Genetic characterization of flowering time in sorghum

Bangbol Sangma, Harriet (2013). Genetic characterization of flowering time in sorghum PhD Thesis, School of Agriculture and Food Sciences, The University of Queensland.

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Author Bangbol Sangma, Harriet
Thesis Title Genetic characterization of flowering time in sorghum
School, Centre or Institute School of Agriculture and Food Sciences
Institution The University of Queensland
Publication date 2013
Thesis type PhD Thesis
Supervisor Ian Godwin
David Jordan
Emma Mace
Erik Van Oosterom
Total pages 158
Language eng
Subjects 0607 Plant Biology
0604 Genetics
Formatted abstract
Sorghum is an important cereal crop that is cultivated in a wide range of geographical locations. Phenology, and in particular time to flowering, is one of the most important traits that determine crop adaptation to the growing conditions. Characterization of genetic factors that control flowering time would be advantageous for the development of germplasm that are better suited to current environmental conditions and are able to cope with any impacts of climate change on crop development. In sorghum, time to flowering is affected by both temperature and photoperiod. Flowering is under a complex genetic control, and although many quantitative trait loci (QTL) for flowering have been identified in sorghum, few studies have dissected this genetic control into QTL that affect either the response to temperature or photoperiod. In this study the genetic control of flowering time as functions of temperature and photoperiod is characterized. This is done in order enhance the genetic information required to optimize sorghum flowering for adaptation. I then continued to study its genetic response to temperature, in particular, to infer the extent of involvement of an identified genetic factor in this response. First, association and composite interval mapping strategies were employed in two previously developed sorghum bi-parental mapping populations (IS8525 x R931945-2-2 and R931945-2-2 x S. bicolor subsp. verticilliflorum), grown across a range of temperature and photoperiod conditions, to identify phenology QTL. A total of 43 QTL were found, including three novel ones, and, based on experimental results function (temperature vs. photoperiod) could be assigned to nineteen of these. Results indicated that temperature and photoperiod control of flowering are relatively independent of each other, suggesting that selection for earliness per se can be done without major effects on photoperiod sensitivity. Nonetheless, some of the identified QTL affected the response to both photoperiod and temperature. Furthermore, I looked at candidate genes co-locating the identified QTL. Putative candidate genes found co-locating QTL that were assigned the photoperiod and temperature functions were counter-intuitive. An interesting example of this was the finding that sorghum SUPRESOR OF OVEREXPRESSION OF CONSTANS 1 (SbSOC1) gene, the putative ortholog of the Arabidopsis photoperiod pathway gene SOC1, co-located with a QTL that was assigned the temperature function. To further investigate the putative role of SbSOC1 in the control of the temperature response, IS8525 and R931945-2-2, the two parents of the mapping population in which SbSOC1 was identified, were grown across four temperature treatments and expression levels of SbSOC1 were measured at two early stages of development. Results showed that expression levels declined rapidly between two and four weeks after emergence. In addition, the early flowering genotype had higher expression levels than the late flowering parent and temperature affected SbSOC1 expression independent of photoperiod. Hence, SbSOC1 plays a role in the temperature response, in addition to its well-known role in the photoperiod pathway, and promotes progress to flowering. In order to infer the extent of the role of SbSOC1 in genetic variability which could help sorghum improvement, the next step was to evaluate its sequence variation across 47 sorghum genotypes, including IS8525 and R931945-2-2, the two parents of the mapping population used in the gene expression analysis. Results suggested a single non-synonymous single nucleotide polymorphism (SNP) on exon 6 of SbSOC1 could be a putative causal polymorphism. Phylogenetic analysis further showed the association between sorghum genotypes containing non-synonymous SNP and genotypes with late flowering alleles. Finally, the finding of signatures of balancing selection at the SNPs in SbSOC1 suggested that the gene diversity is maintained after domestication, most likely to accommodate the requirement for adaptation to new environments. This provides future opportunities of research for its use in designing germplasm that can counteract negative effects of climate change. This study identified QTL and putative candidate genes related to temperature and photoperiod which could be used in molecular breeding strategy to optimize flowering time. I identified a novel role of SbSOC1 as a promoter of flowering associated with temperature response and found a putative SNP in the gene, which once validated could be used to efficiently predict and identify lines suited for different types of environments on the basis of sequence information. 
Keyword Sorghum bicolor
Flowering time
Temperature and photoperiod effects
QTL mapping
Candidate genes identification
SbSOC1 gene expression
Sequence diversity
SNP identification
Phylogenetic Analysis
Sorghum adaptation

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Created: Tue, 08 Apr 2014, 09:47:49 EST by Harriet Bangbol Sangma on behalf of Scholarly Communication and Digitisation Service