The asymmetric ABA response on both sides of root tip is important for tomato root hydrotropism by mediating proton efflux and cell elongation

Summary Hydrotropism is an important adaptation of plant roots to the uneven distribution of water, and the current research on hydrotropism is mainly focused on Arabidopsis thaliana . We examined hydrotropism in tomato ( Solanum lycopersicum ) primary roots. We used RNA sequencing to detect the gene expression on both sides (dry and wet side) of root tips (5 mm from the root cap junction) by splitting root tips longitudinally into two halves. We found that hydrostimulation induced the asymmetric cell elongation between the dry side (lower water potential) and wet side of root tips (higher water potential). ABA biosynthesis gene ABA4 was induced on the dry side as compared to the wet side of root tips. Chemical inhibitors that block ABA biosynthesis can disrupt hydrotropism, and ABA biosynthesis mutant not showed significantly reduced hydrotropism. Furthermore, asymmetric H + efflux was found in wild-type but not in root tips of ABA biosynthesis mutant not after hydrostimulation. Our results suggest that the asymmetric ABA response on both sides of root tip mediate asymmetric H + efflux, and then drive the asymmetric cell elongation, which allows the root to bend towards the wet side to take up more water. Graphical abstract Involvement of ABA-mediated asymmetric H + efflux in root hydrotropism. Compared to the wet side of root tip (higher water potential), the dry side (lower water potential) induces the expression of ABA biosynthesis gene ABA4 , thus enhancing proton efflux to promoting cell elongation on the dry side. Because H + efflux and cell elongation on the dry side of the root tip are higher than that on the wet side, the asymmetric growth of cells on both sides allows the root to bend towards the wet side for taking up more water. In Brief The asymmetric ABA response on both sides of root tip (dry side and wet side) sequently mediates root asymmetric H + efflux, and then drives the asymmetric cell elongation on both sides of root tip, which allows the root to bend towards the wet side for absorbing more water. Highlights Hydrotropic bending requires asymmetric cell elongation on the root two sides. Asymmetric expression of ABA biosynthesis gene ABA4 is required for root hydrotropism. The H + efflux on the dry side of the root is increased. ABA-associated asymmetric H + efflux driven root hydrotropic bending.