Structure and photosynthetic metabolism in green prop roots of C4 sorghum

Plants contain chloroplasts in various organs exposed to sunlight. The C-4 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 delta C-13 values of all organs including the AG portion were within the C-4 range, suggesting that their tissue carbon is derived from C-4 photosynthesis of leaves. These data suggest that the AG portion of prop roots could re-fix internally respired CO2 via C-3 cycle, whereas this photosynthetic function may provide O-2 to heterotrophic tissues of prop roots. This study also demonstrates that different photosynthetic types can function in different organs of a single plant.