The At-Hook Motif Nuclear Localized Protein (AHL) gene family encodes embryophyte-specific nuclear proteins with DNA binding activity. They modulate gene expression and affect various developmental processes in plants. We identify AHL18 (At3G60870) as a developmental modulator of root system architecture and growth. AHL18 is involved in regulation of the length of the proliferation domain and number of dividing cells in the root apical meristem and thereby, cell production. Both primary root growth and lateral root development respond according to AHL18 transcription level. The ahl18 knock-out plants show reduced root systems due to a shorter primary root and a lower number of lateral roots. This change results from a higher number of arrested and non-developing lateral root primordia (LRP) rather than from a decreased LRP initiation. The over-expression of AHL18 results in a more extensive root system, longer primary roots, and increased density of lateral root initiation events. AHL18 is thus involved in the formation of lateral roots at both LRP initiation and their later development. We conclude that AHL18 participates in modulation of root system architecture through regulation of root apical meristem activity, lateral root initiation and emergence; these correspond well with expression pattern of AHL18.

Departamento de Biología Molecular de Plantas Instituto de Biotecnología Universidad Nacional Autónoma de México Avenida Universidad 2001 Colonia Chamilpa Cuernavaca Morelos 62210 Mexico

Departamento de Biología Molecular de Plantas Instituto de Biotecnología Universidad Nacional Autónoma de México Avenida Universidad 2001 Colonia Chamilpa Cuernavaca Morelos 62210 Mexico Laboratorio Nacional de Genómica para la Biodiversidad Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional Irapuato Guanajuato 36821 Mexico

Department of Experimental Biology of Plants Faculty of Science Charles University 128 44 Prague Czech Republic

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