Abstract: Generation of crops with low phytic acid (myo-inositol-1,2,3,4,5,6-hexakisphosphate (InsP₆)) is an important breeding direction, but such plants often display less desirable agronomic traits. In this study, through ethyl methanesulfonate-mediated mutagenesis, we found that inositol 1,3,4-trisphosphate 5/6-kinase 4 (ITPK4), which is essential for producing InsP₆, is a critical regulator of salt tolerance in Arabidopsis. Loss of function of ITPK4 gene leads to reduced root elongation under salt stress, which is primarily because of decreased root meristem length and reduced meristematic cell number. The itpk4 mutation also results in increased root hair density and increased accumulation of reactive oxygen species during salt exposure. RNA sequencing assay reveals that several auxin-responsive genes are down-regulated in the itpk4-1 mutant compared to the wild-type. Consistently, the itpk4-1 mutant exhibits a reduced auxin level in the root tip and displays compromised gravity response, indicating that ITPK4 is involved in the regulation of the auxin signaling pathway. Through suppressor screening, it was found that mutation of Multidrug Resistance Protein 5 (MRP5)5 gene, which encodes an ATP-binding cassette (ABC) transporter required for transporting InsP₆ from the cytoplasm into the vacuole, fully rescues the salt hypersensitivity of the itpk4-1 mutant, but in the itpk4-1 mrp5 double mutant, InsP₆ remains at a very low level. These results imply that InsP₆ homeostasis rather than its overall amount is beneficial for stress tolerance in plants. Collectively, this study uncovers a pair of gene mutations that confer low InsP₆ content without impacting stress tolerance, which offers a new strategy for creating “low-phytate” crops.

Key words: ITPK4, MRP5, phytic acid, root elongation, salt stress