Structure and photosynthetic metabolism in green prop roots of C₄ sorghum

Abstract

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 δ 13 C 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 CO 2 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.

Elevated CO2 increases biomass of Sorghum bicolor green prop roots under drought conditions via soluble sugar accumulation and photosynthetic activity

“…Furthermore, if we consider that glucose and polyols are soluble sugars, the excess CO 2 fixed under E[CO 2 ] may also be translocated as polyols; thus, soluble sugars, mainly glucose and polyols, play a central role in both carbon translocation from the leaves to the GPRs (Figure 6). Ueno and Fuchikami (2019) reported CO 2 transport from leaves to GPRs, even though they did not identify the mechanism behind this flux. Moreover, sorghum GPRs lack stomata, corroborating our hypothesis that the carbon is being translocated from leaves to GPRs via soluble sugars.…”

“…The aboveground portion of the prop roots is green, and the belowground portion is white, similar to the rest of the root system. By analyzing sorghum green and white portions of the prop roots separately, Ueno and Fuchikami (2019) showed that the morphological structure of both tissues is similar, except that GPRs feature granal chloroplasts in the cortex and stele parenchyma (Ueno & Fuchikami, 2019). Experiments conducted with radioactive 13 C demonstrated that carbon is transported from leaves to the GPRs, explaining the carbon source and the maintenance of C3 photosynthesis in organs without stomata (Ueno & Fuchikami, 2019).…”

“…By analyzing sorghum green and white portions of the prop roots separately, Ueno and Fuchikami (2019) showed that the morphological structure of both tissues is similar, except that GPRs feature granal chloroplasts in the cortex and stele parenchyma (Ueno & Fuchikami, 2019). Experiments conducted with radioactive 13 C demonstrated that carbon is transported from leaves to the GPRs, explaining the carbon source and the maintenance of C3 photosynthesis in organs without stomata (Ueno & Fuchikami, 2019). Studies with maize and the C4 model plant species Setaria viridis have reported that prop roots are highly sensitive to drought at the physiological and molecular levels and could be involved in tolerance mechanisms (Sebastian et al, 2016; Zhan et al, 2015).…”

Integrative Analysis ofSorghum BicolorGreen Prop Roots Under Elevated CO2and Water Deficit Conditions

“…The above-ground portion of prop roots is green and the below-ground portion is white, similar to the rest of root system. Ueno and Fuchikami (2019), by analyzing the sorghum GPR and white portion separately, demonstrated that the morphological structure for both parts is similar, except that GPR present lack of stomata and the presence of granal chloroplasts in the cortex and stele parenchyma (Ueno and Fuchikami, 2019). Experiments conducted with radioactive 13 C demonstrated that the carbon is transported from leaves to the GPR, explaining the carbon origin and the maintenance of the C3 photosynthesis in an organ without stomata (Ueno and Fuchikami, 2019).…”

“…Ueno and Fuchikami (2019), by analyzing the sorghum GPR and white portion separately, demonstrated that the morphological structure for both parts is similar, except that GPR present lack of stomata and the presence of granal chloroplasts in the cortex and stele parenchyma (Ueno and Fuchikami, 2019). Experiments conducted with radioactive 13 C demonstrated that the carbon is transported from leaves to the GPR, explaining the carbon origin and the maintenance of the C3 photosynthesis in an organ without stomata (Ueno and Fuchikami, 2019). Studies with maize and the C4 plant-model Setaria viridis reported that prop roots are described to be highly sensitive to drought at physiological and molecular levels, and could be involved in tolerance mechanisms (Zhan et al, 2015; Sebastian et al, 2016).…”