Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, ON M5S3B2, Canada.
Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, ON M5S3B2, Canada.
Curr Opin Plant Biol. 2016 Jun;31:58-65. doi: 10.1016/j.pbi.2016.03.016. Epub 2016 Apr 5.
Photorespiratory limitations on C3 photosynthesis are substantial in warm, low CO2 conditions. To compensate, certain plants evolved mechanisms to actively concentrate CO2 around Rubisco using ATP-supported CO2 pumps such as C4 photosynthesis. Plants can also passively accumulate CO2 without additional ATP expenditure by localizing the release of photorespired and respired CO2 around Rubisco that is diffusively isolated from peripheral air spaces. Passive accumulation of photorespired CO2 occurs when glycine decarboxylase is localized to vascular sheath cells in what is termed C2 photosynthesis, and through forming sheaths of chloroplasts around the periphery of mesophyll cells. The peripheral sheaths require photorespired CO2 to re-enter chloroplasts where it can be refixed. Passive accumulation of respiratory CO2 is common in organs such as stems, fruits and flowers, due to abundant heterotrophic tissues and high diffusive resistance along the organ periphery. Chloroplasts within these organs are able to exploit this high CO2 to reduce photorespiration. CO2 concentration can also be enhanced passively by channeling respired CO2 from roots and rhizomes into photosynthetic cells of stems and leaves via lacunae, aerenchyma and the xylem stream. Through passive CO2 concentration, C3 species likely improved their carbon economy and maintained fitness during episodes of low atmospheric CO2.
在温暖、低 CO2 条件下，光呼吸对 C3 光合作用的限制是很大的。为了弥补这一限制，某些植物进化出了使用 ATP 支持的 CO2 泵（如 C4 光合作用）将 CO2 主动集中在 Rubisco 周围的机制。植物还可以通过将光呼吸和呼吸释放的 CO2 本地化，使其与周围的空气空间扩散隔离，从而无需额外的 ATP 支出而被动地积累 CO2。当甘氨酸脱羧酶定位于被称为 C2 光合作用的维管束鞘细胞中时，光呼吸的 CO2 就会被动积累，同时形成围绕叶肉细胞周边的叶绿体鞘。周边的叶绿体鞘需要光呼吸的 CO2 重新进入叶绿体，在那里它可以被重新固定。由于富含异养组织和器官周边较高的扩散阻力，呼吸 CO2 的被动积累在茎、果实和花朵等器官中很常见。这些器官中的叶绿体能够利用高 CO2 来减少光呼吸。通过腔隙、通气组织和木质部流，将根部和根茎呼吸产生的 CO2 引导到茎和叶片的光合作用细胞中，也可以被动地增加 CO2 浓度。通过被动 CO2 浓缩，C3 物种可能在大气 CO2 含量低的时期改善了它们的碳经济，并维持了适应能力。
