AUXIN-BINDING PROTEIN 1: Identification of signalling components and role in cell-wall development in Arabidopsis thaliana

Abstract

AUXIN-BINDING PROTEIN 1 (ABP1) is an auxin receptor essential for plant growth and development, including cell division and expansion; however, little is known about its signalling pathway. This research investigated ABP1 signalling by focusing on how ABP1 inactivation affects transcription using a conditional abp1 mutant and microarrays, followed by analysis of mutants in signalling genes affected by ABP1 inactivation to identify ABP1 pathway components. Analysis of T-DNA mutants identified three genes as components of ABP1 signalling. Two receptor-like kinase genes, AT1G25320 and AT4G25390, were identified as repressors of the ABP1 pathway based on epistatic analysis with the conditional abp1 mutant. It is proposed that they are components of ABP1-ROP signalling in a plasma membrane auxin-sensing complex with ABP1. In addition, the calmodulin-like gene TOUCH 3 (TCH3) was down-regulated with ABP1 inactivation and epistatic analysis indicated that TCH3 was involved in ABP1-regulated growth, providing a link between ABP1 and calcium signalling. ABP1-regulated TCH3 signalling was also linked to differential responses of organs to auxin. Crosses between all three genes and the conditional abp1 mutant all exhibited similar resistance to IAA, indicating that these genes are in the same pathway. ABP1 was found to regulate the transcription of many genes. Genes involved in the pathway of the other auxin receptor, TRANSPORT INHIBITOR RESPONSE PROTEIN 1 (TIR1), were affected, indicating that the two pathways interact indirectly; however, only 20% of auxin-regulated genes were affected by ABP1 inactivation, indicating that ABP1 signals to the nucleus independently of TIR1. The other major focus of this research was investigation of ABP1’s role in cell-wall development, since the cell wall is essential for auxin-regulated growth and development. Microarray and real-time RT-PCR analysis showed that ABP1 is required for the transcriptional control of auxin-regulated cell-wall expansion and lignification. Histochemical and Fourier transform Infrared microspectroscopy techniques showed that lignification was reduced in the interfascicular fibre cell walls of ABP1-inactivated stems, corresponding to reduced transcription of lignification pathway genes. ABP1-inactivated stems displayed stunted growth and iTRAQ analysis of the cell-wall proteome showed that ABP1 regulates the concentrations of proteins involved in cell-wall expansion and lignification at the cell wall.