Mitsuhashi
et al. (2005) reported that exposing the cultured cells ofCatharanthus roseus and Arabidopsis to high Pi conditions
stimulates phytic acid synthesis and accumulation phytic acid in both
cytosol and vacuoles. Based on this observation, we examined whether
high Pi accumulation stimulates phytic acid synthesis in leaves. Perera
et al. (2018) summarized the putative genes involved in phytic acid
synthesis in rice plants. Among those, we selected the genes that are
expressed in leaves using the Rice-XPro data base and quantified their
mRNA expressions. Figure 8a shows the phytic acid synthesis pathways as
well as the genes involved in those pathways (Suzuki et al., 2007;
Perera et al., 2018). The INO1, IPK1, IPK2, 2-PGK, ITPK3-1,
ITPK3-2, ITPK5, and ITPK6 transcript levels tended to increase
with increasing in Pi application (Figure 8b). On the other hand, theIMP1-1 transcript levels increased only under low-Pi conditions.
The IMP1-2 , ITPK1, and ITPK2 transcript levels
showed no clear response to the Pi application.
To certify that the change in mRNA expression of the genes involved in
phytic acid synthesis actually changes the phytic acid content in
leaves, we quantified the phytic acid content in leaves. The leaf phytic
acid content was comparable between the low-Pi and control-Pi plants
(Figure 8c, d). As with the increase in Pi application, the leaf phytic
acid content increased and the 2.4 and 3.0 mM Pi plants showed
significantly higher phytic acid content than the control-Pi plants
(Figure 8c, d). Figure 8e shows the phytic acid/free Pi ratio in the
leaves. Under 0.06 mM Pi conditions, the phytic acid/free Pi ratio was
significantly higher than that under other Pi application conditions. In
contrast, the increase in Pi application from the control-Pi to 3.0 mM
Pi did not change the phytic acid/free Pi ratio in the leaves. These
results indicated that the phytic acid content increases with increasing
Pi content in the leaves of all Pi-treated plants, except for the low-Pi
plants.