AT-HOOK MOTIF NUCLEAR LOCALIZED transcription factors AHL19, AHL21 and AHL23 - the modulators of the root system...

AT-HOOK MOTIF NUCLEAR LOCALIZED (AHL) transcription factors regulate diverse developmental processes in plants, yet their roles in roots remain poorly understood. Three closely related, co-expressed paralogs AHL19, AHL21, AHL23 display distinct expression patterns in Arabidopsis root tip; AHL19 and AHL23 are specifically active during lateral root (LR) primordia development. Insertion mutants exhibit a markedly reduced root system (total length decreased by ∼25-84 %), with additive effect in double and triple mutants. The reduced LR number arises primarily from arrested outgrowth rather than impaired initiation. Mutant roots show diminished proliferative activity and cell size in the root apical meristem, together with reduced cell elongation. Overexpression (OE) of tested AHLs phenocopies insertion mutant defects due to localized co-suppression in LR primordia. Introducing AHL19 under CaMV 35S promoter into rdr6 background restores AHL19 expression in LRPs and enhances root growth beyond WT, confirming a dosage-dependent role of AHLs. Sucrose supplementation broadly rescues insertion mutant root growth; notably, ahl19 shows sucrose-specific hyper-responsiveness, whereas glucose and fructose do not reproduce this effect. Collectively, our data identify AHL19/21/23 as positive regulators of root system architecture that act at the interface of root carbon status.