The specificity of the metabolic pathway of photosynthesis in conifers was revealed

Low assimilation efficiency of photorespiratory ammonia in conifer leaves

Shin-Ichi Miyazawa (a), Mitsuru Nishiguchi (a), Norihiro Futamura (a), Tomohisa Yukawa (b), Mitsue Miyao (c), Tsuyoshi Maruyama (d), Takayuki Kawahara (e)

(a) Department of Forest Molecular Genetics and Biotechnology, FFPRI, Tsukuba, Ibaraki, Japan.
(b) National Museum of Nature and Science, Tsukuba, Ibaraki, Japan.

(c) Tohoku University, Sendai, Miyagi, Japan.

(d) Research Planning and Coordination Department, FFPRI, Tsukuba, Ibaraki, Japan.

(e) Hokkaido Research Center, FFPRI, Sapporo, Hokkaido, Japan.

Journal of Plant Research, Springer, July 2018, DOI 10.1007/s10265-018-1049-2( External link )

Plants absorb carbon dioxide and then assimilate it by photosynthesis ^{(Note 1)}.

Photosynthesis comprises a series of complex reactions, including the metabolic process known as “photorespiration,” which produces carbon dioxide and ammonia.

In the established theory, ammonia generated during photorespiration is immediately assimilated through catalysis by glutamine synthetase (GS2) in chloroplasts ^{(Note 2)} into an amino acid called glutamine, which is then available for reuse as a nitrogen source. However, it has been reported that GS2 does not exist in Scots pine, a species of conifer. It is unclear whether the lack of GS2 is universal in conifer trees and how the physiological properties of plants lacking GS2 differ from those of plants with GS2.

Therefore, we tested for the presence or absence of GS2 in various conifer leaves, finding only the cytosolic glutamine synthetase isoform (GS1); GS2 was not detected. Furthermore, we found that the leaves of conifers such as Japanese cedar (Cryptomeria japonica) and Japanese red pine (Pinus densiflora) have very low assimilation efficiency of photorespiratory ammonia in comparison to crops and leaves of broadleaf trees. Low assimilation efficiency of photorespiratory ammonia in conifer leaves is thought to be associated with the absence of GS2.

GS2 is normally one of more than 20 enzymes involved in photosynthesis, yet it appears to be absent in conifers. The mechanism of photosynthesis has been thought to differ minimally between broadleaf trees and conifers; however, future studies must focus on re-examining this mechanism in conifers.

^{Note 1)} Assimilation: The reaction in which inorganic substances such as carbon dioxide and ammonia are converted into organic substances such as sugars and amino acids.

^{Note 2)} Chloroplast: An intracellular organelle containing the enzymes necessary for assimilation.

Fig. (Top): Schematic showing the difference between photosynthesis in broadleaf tress and conifers. CO₂: carbon dioxide, NH3: ammonia.
(Bottom): Detection of GS1 and GS2 by antibodies. Only GS1, not GS2, is detected in Japanese cedar (right-hand column).

This figure has been modified from Figure 2 in “The mystery of photosynthesis of conifer that has been revealed from ammonia research.” published in the Quarterly Forest Research No. 41, page 7.