ABA is required for differential cell wall acidification associated with root hydrotropic bending in tomato

Hydrotropism is an important adaptation of plant roots to the uneven distribution of water, with current research mainly focused on Arabidopsis thaliana. To examine hydrotropism in tomato (Solanum lycopersicum) primary roots, we used RNA sequencing to determine gene expression of root tips (apical 5 mm) on dry and wet sides of hydrostimulated roots grown on agar plates. Hydrostimulation enhances cell division and expansion on the dry side compared with the wet side of the root tip. In hydrostimulated roots, the abscisic acid (ABA) biosynthesis gene ABA4 was induced more on the dry than the wet side of root tips. The ABA biosynthesis inhibitor Fluridone and the ABA-deficient mutant notabilis (not) significantly decreased hydrotropic curvature. Wild-type, but not the ABA biosynthesis mutant not, root tips showed asymmetric H⁺ efflux, with greater efflux on the dry than on the wet side of root tips. Thus, ABA mediates asymmetric H⁺ efflux, allowing the root to bend towards the wet side to take up more water.