Miyazawa Shin-Ichi, Nishiguchi Mitsuru, Futamura Norihiro, Yukawa Tomohisa, Miyao Mitsue, Maruyama Tsuyoshi Emilio, Kawahara Takayuki
Department of Forest Molecular Genetics and Biotechnology, Forestry and Forest Products Research Institute (FFPRI), 1 Matsunosato, Tsukuba, 305-8687, Japan.
Tsukuba Botanical Garden, National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, 305-0005, Japan.
J Plant Res. 2018 Sep;131(5):789-802. doi: 10.1007/s10265-018-1049-2. Epub 2018 Jun 9.
Glutamine synthetase (GS) localized in the chloroplasts, GS2, is a key enzyme in the assimilation of ammonia (NH) produced from the photorespiration pathway in angiosperms, but it is absent from some coniferous species belonging to Pinaceae such as Pinus. We examined whether the absence of GS2 is common in conifers (Pinidae) and also addressed the question of whether assimilation efficiency of photorespiratory NH differs between conifers that may potentially lack GS2 and angiosperms. Search of the expressed sequence tag database of Cryptomeria japonica, a conifer in Cupressaceae, and immunoblotting analyses of leaf GS proteins of 13 species from all family members in Pinidae revealed that all tested conifers exhibited only GS1 isoforms. We compared leaf NH compensation point (γ) and the increments in leaf ammonium content per unit photorespiratory activity (NH leakiness), i.e. inverse measures of the assimilation efficiency, between conifers (C. japonica and Pinus densiflora) and angiosperms (Phaseolus vulgaris and two Populus species). Both γ and NH leakiness were higher in the two conifers than in the three angiosperms tested. Thus, we concluded that the absence of GS2 is common in conifers, and assimilation efficiency of photorespiratory NH is intrinsically lower in conifer leaves than in angiosperm leaves. These results imply that acquisition of GS2 in land plants is an adaptive mechanism for efficient NH assimilation under photorespiratory environments.
定位于叶绿体中的谷氨酰胺合成酶（GS），即GS2，是被子植物光呼吸途径产生的氨（NH）同化过程中的关键酶，但在一些松科针叶树种（如松树）中不存在。我们研究了GS2的缺失在针叶树（松科）中是否普遍存在，还探讨了在可能缺乏GS2的针叶树和被子植物之间，光呼吸产生的NH同化效率是否存在差异。搜索柏科针叶树日本柳杉的表达序列标签数据库，并对松科所有属的13个物种的叶片GS蛋白进行免疫印迹分析，结果显示，所有测试的针叶树仅表现出GS1同工型。我们比较了针叶树（日本柳杉和赤松）和被子植物（菜豆和两种杨树）叶片的NH补偿点（γ）以及单位光呼吸活性下叶片铵含量的增加量（NH泄漏率），即同化效率的反向指标。在两种针叶树中，γ和NH泄漏率均高于测试的三种被子植物。因此，我们得出结论，GS2的缺失在针叶树中很常见，并且针叶树叶片中光呼吸NH的同化效率本质上低于被子植物叶片。这些结果表明，陆地植物中GS2的获得是在光呼吸环境下高效NH同化的一种适应性机制。
