Arabidopsis thaliana’da Kükürt Teşvikli Bor Toleransının İncelemesi
Özet
Boron (B) is a micronutrient element that is essential for plant growth and development and its high concentrations have highly toxic effect. Within the scope of this thesis, the acquirement of tolerance to boron toxicity by sulphate applications in the model plant Arabidopsis thaliana; 1) some physiological, photochemical, and antioxidant enzyme activities, and 2) the expression changes of genes associated with transporters that play a role in boron uptake and sulfate uptake/excretion were aimed to investigate.
After the Arabidopsis seeds were subjected to stratification in petri dishes containing nutrient medium (MS) for 3 days at +4°C in dark, then the germinated plants were grown in a controlled growth cabinet (22± 1 °C temperature, 16/8 h photoperiod, 200 µmol.m-2s-1 light intensity and 50-60 % humidity) for 12 days. And then, sulphate pre-treatment groups (PS-B) were transferred to MgSO4- type sulphate-containing media for 3 days, and at the end of 3 days, combined (S+B) and un-treated application groups were transferred to petri dishes containing boron with different concentrations (3 and 5 mM H3BO3) to be exposed to boron toxicity for 10 days.
Plant biomass and leaf surface area decreased significantly depending on the accumulation of boron which was uptake from the root and transported to the leaves in the Arabidopsis plant. In addition, it was determined that the amount of SO4- in the leaves of the plants decreased under toxic boron conditions. Toxic levels of boron transported to the leaves adversely affected the photosynthetic apparatus and photochemical activity. Boron toxicity caused the changes in chlorophyll a fluorescence transients (OJIP curve) in thylakoid membranes of plants, specific and phenomenological energy fluxes, quantum yields, and the efficiencies of PSII's donor and PSI's acceptor sides. These changes have led to decreases in the photosynthetic performance of plants. It has been proved that the decrease in photochemical efficiency of plants with toxic boron application is associated with the thylakoid dissociation, increase in the non-QA-reducing reaction center (inactive RC), and dissipation of energy in the form of heat. In addition, B toxicity disrupted the cooperation between both photosystems, and it was determined that PSII was affected more than PSI. Although boron toxicity in Arabidopsis adversely affects the integrity of the membrane and the amount of photosynthetic pigment in the antenna and reaction centers, it has been determined that this effect is not at a level to completely block the functionality of the photosystems, and this negative effect can be alleviated by sulphate applications.
In the Arabidopsis plant, the increase in the antioxidant enzyme activities and the content of phenolic compounds (anthocyanin and flavonoid) with sulphate applications provided tolerance against boron toxicity by regulating the defense systems. Increases in peroxidase and glutathione-S-transferase activities at toxic boron levels reveal that they play an active role in protecting the plant against oxidative damage. The activities of the investigated enzymes and the accumulation of phenolic compounds were found to be higher in sulfate pre-treatment groups.
Relative expression changes of genes that synthesize some metabolites involved in sulphate uptake and transport and sulfur metabolism were investigated. Against boron toxicity, with sulfate pre-application, the 3rd group sulfate carriers, especially SULTR 3;1, SULTR 3;3, SULTR 3,5 genes, were more expressed. It is thought that the increase in the expression of the SULTR3 and SULTR4 group genes may have contributed to the sulfur assimilation by taking the sulfate from the vacuole to the cytosol in the pre-treatment groups with the SULTR3 transporters localized on the chloroplast. Glutamate cysteine ligase and GSH synthase enzymes, which catalyze the first and last step of the glutathione synthesis pathway respectively, are encoded by the GSH1, and GSH2 genes, respectively, and the relative expression increases of these genes in the treatment groups compared to the control indicate that B toxicity induced glutathione biosynthesis.
Plants may have acquired tolerance against boron toxicity by either assimilating the sulphate taken by sulphate applications (especially sulphate pre-treatment) in sulphate metabolism or by storing them in their vacuoles.
