Boronic acids induce monopteros phenocopies by affecting PIN1 membrane stability and polar auxin transport in Arabidopsis thaliana embryos.

Abstract

Several observations suggest that the micronutrient boron (B) has a stabilizing role in the plasma membrane (PM), supporting functions in PM-linked (hormone) signalling processes. However, this role is poorly characterized. This study shows, that boronic acids, specific competitors of B, induce phenocopies of the Arabidopsis thaliana rootless pattern mutant monopteros. At least in part, this is caused by phenylboronic acid (PBA)-induced internalisation of the membrane-localized auxin efflux carrier PINFORMED1 (PIN1) in the early embryo. PIN1 internalisation interrupts the feedback signal transduction cascade involving the phytohormone auxin, PIN1 and the transcription factor MONOPTEROS. This entails several effects including abnormal development of vascular cell precursors, suppression of MONOPTEROS downstream targets and loss of the root auxin maximum - essential signals for root meristem development. While PIN1 is internalised, we observe a differential effect of PBA on other proteins, these being either unaffected, internalised or, as in the case of the B transporter BOR1, stabilised at the PM. These findings suggest a competition of PBA with B for plant membrane proteins and may shed light on B's function at the PM.