Characterization of At-RLK3, a putative receptor-like protein kinase from Arabidopsis thaliana

[摘要] English: An attempt was made during this study to assign a putative role for At-RLK3 in Arabidopsis thaliana. Receptor-like protein kinases (RLKs) are a group of proteins that was described in various plants. These proteins have a very characteristic structure. They are integral plasma membrane proteins where a hydrophobic transmembrane domain links an extracellular ligand binding domain with an intracellular protein kinase domain. RLKs are involved in various plant processes including development, disease resistance and hormonal signaling. At-RLK3 is a single copy gene in A. thaliana that is expressed during various stress conditions. Oxidative and osmotic stress, infection with pathogens and exposure to salicylic acid (SA) all led to the induced expression of the gene. A similar increase in At-RLK3 protein levels was found after the respective treatments. The kinase domain shares high homology with several other protein kinases and has all 11 conserved sub-domains characteristic of plant protein kinases. It was proven that At-RLK3 is an active protein kinase that specifically phosphorylates serine and threonine amino acids within its active domain. It preferentially uses Mg 2+ as cofactor. The enzyme is located at the plasma membrane and is rapidly activated upon treatment with hydrogen peroxide (H2O2), menadione and osmotic stress conditions, but not after SA treatment. The extracellular domain has two conserved cysteine rich regions that form part of two 'domain of unknown function (DUF) regions. These cysteines are thought to be involved in the formation of disulphide bridges and to be responsive to changes in the redox conditions surrounding the cell. Plants expressing an antisense copy of the gene showed an altered response to treatments with H2O2 and SA compared to the wild type plants. Whereas the wild type plants activated the plant defense shortly after treatment with both H2O2 and SA, only SA managed a similar activation in the transgenic plants. The transgenic plants treated with H2O2 did not show this initial activation. In addition, after both treatments the wild type plants exhibited an amplified and sustained activation of the defense reaction 48 h after treatment which was completely absent in the transgenic plants. This led to the hypothesis that At-RLK3 is responsible to detect changes in extracellular H2O2 levels in A. thaliana most probably due to a change in redox conditions. This then leads to the activation of the enzyme and the subsequent activation of the plant defense response.

[发布机构] University of the Free State