Title: Effect of enzyme activity on the starch structure and processing quality of selected rice varieties Authors
Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: December 14, 2013
Publication Date: N/A
Technical Abstract: Although most commercialized long grain rice varieties have intermediate amylose content (~22%), high amylose (>25%) varieties are important for the canning and parboiling industry. Research has shown that high amylose rice varieties that have the best processing quality have high setback and low breakdown paste viscosity as measured by the Rapid Visco Analyzer (RVA). Genetic markers have been developed that are associated with the granule bound starch synthase gene (GBSS) and can be used to generally categorize rice varieties for amylose content and starch paste viscosity profiles, thus predicting processing quality. However our research has demonstrated that high amylose rice cultivars with the same genetic haplotype for GBSS may differ in RVA profiles. Moreover, various environmental parameters such as high nighttime temperatures, in particular, have been shown to be related to lower head rice yield, higher chalk percentages, lower amylose contents, and poor processing quality. This phenomenon is believed to be caused by the changes in the activity of the starch synthesizing enzymes during grainfill (starting at R6 growth stage). This study was conducted to determine how intermediate and high amylose rice varieties respond to different growing environments for factors associated with processing quality and to what degree this may be explained by changes in starch enzyme activity and starch structure. Ten rice varieties were grown in the field in Stuttgart, AR during 2010 and 2012 using a randomized complete block design with two replications and two planting dates, about 1 month apart. Intermediate amylose varieties included Cypress, Francis, and Lagrue and the high amylose varieties included Rondo, Shu 121, Teqing, Zhe 733, Dixiebelle, Sabine, and Bowman. Grains were harvested at the R6 growth stage and the activity of seven starch synthesizing enzymes (sucrose synthase (SS), uridine 5’-diphosphatase (UDGP), adenosine 5’-diphosphatase glucose pyrophosphorylase (ADGP), granule-bound starch synthase (GBSS), soluble starch synthase (SSS), branching enzyme (BE), and debranching enzyme (DB)) were determined. At maturity (R8) grain was harvested, dried to 12 % moisture, and milled. Apparent amylose content, protein content, DSC, RVA, and chain-length distributions of amylopectin were determined. A two-way ANOVA was performed with the plant date, variety, and their interaction treated as fixed effects. The interaction source of variation was not significant for any parameter measured. The delayed planting date was found to decrease gelatinization temperature, increase protein content, and increase UDGP activity. Varieties were significantly different for amylose and protein contents, gelatinization temperature, and RVA parameters. Rondo and Shu 121 had significantly higher final RVA paste viscosity and a higher proportion of short chains (db 6-12) in the amylopectin assay than all other varieties. Rondo and Shu 121 were also characterized as having the lowest gelatinization temperature. The other high amylose varieties (Dixiebelle, Sabine, Bowman, Teqing, and Zhe 733) had significantly greater final RVA paste viscosities as compared to the intermediate amylose cultivars (Cypress, Lagrue, and Francis). There was a significant varietal difference in DB activity, however, no significant difference was found among the cultivars for the activity of any of the other enzymes. Dixiebelle had significantly greater DB activity than Zhe 733, Rondo, Bowman, and Teqing. These initial results indicate that factors associated with processing quality and starch enzyme activity were quite stable across the two planting dates that were one month apart. Varieties with the same genetic haplotype for several genetic markers associated with GBSS were also consistent for measures associated with processing quality. Differences were observed between intermediate and high amylose varieties for factors associated with processing quality however these differences were not related to six of the seven starch enzymes activities that were analyzed. Debranching enzyme, which determines the degree of branching of the amylopectin molecule, was the only enzyme to significantly differ among the varieties. The data will be further explored using various modeling approaches to better understand the relationship of enzyme activities, starch structure, and processing quality.