The objective of this thesis was to study the response of Eutrema salsugineum (Yukon) plants to low phosphate (Pi) using seedlings and four- week-old plants grown on media formulated with variable phosphate (Pi). Seedlings showed similar root architecture whether grown with high Pi or without added Pi. Four-week-old plants grown with 0 or 2.5 mM Pi added to the soil had the same shoot biomass and relative growth rates. Confirmation that plants on low Pi were Pi-deficient despite lacking a Pi-starvation phenotype was provided by the increased expression of Pi-starvation-inducible genes (notably EsIPS2) in Pi-deprived plants. We also found that seedling roots on media lacking Pi did not acidify their rhizosphere nor did they show increased phosphatase secretion or phosphatase activity relative to roots of Pi-sufficient seedlings. In soil-grown plants, leaf P remobilization was slower during dark-induced senescence of Eutrema relative to similarly treated, Pi-starved Arabidopsis. Also related to metabolism, in vitro assays showed that the ratio of maximal PPi- and ATP- dependent phosphofructokinase activities approximated 1:1 and 2:1 for Eutrema leaf and root extracts, respectively, with no Pi-responsive changes found and, relative to Arabidopsis, Eutrema phosphoenolpyruvate carboxylase activities were high. The enzyme activities suggest Eutrema operates glycolytic by-passes under Pi sufficient and deficient conditions. Finally, transcripts for the transcription factors Phosphate Starvation Response 1 (PHR1) and WRKY75 were not Pi-starvation-inducible and were more abundant in Eutrema leaves than in leaves of Pi-deprived Arabidopsis. Global gene expression showed the leaf and root transcriptomes to be about 90% similar between 0 and 2.5 mM Pi- treated plants with 2,901 differentially expressed genes detected by DESeq2. In summary, Eutrema displays few Pi-starvation responsive traits whether those traits reflect changes at the level of gene expression or plant morphology, behaviour consistent with a specialist that is continuously primed for Pi starvation. / Thesis / Doctor of Philosophy (PhD)

<p> Abiotic stresses such as extreme temperatures, drought and high salinity severely compromise plant productivity, and have placed selective pressure for the acquisition of traits enabling plants to adjust to and recover from these unfavorable environmental conditions. Thellungiella salsuginea is a plant that is native to highly saline and semiarid environments and exhibits an exceptional ability to tolerate abiotic stress. In this thesis, I report on laboratory and field studies aimed at identifying traits that allow Thellungiella to tolerate harsh environmental conditions. It was found that Thellungiella accumulates organic solutes in response to abiotic stress. Transcript and metabolite profiling approaches were used to identify metabolic pathways important for the accumulation of compatible organic solutes in Thellungiella in response to sub-optimal environmental conditions. The relative abundance of transcripts encoding enzymes associated with the biosynthesis of compatible organic solutes such as proline or galactinol showed stress-responsive increases in cabinet-grown material and these metabolites were accumulated in salt or drought treated plants, respectively. However, proline and galactinol were found to be of low relative abundance in leaves of field plants. In contrast, several carbohydrates including sucrose, glucose, and fructose made a greater relative contribution to the field plant profiles suggesting that carbohydrates play an important role in plant abiotic stress tolerance during growth under field conditions. The identification of stress-specific metabolic changes can be used to identify important biochemical traits underlying environmental stress tolerance in Thellungiella. This information can be used to improve the tolerance of stress -sensitive crops (including a related crucifer species, canola) that are grown in areas where persistent droughts, saline soils and early or late frosts frequently occur. </p> / Thesis / Doctor of Philosophy (PhD)

<p><em>Eutrema salsugineum</em> is an extremophile plant native to the Yukon Territory and coastal China. As an extremophile, Yukon <em>Eutrema</em> is tolerant to highly saline, drought conditions and cold temperatures while Shandong <em>Eutrema</em> can survive in highly saline conditions (Griffith et al., 2007; Guevara et al., 2012; Inan et al., 2004). The disease resistance responses of the Yukon and Shandong accessions of <em>Eutrema</em> were investigated to understand how an abiotic stress-tolerant plant responds to biotic stress. A pathosystem was developed using <em>Pseudomonas</em> <em>syringae</em> pv. <em>tomato</em> DC3000 (<em>Pst</em>) to examine <em>Eutrema</em> defense responses. Compared to <em>Arabidopsis </em>(Col-0), both <em>Eutrema</em> accessions exhibited resistance to <em>Pst,</em> with Shandong <em>Eutrema</em> displaying greater resistance than Yukon <em>Eutrema</em>. Resistance to <em>P. syringae</em> pv. <em>maculicola</em> (<em>Psm</em>) was also observed in both accessions. Furthermore, both <em>Eutrema</em> accessions displayed a differential capacity for effector-triggered immunity (ETI). RNA-Seq data of uninoculated Shandong vs. Yukon <em>Eutrema</em> revealed an overrepresentation of defense genes including <em>PR1</em> (<em>pathogenesis-related1</em>; Champigny et al., 2013). Expression of the <em>Eutrema</em> <em>PR1</em> ortholog in uninoculated Shandong leaves combined with enhanced resistance to <em>Pst</em> compared to Yukon <em>Eutrema</em> or Col-0 <em>Arabidopsis</em> suggests that Shandong plants exist in a defense-primed state. Resistance to other pathogens such as <em>Pectobacterium</em> <em>carotovorum</em> ssp. <em>wasabiae</em> (<em>Pcw</em>) further supported the hypothesis that Shandong <em>Eutrema</em> is primed for pathogen tolerance. The <em>Eutrema</em>-<em>P. syringae</em> pathosystem will facilitate future studies to understand how <em>Eutrema</em> deals with multiple or concurrent stresses and this knowledge will contribute to efforts to improve tolerance to both abiotic and biotic stress in crop plants.</p> / Master of Science (MSc)