A key enzyme for nitrogen assimilation is found to be essential for chloroplast development in roots

February 17, 2020

Subtle functional differences of rice glutamine synthetase isozymes are revealed

A research group of the University of Tsukuba, Tohoku University, RIKEN CSRS, JIRCAS and Gifu University found that OsGS1;1, an isozyme of cytoplasmic localized glutamine synthetase (OsGS1) that is essential for nitrogen assimilation in rice, plays a significant role in chloroplast development in roots where photosynthesis does not normally occur. Nitrogen is one of the three macronutrients of fertilizers and one of the materials required to form chlorophyll and amino acids, which are essential for plant survival. Nitrogen incorporated into plants is converted into ammonium, which is then assimilated by glutamine synthetase (GS) into glutamine, a type of amino acid. Plants have multiple genes encoding cytoplasmic localized GS1, although the reason they have many GS1 isozymes remains unclear.

In the present study, the research group focused on OsGS1;1 and OsGS1;2 expressed in the early stage of development, which is the appropriate timing for effective nitrogen fertilizing among the GS1 isozymes of rice, one of the essential crops for food. An analysis of rice mutants with either of the genes disrupted revealed that the roots of the Osgs1;1 mutant lost the balance of accumulation of sugars and amino acids in central metabolism, whereas the roots of the Osgs1;2 mutant exhibited only a decrease in the levels of amino acids. Further, the group found that in the Osgs1;1 mutant, chloroplast was synthesized in the roots where photosynthesis does not normally occur. These results indicate that OsGS1;1 associates with a broad range of events including the maintenance of metabolic homeostasis of carbon and nitrogen and chloroplast development, whereas OsGS1;2 has a more specific role in the regulation of amino acid biosynthesis during metabolism.

This study suggests that the regulation of nitrogen assimilation and carbon and nitrogen metabolisms that have been considered unrelated to chloroplast development may lead to the development of new approaches that will provide roots with a photosynthetic ability.

Original article
Plant Physiology doi:10.1104/pp.19.01118
M. Kusano, A. Fukushima, M. Tabuchi-Kobayashi, K. Funayama, S. Kojima, K. Maruyama, Y. Y. Yamamoto, T. Nishizawa, M. Kobayashi, M. Wakazaki, M. Sato, K. Toyooka, K. Osanai-Kondo, Y. Utsumi, M. Seki, C. Fukai, K. Saito, T. Yamaya,
"Cytosolic GLUTAMINE SYNTHETASE 1;1 modulates metabolism and chloroplast development in roots".
Miyako Kusano; Senior Visiting Scientist
Kazuki Saito; Group Director
Metabolomics Research Group