sn-Glycerol-3-phosphate acyltransferase (GPAT) catalyzes the acylation of sn-1 position of sn-glycerol-3-phosphate to produce lysophosphatidic acid and Coenzyme A. GPATs are involved in several lipid synthetic pathways and play important physiological roles in plant development. The present doctoral thesis includes three related studies, which aim to molecularly and functionally characterize several plant GPAT genes and the encoded enzymes. The first study characterized three endoplasmic reticulum-bound GPAT4s encoded by three homologous GPAT4 genes of Brassica napus (oilseed rape), focusing primarily on their functional divergence and physiological roles in plant development and lipid biosynthesis. The three homologous GPAT4 genes exhibited different expression patterns and altered epigenetic features. Phenotypic rescue of a gpat4 gpat8 Arabidopsis double mutant and analysis of the gpat4 RNAi B. napus lines suggested physiological roles for the GPAT4s in cuticle formation of the rosette leaves, early flower development, pollen development and storage lipid biosynthesis. The second study investigated stable internal reference genes for gene expression studies in B. napus. This project identified four reliable reference genes to be used in gene expression analysis of BnGPAT4 homologues in both vegetative tissues and developing seeds. The third study focused on molecular cloning and biochemical characterization of a soluble plastidial GPAT isolated from a chilling-tolerant plant, western wallflower (Erysimum asperum). A truncated form of recombinant EaGPAT, with the putative transit peptide deleted, was functionally expressed in yeast. A series of enzymatic assays were performed in order to determine the optimum in vitro reaction conditions for the recombinant EaGPAT. The recombinant EaGPAT was further assayed with different acyl-CoAs and exhibited a substrate preference for 18 carbon unsaturated acyl-CoAs. With this substrate preference, the EaGPAT could potentially be used as a biotechnological tool for improving plant chilling-tolerance or increasing unsaturated fatty acid content of seed oil. Overall, the present doctoral studies revealed the functional divergence and important physiological roles of the GPAT4s in B. napus, and biochemically characterized a plastidial GPAT from E. asperum. The knowledge obtained from these studies provides new insights into the role of GPAT in plants and will be useful for further development of biotechnological approaches to modify seed oil biosynthesis in oleaginous crops. / Plant Science

No description available.

Master of Science / Department of Agronomy / S.V. Krishna Jagadish / Low temperature is one of the most limiting stresses to crops that are adapted to tropical and subtropical regions, such as maize (Zea mays L.) and sorghum [Sorghum bicolor (L.) Moench], when introduced into temperate regions. However, no studies have compared the chilling tolerance of maize and sorghum grown together. Therefore, the objective of this research was to screen maize hybrids and sorghum genotypes for chilling tolerance at the germination and seedling stages. With the hypothesis that grain composition of maize and sorghum could lead to varying chilling tolerance, the seeds were analyzed for concentrations of protein, starch, and amylose. Five commercial hybrids of maize and 18 genotypes of sorghum were maintained in growth chambers for 31 days at two temperatures: a control temperature (25/20 °C, day/night) and at chilling temperatures (11/8 °C for 14 days; 12.5/9.5 °C for 14 days, and 14/11 °C for 3 days). Emergence and seedling height were measured during the experiment. At the end of the experiment, shoot dry weight, root dry weight, and leaf area were determined. Emergence of sorghum under the chilling temperature regime was low (18%). Average height of the emerged sorghum seedlings in the cold temperatures at the end of the experiment was 1.4 cm compared to 55.5 cm in the control treatment. All maize hybrids emerged, but emergence and growth were slowed by the cold temperatures, and average height at the end of the experiment was 4.6 cm compared to 96.1 cm in the control treatment. Shoot dry weight, root dry weight, and leaf area of the sorghum under the chilling temperatures were too small to measure, and, for maize, they were greatly reduced. The results showed that, for sorghum, temperatures should be above 14 °C for emergence, while maize could emerge at lower temperatures. The analyses of the sorghum seeds showed that Redbine 60 and RTx430 had the highest protein concentrations (15.71% and 15.35%, respectively), and Segaolane had the lowest protein concentration (9.83%). Segaolane had the highest starch concentration (72.71%), and RTx430 had the lowest starch concentration (65.31%). There was an inverse relationship between protein and starch concentrations in the sorghum seeds (R2 = 0.69). Amylose concentrations did not vary significantly among the sorghum seeds. The analyses of the maize seeds showed that Dekalb 51-20 and Pioneer 1151 had the highest protein concentrations (10.98% and 10.95%, respectively), and Pioneer 1105 had the lowest protein concentration (9.26%). Starch and amylose concentrations did not vary significantly among the maize seeds.

Forage sorghum (FS) [Sorghum bicolor (L.) Moench] is a warm-season biomass crop with the potential to become a bioenergy feedstock. The objective of this study was to screen potential FS genotypes for increased chilling tolerance and biomass productivity. The experiments were conducted in Fargo and Hickson, ND, in 2017 and 2018. Seventy-two genotypes of FS were tested at 24, 12, and 10℃. The genotypes were ranked from high to low vigor index and 12 genotypes were planted on two seeding dates: early (10 May) and late (27 May). Field emergence index values were greater for the late-seeding compared with the early-seeding date. Stand establishment and seed mortality were affected by the seeding date. Biomass yield correlated with emergence index and normalized vegetative index. Some of the genotypes tested had increased chilling tolerance and biomass yield when seeded earlier than normal, and may be used for breeding chilling tolerance into FS.

Thesis (Ph.D. (Botany))--North-West University, Potchefstroom Campus, 2009.

Thesis (Ph.D. (Botany))--North-West University, Potchefstroom Campus, 2009.

Doctor of Philosophy / Department of Agronomy / S.V. Krishna Jagadish / Sorghum (Sorghum bicolor (L.) Moench) is one of the hardiest crop to abiotic stresses compared with other grain crops. However early stage chilling, terminal heat and drought stress are three most damaging abiotic stresses that have limited sorghum productivity in the US Great plains and other locations having similar environmental conditions. Three studies were conducted with an overall goal aimed at increasing grain sorghum’s resilience to harsh climatic conditions. In the first study, four promising chilling stress tolerant sorghum advanced breeding lines, a known early stage chilling tolerant Chinese landrace (Shan Qui Red - SQR) and a susceptible US elite cultivar (RTx430) as checks were assessed for chilling tolerance during emergence and early growth under field and controlled environments. Aerial phenotyping using unmanned aircraft systems (UAS) fitted with multispectral camera was used to capture reflectance-based vegetation indices (NDVI and NDRE) in field experiments. Some advanced breeding lines with superior agronomic background also recorded significantly better emergence, seedling growth and vigor compared to SQR under chilling conditions. Aerial phenotyping indices from images taken between 30 and 60 days after emergence were consistently correlated with destructive measurements under early plantings, indicating their effectiveness in differentiating chilling responses. Second study was conducted to understand physiological mechanisms inducing heat stress resilience in sorghum during flowering. A diverse set of sorghum inbreds and selected hybrids were tested under greenhouse, growth chamber facilities and field conditions. A highly conserved early-morning-flowering mechanism was observed across all the inbreds and hybrids, with the peak anthesis wherein >90% of florets completed flowering within 30 min after dawn. The conserved response was consistent even under drought stress and heat stress exposure imposed at different times of the day. Our findings report a novel heat escaping early-morning-flowering mechanism effectively employed by sorghum to minimize heat stress impact at anthesis. Another experiment with sequential increase in daytime temperature treatments suggest heat stress induced loss in pollen viability to be a key factor resulting in reduced seed-set and grain yield. The findings suggest heat stress could have a greater impact on post-pollen germination processes such as fertilization, embryo formation and development. We identified a heat tolerant genotype “Macia” which appears to be a promising donor for developing improved heat tolerant sorghum hybrids. In the third study, a bi-parental recombinant inbred lines (RILs) mapping population developed from elite post flowering drought susceptible cultivar (RTx430) and a known drought tolerant cultivar (SC35) were evaluated under wide spectrum of environments and moisture conditions. Several novel and major QTL for grain yield, panicle neck diameter, effective quantum yield of photosystem II and chlorophyll content were identified. The genomic regions and the candidate genes within these regions can potentially help in improving source and sink dynamics in sorghum under diverse environments. The findings from these studies will complement ongoing efforts in developing future sorghum with enhanced resilience to different abiotic stresses that continue to limit sorghum productivity.