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)

Abiotic and biotic stresses lead to crop yield losses and ultimately negatively affect agriculture production. Elucidation of the mechanisms of plant stress responses and their regulation could help to understand plant defence and improve stress tolerance. Phytohormones stand behind both plant growth and developmental changes as well as stress signalling. This thesis summarises the results published in two articles focused on phytohormone dynamics in response to abiotic and biotic stresses, namely phosphate shortage and Plasmodiophora brassicae infection. A review article focuses in depth on strigolactones as master regulators of phosphate deficiency responses. The main emphasis is put on organ-specific reactions and exogenous phytohormone treatment with the potential to convey stress tolerance. In the case of phosphate shortage, the universal reaction in all organs was a decrease in active cytokinin trans-zeatin and gibberellin GA4 with a concurrent elevation of abscisic acid. Also, the high- affinity phosphate transporters (PHT1;4 and PHT1;7) exhibited increased gene expression within the whole plant. Shoot apical meristems showed numerous changes in gene expression and were the most affected organ during the lack of phosphate. Only in roots, we observed a substantial elevation of low active...

Masters of Science / In this study, we observed that phosphate (P) deficiency stunted plant growth and produced plants with poor morphological characteristics (yellow and small leaves). Furthermore, we treated plants with 0.8 mM (control) and 0.02 mM P (deficient) in addition to 6 μM methylglyoxal (MG) and we observed that the plants treated with MG had a higher germination, and better morphological characteristics (the leaves were more dark green and bigger in size) compared to the P deficient plants. However, we also observed that the P deficient plants treated with MG had low levels of both O2- and H2O2 and this could be a possible reason for the improved growth and morphological characteristics. In contrast, the P deficient plants not treated with MG had high levels of O2- and H2O2 which could be the possible reason for the observed cell death. We also performed biochemical assays including superoxide dismutase, ascorbate peroxidase, malondialdehyde content, ascorbic acid content, catalase, and most of the assays showed high levels of reactive oxygen species (ROS) and low levels of antioxidant activities in plants not treated with MG while high levels of antioxidant activities and low levels of ROS were observed in plants treated with exogenous MG. Since nitric oxide (NO) is also known to be a signalling molecule, we did a NO assay and observed that NO content increased under low exogenous doses of MG. From our findings we came to a hypothesis that MG modulates P deficiency stress in P. vulgaris through NO signalling or it might be that NO and MG work in tandem to modulate signalling pathways under P deficiency. Finally, we looked at the nutrient profile and the results showed that while there was a poor nutrient profile generally under P deficiency, there was an improvement in nutrient profile when MG was administered at low doses.