Characterization of AFB5 in Arabidopsis Auxin Signaling

Abstract

Auxin is a pivotal hormone that regulates nearly every aspect of plant growth and development, both genomic as well as non-genomic responses. Genomic responses are regulated through the degradation of a group of transcriptional repressors called Aux/IAA proteins. These repressors are degraded through the ubiquitin-proteasome pathway involving SCF TIR1/AFBs in which TIR1/AFBs function as auxin co-receptors. TIR1 gene family in Arabidopsis consists of 6 genes, of which AFB4 and AFB5 are distantly related to TIR1. Two mutant alleles of Arabidopsis AFB5 (pic3 and pic59) were isolated through a genetic screen for picloram resistance, a synthetic auxin commonly used as an herbicide.

Both mutant alleles exhibit primary root growth resistance to picloram and indole-3-butyric acid (IBA), but not to indole-3-acetic acid (IAA) and results indicate that AFB5 may not promote Aux/IAA degradation, suggesting that it functions partially or completely differently from TIR1. afb5 resistance to IBA in primary root elongation, lateral root proliferation, as well as altered responses to IBA-induced gene expression raises the possibility that AFB5 is involved in IBA rather than IAA signaling, however the role of IBA in auxin signaling still remains unclear. pic3 and pic59 display altered lateral root densities and primary root elongation, and expression of cell division reporter CyclinB::GUS is lower in pic3 and higher in pic59, indicating that AFB5 regulates cell division in primary and lateral roots. afb5 mutants also exhibit altered responses to abscisic acid (ABA) in seed germination and primary root growth and it was shown that ABA downregulates AFB5 expression, supporting a role for AFB5 in auxin and ABA cross-talk.

Additionally, recent published data suggests that ROP GTPases regulate both auxin and ABA signaling. Two members of this family, ROP2 and ROP6 express highly in actively dividing tissues and mutants show defects in lateral root development. ROP2 and ROP6 have also been shown to interact with IBR5, a phosphatase which regulates auxin signaling and displays both ABA and IBA response. Based on these data, it was hypothesized that the functions of AFB5 may be regulated through ROP GTPases. Results indicate that basal expression of AFB5 is significantly higher in rop2 and rop6 and that ROP2/6 may functionally interact with AFB5 in regulating primary root elongation.