ABCB ‐mediated shootward auxin transport feeds into the root clock

Abstract

Although strongly influenced by environmental conditions, lateral root (LR) positioning along the primary root appears to follow obediently an internal spacing mechanism dictated by auxin oscillations that prepattern the primary root, referred to as the root clock. Surprisingly, none of the hitherto characterized PIN‐ and ABCB‐type auxin transporters seem to be involved in this LR prepatterning mechanism. Here, we characterize ABCB15, 16, 17, 18, and 22 (ABCB15‐22) as novel auxin‐transporting ABCBs. Knock‐down and genome editing of this genetically linked group of ABCBs caused strongly reduced LR densities. These phenotypes were correlated with reduced amplitude, but not reduced frequency of the root clock oscillation. High‐resolution auxin transport assays and tissue‐specific silencing revealed contributions of ABCB15‐22 to shootward auxin transport in the lateral root cap (LRC) and epidermis, thereby explaining the reduced auxin oscillation. Jointly, these data support a model in which LRC‐derived auxin contributes to the root clock amplitude. image A group of related ATP‐binding cassette subfamily B (ABCB) transporters contributes to shootward auxin transport that feeds into the root clock. ABCB15, 16,17,18, and 22 stimulate cellular auxin efflux. ABCB15,16,17,18, and 22 have functional redundant functions in plant development. Outer root tissue‐expressed ABCB15‐22 cluster genes contribute to shootward auxin transport that feeds into the root clock.