Glutamine Synthetases GLN1;2 and GLN2 in Relation to Arabidopsis Growth Response to Elevated Atmospheric Carbon Dioxide and Varying Nitrogen Forms

Research output: Book/ReportPh.D. thesisResearch

  • Swathi Vurrakula
Carbon and nitrogen are the most abundant elements in plants, together making up around 40-50% and 2-6% of dry matter respectively. Elevated atmospheric CO2 levels are predicted to double by the end of this century, increasing carbon fixation by C3 plants like Arabidopsis and, hence, their carbon content while diluting nitrogen concentrations. Such a reduction in nitrogen concentration will affect plant response to stress and seed/grain yield. Glutamine synthetase (GS) is the central nitrogen-assimilatory enzyme, performing primary and secondary nitrogen assimilation, in response to environmental cues and adjusting it to the plant internal status. The two major types of GS include cytosolic GS1 (five isoforms in Arabidopsis, GLN1;1 to GLN1;5) and a single chloroplastic GS2. GS draws its substrates from carbon skeletons to synthesize amino acids. Thus, carbon and nitrogen metabolisms are closely connected and interdependent. Despite many years of research, gaps exist in our understanding of these interactions. This study aims to further our knowledge in relation to the specific roles of GLN1;2 (major GS1 isoform in Arabidopsis shoots) and GLN2 isoforms in response to elevated CO2 and varying nitrogen species (ammonium and nitrate) and availability.
Arabidopsis gln1;2 knock-out and gln2 (reduced GS2 activity by 50%) mutants were studied in response to elevated CO2 (800 ppm) when provided with moderate and high levels of ammonium or nitrate alone, or in combination. Elevated CO2 promoted plant growth, furthermore with an increasing nitrate supply. Elevated CO2 made plants ammonium-sensitive. This happened to a greater extent when supplied with ammonium in solo. Reduced GS activity (both GS1 and GS2) lowered plant nitrogen assimilation capacity in response to excess ammonium as well as CO2.
Plants grown under elevated CO2 absorbed ammonia from the atmosphere, except with a high ammonium supply.
GLN1;2 had a non-redundant role in determining vegetative growth and ammonium tolerance in response to elevated CO2. Under elevated CO2, GLN1;2 was compensable by GLN2 in assimilating nitrate but not ammonium. Reduced GS1 activity correlated with increased ammonia emissions from leaf surface, markedly so with an increased supply of both ammonium and CO2. GLN1;2 was also found to play a vital role in assimilating high levels of nitrate.
Under current CO2 levels (400ppm) GLN2 had a non-redundant role in nitrate assimilation. This was the case as the gln2 mutant biomass was highly reduced with an increased supply of nitrate in solo. Nitrate assimilation was impaired in the mutant in parallel with reduced NR activity and activation state (%), leading to reduced glutamine content while glycine accumulated. When both ammonium and nitrate were provided, the mutant preferably assimilated ammonium as evidenced by decreasing tissue levels of ammonium while tissue nitrate increased. The mutant responded similarly to the Wt under elevated CO2 and played no role in determining ammonia emissions from plants.
In conclusion, our study adds new insights into the individual roles of GLN1;2 and GLN2 in relation to growth responses to elevated CO2 and deciphers their specific redundant and non-redundant roles in response to differing nitrogen species availability. Major gaps still exist in our understanding of the complicated interactions between nitrogen and carbon, pointing towards the need for a deeper understanding of the same.
Original languageEnglish
PublisherDepartment of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen
Number of pages165
Publication statusPublished - 2015

ID: 141361591