Characterisation of potential receptor-like kinases (RLKs) involved in the AUXIN BINDING PROTEIN 1 (ABP1) signalling pathway

Abstract

Auxin is a key regulatory hormone implicated virtually in every aspect of plant growth and development. At the cellular level, auxin is perceived by two receptors: AUXIN BINDING PROTEIN 1 (ABP1) at the cell surface and TRANSPORT INHIBITOR RESPONSE 1(TIR1) in the nucleus. Currently, the signalling pathway for the ABP1 receptor is poorly understood as molecular mechanisms for ABP1 signalling have not been established. ABP1 is a soluble protein receptor with no transmembrane region. Thus how the auxin signal is transmitted across the plasma membrane remains unanswered. Recently, it was shown that the ABP1 signalling pathway involved ROPs (Rho proteins of plants) which in other pathways are regulated by receptor-like kinases (RLKs). The aim of this project was to investigate if RLKs could be part of an auxin signalling complex with ABP1 at the plasma membrane. Seven RLK genes (BRL3, CCR1, CCR3 MEE62, AT1G25320, AT2G36570 and AT4G25390) which had been previously identified in a microarray as co-regulated with ABP1 expression were characterised. The focus of this research was to determine whether the selected RLK genes were involved in auxin signalling and assess whether the RLK proteins could interact with ABP1. T-DNA mutants retrieved from mutant collections were genotyped and screened for auxin resistance and growth defective phenotypes. Bioinformatic strategies were conducted to analyse gene expression profiles and to determine subcellular localisation. Protein-protein interactions of RLK extracellular domains with ABP1 were also conducted. Six T-DNA mutants (brl3, ccr1, mee62, at1g25320, at2g36570 and at4g25390) were found to be resistant to IAA. Bioinformatic analysis revealed that AT1G25320 had an expression profile that was similar to ABP1 across development which indicates co-expression. The extracellular domain of AT1G25320 was found to interact with the ABP1 protein and the Cterminal of ABP1 in an auxin dependent manner. These findings suggest that AT1G25320 may act as a putative co-receptor for ABP1 at the plasma membrane and could potentially relay the auxin signal across the plasma membrane.