. Kinmaze, IR36 and their respective waxy mutants, EM21 and EM2009, were used as controls. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of crude proteins extracted from seed samples were performed by the method of Laemmli (1970). Apparent amylose content in endosperm starch was colorimetrically determined by DU 7500 Spectrophotometer (Beckman).

Kinmaze and IR36 showed low and high GBSS level, respectively. GBSS were absent in their waxy mutants. Myanmar local rice cultivars were classified into four types of GBSS levels; high, intermediate, low and absent, based on staining intensity of 60 kD band and western blot analysis using antiserum raised against the waxy protein from wheat endosperm (Figs. 1A and B). This result indicates that GBSS level is diverse in Myanmar local rice cultivars.

The apparent amylose content of Myanmar local rice cultivars varied from 0 to 30%. The 350 accessions analyzed were classified into five apparent amylose content groups: waxy (0-6%), very low (6-12%), low (12-18%), intermediate (18-24%) and high (>24%). The relationship between GBSS level and the apparent amylose content was investigated by half seed

analysis (Fig. 2). Amylose content ranged from 0 to 8% among the cultivars without GBSS, and from 6 to 24% among cultivars with low level of GBSS. Cultivars with intermediate and high levels of GBSS had 6 to 24% and 13 to 30% amylose content, respectively. Apparent amylose content estimated by colorimetric method was influenced not only by net amylose content but also by fine structure of amylopectin (Nishi et al. 2001). The amylopectin structure is clearly different between japonica and indica rice cultivars (Nakamura et al. 2002). Variation in apparent amylose content of Myanmar local rice cultivars may be caused not only by net amylose content but also by amylopectin structure, even though each GBSS level shows variation in apparent amylose content.

The regional distribution pattern of GBSS level is shown (Table 1). Cultivars with intermediate GBSS level were localized in three mountain regions, i.e. Western Mountain Region (WMR), Northern Mountain Region (NMR) and Eastern Mountain Region (EMR). Cultivars showing high level of GBSS were widely distributed throughout; 59% of Myanmar local rice cultivars belong to this type. Variation in GBSS level of cultivars from the three mountain regions was much wider than these from other regions.

These results suggest that the mountain regions in Myanmar are important for maintaining

the genetic resources of amylose properties, as reported by Nakagahra (1978).

We thank Dr. N. Fujita, Faculty of Bioresource Sciences, Akita Prefectural University, for the gift of antiserum raised against the waxy protein of wheat endosperm.


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