Molecular Mechanisms Of Root Hydrotropism In Arabidopsis Thaliana

Root tips sense moisture gradient and grow towards more water availability,which is designated as root hydrotropism in plants.This unique biological characteristic is crucial for plants to efficiently obtain water from soil.Molecular mechanisms regulating such a fundamental process,however,is largely unknown.For example,what is the asymmetrically distributed signaling molecule and what kind of cell activities determine root hydrotropism?Here we report our identification that cytokinins are key signaling molecules directing root growth orientation in a hydrostimulation condition(moisture gradient).We discovered lower water potential side of the roots accumulates more cytokinins.Consequently,two type-A response regulators,ARR16 and ARR17,are specifically up-regulated at the lower water potential side,causing more cell division within the meristem zone,and utmately directing root to bend towards higher water potential side.Physiological and genetic experiments were performed to verify the essential role of cytokinins in regulating root hydrotropism in Arabidopsis.Firstly,artificially made cytokinin gradient can enhance cell division activities at the higher cytokinin side within root meritem,causing root tips grow towards lower cytokinin side,suggesting cytokinins are likely the signaling molecules involved in root hydrotropism.Secondly,incuced asymmetric expression of ARR16 or ARR17 in root meristem zone can significantly promote the cell division at the expressed side and leads more growth relative to uninduced side,indicating asymmetric exprssion of these genes is the key to hydrostimulation.Thirdly,arr16 arr17 double mutant,is significantly less responsive to hydrostimulation,suggesting that root hydrotropism is indeed dependent of ARR16 and ARR17.Finally,inhibitors interferring either cytokinin biosynthesis or cell division completely abolish root hydrotropism but not gravitropism,demonstrating that uneven cell division activities between higher and lower water potential sides in the root meristematic zone is the cellular basis of root hydrotropism.In summary,these studies demonstrated uneven cell division,caused by asymmetric distribution of cytokinins,is the basis of root hydrotropic response.These analyses contribute to our better understanding of the detailed molecular mechanisms of plant root hydrotropism,and provide novel strategies for improving crop drought tolerance by using genetic engineering and traditional breeding approaches in the near future.