ABSTRACT Plants contain chloroplasts in various organs exposed to sunlight. The C4 crop, sorghum (Sorghum bicolor), develops prop roots on stems above the soil level with the progression of growth. These roots penetrate in the soil and form green above-ground (AG) and non-green below-ground (BG) portions. We investigated the structure and photosynthetic metabolism in the AG portion of prop roots in comparison with leaf blades and the BG portion. The AG portion lacked stomata on the epidermis and showed typical root structure as in the BG portion. However, they contained granal chloroplasts in the cortex and stele parenchyma. The chlorophyll content was much lower in the AG portion than in leaf blades. Western blot analysis showed that the AG portion accumulates ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) but lacked substantially phosphoenolpyruvate carboxylase and pyruvate, Pi dikinase. An immunolocalization study confirmed that Rubisco is accumulated in the chloroplasts of AG portion. The AG portion accumulated only small amount of malate without diurnal change as in other organs, indicating that crassulacean acid metabolism (CAM) is inactive. The δ13C values of all organs including the AG portion were within the C4 range, suggesting that their tissue carbon is derived from C4 photosynthesis of leaves. These data suggest that the AG portion of prop roots could re-fix internally respired CO2 via C3 cycle, whereas this photosynthetic function may provide O2 to heterotrophic tissues of prop roots. This study also demonstrates that different photosynthetic types can function in different organs of a single plant. GRAPHICAL ABSTRACT

中文翻译：

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C4高粱绿色支柱根的结构和光合代谢

摘要 植物暴露在阳光下的各个器官中都含有叶绿体。C4 作物高粱 (Sorghum bicolor) 随着生长的进行，在高于土壤水平的茎上形成支撑根。这些根穿透土壤并形成地上绿色 (AG) 和非绿色地下 (BG) 部分。与叶片和BG部分相比，我们研究了支柱根AG部分的结构和光合代谢。AG 部分在表皮上没有气孔，显示出与 BG 部分一样的典型根结构。然而，它们在皮质和石柱薄壁组织中含有颗粒叶绿体。AG 部分的叶绿素含量远低于叶片。Western印迹分析显示AG部分积累核酮糖1，5-二磷酸羧化酶/加氧酶（Rubisco）但基本上缺乏磷酸烯醇式丙酮酸羧化酶和丙酮酸、Pi双激酶。一项免疫定位研究证实 Rubisco 在 AG 部分的叶绿体中积累。AG 部分仅积累少量苹果酸，没有其他器官的昼夜变化，表明景天酸代谢 (CAM) 不活跃。包括AG部分在内的所有器官的δ13C值均在C4范围内，表明它们的组织碳来自叶片的C4光合作用。这些数据表明，支柱根的 AG 部分可以通过 C3 循环重新固定内部呼吸的 CO2，而这种光合功能可以为支柱根的异养组织提供 O2。这项研究还表明，不同的光合类型可以在单一植物的不同器官中发挥作用。图形概要