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.