To evaluate the effects of excessive Pi application on sugar-phosphate
metabolism, we analyzed the content of major sugar-phosphates and
adenylates in leaves under
illumination conditions (see Materials and Methods). In the low-Pi
plants, a large number of sugar-phosphates were decreased compared to
the control-Pi plants (Figure 3b). However, ribose-5-phosphate (R5P)
showed similar levels in both low-Pi and control-Pi plants, and the
levels of ribulose-5-phosphate (Ru5P) and glycerol-3-phosphate (G-3-P)
were higher in the low-Pi plants than in the control-Pi plants. The
total adenylate content was lower in the low-Pi plants than in the
control-Pi plants, and the ATP/ADP ratio was also decreased (Figure 3c,
d). These results indicated that the substrates for photosynthesis in
the Calvin cycle decreased, and that the ATP consumption exceeded the
ATP synthesis owing to lower adenylate content in low-Pi plants than in
the control-Pi plants. These observations were clearly consistent with
the photosynthetic kinetics in the low-Pi plants (Figure 2a) (Sharkey,
1985; Fabre et al., 2019). The increases in Ru5P and R5P levels in the
low-Pi plants might be caused by limited ATP supply (Figure 3a, b). The
increase in G-3-P has been observed under P deficiency for remodeling
lipid composition (Cheng et al., 2011; Nakamura, 2013), that is, an
increase in G-3-P represents the P starvation response (Figure 3b). An
increase in Pi application increased the sugar-phosphate content (Figure
3b). Especially, the levels of glycelaldehyde-3-phosphate (GA3P),
dihydroxyacetone-phosphate (DHAP), fructose-6-phosphate (F-6-P),
6-phospho gluconate (6-PG), glucose-1-phosphate (G-1-P), and
glucose-6-phosphate (G-6-P) were increased with an increase in Pi
application (Figure 3b). The total adenylate levels were also increased
with an increase in Pi application (Figure 3c). In contrast, the ATP/ADP
ratio was enhanced with increasing Pi application (Figure 3d). These
results indicated that the ATP consumption is suppressed more than the
ATP synthesis under high Pi application conditions. This observation
clearly matched the decrease in the H+-conductance of
the chloroplastic ATP synthase as indicated by the value of
gH+ (Figure S2b).