Efficient soil exploration by roots represents an important target for crop improvement and food security [1, 2]. Lateral root (LR) formation is a key trait for optimizing soil foraging for crucial resources such as water and nutrients. Here, we report an adaptive response termed xerobranching, exhibited by cereal roots, that represses branching when root tips are not in contact with wet soil. Non-invasive X-ray microCT imaging revealed that cereal roots rapidly repress LR formation as they enter an air space within a soil profile and are no longer in contact with water. Transcript profiling of cereal root tips revealed that transient water deficit triggers the abscisic acid (ABA) response pathway. In agreement with this, exogenous ABA treatment can mimic repression of LR formation under transient water deficit. Genetic analysis in Arabidopsis revealed that ABA repression of LR formation requires the PYR/PYL/RCAR-dependent signaling pathway. Our findings suggest that ABA acts as the key signal regulating xerobranching. We conclude that this new ABA-dependent adaptive mechanism allows roots to rapidly respond to changes in water availability in their local micro-environment and to use internal resources efficiently.

Department of Forest Genetics and Plant Physiology Umeå Plant Science Centre 901 83 Umeå Sweden

Department of Forest Genetics and Plant Physiology Umeå Plant Science Centre 901 83 Umeå Sweden; Centre of the Region Haná for Biotechnological and Agricultural Research Laboratory of Growth Regulators Palacký University and Institute of Experimental Botany AS CR Šlechtitelů 11 78371 Olomouc Czech Republic

Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Ghent Belgium; Department of Plant Systems Biology VIB 9052 Ghent Belgium

Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Ghent Belgium; Department of Plant Systems Biology VIB 9052 Ghent Belgium; Division of Plant and Crop Sciences School of Biosciences University of Nottingham Leicestershire LE12 5RD UK; Centre for Plant Integrative Biology School of Biosciences University of Nottingham Sutton Bonington LE12 5RD UK

Department of Plant Science The Pennsylvania State University University Park PA 16802 USA

Division of Plant and Crop Sciences School of Biosciences University of Nottingham Leicestershire LE12 5RD UK; Centre for Plant Integrative Biology School of Biosciences University of Nottingham Sutton Bonington LE12 5RD UK

Division of Plant and Crop Sciences School of Biosciences University of Nottingham Leicestershire LE12 5RD UK; Centre for Plant Integrative Biology School of Biosciences University of Nottingham Sutton Bonington LE12 5RD UK; Department of Plant Science The Pennsylvania State University University Park PA 16802 USA

Earth and Life Institute Université catholique de Louvain 1348 Louvain la Neuve Belgium

Earth and Life Institute Université catholique de Louvain 1348 Louvain la Neuve Belgium; Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Ghent Belgium; Department of Plant Systems Biology VIB 9052 Ghent Belgium

InBioS PhytoSYSTEMS Laboratory of Plant Physiology University of Liège 4000 Liège Belgium

Instituto de Biología Molecular y Celular de Plantas Consejo Superior de Investigaciones Cientificas Universidad Politecnica de Valencia 46022 Valencia Spain

Louvain Institute for Biomolecular Sciences and Technologies Université catholique de Louvain 1348 Louvain la Neuve Belgium

State Key Laboratory of Crop Genetics and Germplasm Enhancement MOA Key Laboratory of Plant Nutrition and Fertilization in Lower Middle Reaches of the Yangtze River Nanjing Agricultural University Nanjing 210095 PR China

The Lancaster Environment Centre Lancaster University LA1 4YQ UK

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