Characterization of Stem Growth and Chemical Composition in Sorghum Bicolor

Sorghum bicolor is a subtropical grass grown throughout the world for human consumption, animal feed and for the growing biofuels industry. In this thesis I characterize sorghum stem growth and chemical composition, and identify QTL and candidate genes which may regulate stem development. In addition, I attempt to correlate variation in stem composition with saccharification efficiency in two sorghum populations. Under greenhouse conditions, stem length in the vegetative phase typically accelerated starting with the 10th internode, possibly reaching a maximum elongation rate by the time the 20th internode stopped growing. The rate of stem diameter growth increased steadily and achieved a maximum diameter growth rate by the time the 11th internode stopped growing. Under these growth conditions, the plateau for the internode diameter growth rate occurred at 60 days after seedling emergence.

Variation in internode growth was driven primarily by cell division, though differences in cell size began to play a significant role as cell number increased. Internode growth was positively correlated with GRAS, bHLH and B3 transcription factor expression and was negatively correlated with AP2/EREBP, MYB and WRKY transcription factors. In addition, the outer rind of the internode had gene expression that resembled expression in young tissue while the central tissue had gene expression that resembled expression in mature tissue. Sorghum accessions displayed a wide range of internode chemical compositions and saccharification efficiencies. However, no clear patterns were present between variation in composition from the NIR spectra and variation in saccharification efficiency on the iWALL system. Multiple linear regression analysis, however, revealed that high biomass density appeared to inhibit saccharification in these populations.

Overall, sorghum stem growth mirrors that seen in other monocots. The gene expression information presented here should be useful for future studies on the roles of various transcription factors in plant development and for the identification of transcription factor binding sites within the genome. Such information will be important for future success in molecular breeding and marker assisted selection. This information will also be invaluable for designing genotypes with novel transcripts that can be activated at specific times, in specific organs and under specific conditions.