An investigation into the genetics and physiology of sugar accumulation in sweet sorghum as a potential model for sugarcane.

Ritter, Kimberley Belle (2007). An investigation into the genetics and physiology of sugar accumulation in sweet sorghum as a potential model for sugarcane. PhD Thesis, School of Land, Crop and Food Sciences, University of Queensland.

       
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Author Ritter, Kimberley Belle
Thesis Title An investigation into the genetics and physiology of sugar accumulation in sweet sorghum as a potential model for sugarcane.
School, Centre or Institute School of Land, Crop and Food Sciences
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Associate Professor Ian Godwin
Abstract/Summary Obtaining higher sugar yield is a major focus of sugarcane variety improvement programs, however the complex genome of sugarcane has hindered research and development of the crop. Sorghum and sugarcane are both members of the Andropogoneae tribe and comparative mapping has revealed a high level of synteny among the two closely related species. Sorghum is a diploid species with a small genome unlike sugarcane which has one of the most complex genomes of any organism. Particular varieties of sorghum, known as sweet sorghums or sorgos, accumulate high levels of sugar in stalk juice near the time of maturity, as does sugarcane. Based on this relationship between sorghum and sugarcane, this study aimed to investigate sweet sorghum as a genetic and physiological model for sugar accumulation in sugarcane. An F6 recombinant inbred line (RIL) segregating population with 184 progeny lines was developed from a cross between R9188, a dwarf conversion of the sweet sorghum line Rio, and R403463-2-1, a Queensland Department of Primary Industries and Fisheries (QDPIF) elite grain sorghum R-line. The population was evaluated in two field trials for 16 agronomic traits and eight sugar-related traits. A genetic linkage map of R9188 x R403463-2-1 was constructed integrating 225 polymorphic bands produced by 38 AFLP™ primer pairs, 37 Xtxp SSR and six sugarcane SSR markers. The constructed map had 16 linkage groups (LG), of which 10 could be assigned a chromosome on the basis of shared markers between this map and previously published maps of sorghum and sugarcane, and spanned a total length of 2012.9 cM (including unknown LG). Eleven traits were analysed for QTL identification; seven sugar-related traits (sucrose content, glucose content, fructose content, sugar content, sucrose yield, sucrose to sugar ratio and Brix) and four agronomic traits (height, DTF, total dry matter and grain yield). Fifty-five marker were associated with the 11 traits from the two field trials, of which 15 were identified in both trials which is not unexpected considering the high heritability of the traits in the field trials. QTL generally colocated to five major locations. QTL from R9188 were found for sucrose content, sugar content and sugar yield on chromosomes SBI-01, SBI-05 and LG-U6. R9188 also contributed QTL for Brix on SBI-05 and LG-U6, and sugar content on SBI-03. QTL from R403463-2-1 were found for sucrose content and sucrose yield on SBI-10, and glucose content on SBI-07. QTL for height, days to flowering and total dry matter, were located on SBI-01 from R403463-2-1, and on LG-U6 from R9188. QTL for grain yield from both R403463-2-1 and R9188 were found on SBI-03. Three of the sorghum chromosomes (SBI-01, SBI-03 and SBI-05) that contained QTL for sugar-related traits, also contained sugarcane SSRs that mapped in sugarcane to three homology groups (2, 3 and 4) with strong QTL for sugar-related traits suggesting that similar loci for sugar-related traits are being detected between sweet sorghum and sugarcane. Field and glasshouse trials were conducted to investigate the accumulation of sugars spatially within the stem and developmentally at anthesis and post-black layer in sweet sorghums. A stem sugar profile in sweet sorghums had not previously been created at post-black layer and it was found that in the glasshouse trial, the sucrose increased toward the base of the stem, in the same manner of accumulation seen in sugarcane. Like sugarcane, the sucrose in the sweet sorghums accounted for 90% of the soluble solids (sucrose to total ratio) at the end of the crop cycle. A comparison of profiles was also conducted between a sweet and grain sorghum, fertile sorghum and their sterile counterpart, sweet sorghum and the dwarf conversion, and high Brix and low Brix progeny of the R9188 x R403463-2-1 population. At post-black layer, the sweet sorghum had higher sucrose than the grain sorghum, and higher sugars were found in the sterile versions compared to the fertile versions. Sugar accumulation had not been previously investigated in dwarf converted sweet sorghums; R9188, the dwarf version of Rio, was significantly lower in whole-plant sucrose at post-black layer, but no difference was noted in individual stem sections. No clear distinction could be made between the high Brix lines and the low Brix lines, sampled at post-black layer, for sugar-related traits at a whole-plant or stem section level. The genetic relationship of sweet and grain sorghums, within S. bicolor ssp. bicolor, was investigated for the first time. Ninety-five genotypes, including 31 sweet sorghums and 64 grain sorghums, representing all five races within the subspecies bicolor, were screened with 277 polymorphic AFLP™ markers. The sweet sorghum lines were largely distinguished from the others, particularly by a group of markers located on SBI-08 and SBI-10, and although the clusters obtained did not group clearly on the basis of racial classification; the sweet sorghum lines often clustered with grain sorghums of similar racial origin thus suggesting that the high stem sugar in sweet sorghums occurred independently or was independently selected in several different genetic backgrounds within S. bicolor ssp. bicolor. In this study, great steps have been made in determining the use of sweet sorghum as a genetic and physiological model for sugar accumulation in sugarcane. From a genetic perspective, similar loci for sugar-related traits were detected between sweet sorghum and sugarcane, thus indicating that sweet sorghum is likely to be a good genetic model for sugar accumulation in sugarcane. In sweet sorghum, the markers associated with the sugar-related traits will be a valuable resource for future markerassisted selection for these traits in a sweet sorghum breeding program. However, the diversity study suggested that the high stem sugar in sweet sorghum occurred independently or was independently selected in several different genetic backgrounds, which is in contrast to the possible monophyletic origin of high stem sugar in sugarcane. From a physiological perspective, the sucrose levels increased toward the base of the stem in sweet sorghum and accounted for 90% of the soluble solids at the end of the crop cycle, in the same manner of accumulation seen in sugarcane, suggesting that sweet sorghum could be a physiological model for sugarcane. Thus, sweet sorghum could be used as both a genetic and physiological model for sugarcane.

 
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