The slow response of auxin signalling is not conserved in streptophyte algae, hence the molecular basis for their known responsiveness to auxin remained unclear. Recently, it became evident that components of the fast response, namely the auxin receptors AUXIN BINDING PROTEIN 1 (ABP1) and its homologs as well as the interacting TRANSMEMBRANE KINASE 1 (TMK1), are conserved and are therefore probably part of a more ancient evolutionary branch. Cytoplasmic streaming, which is regulated by this fast response, is similarly conserved from algae to flowering plants and is important for the efficient transport of proteins and nutrients within the cell. To uncover novel components of this conserved auxin signalling pathway, the researchers performed phosphoproteomics in Arabidopsis, two moss and two streptophyte algae species after short auxin incubations.
The researchers found that in most species short auxin treatment leads to a general hyperphosphorylation and that there is a conserved set of phosphotargets in all species. A network analysis of the phosphosite changes in an Arabidopsis time series of auxin treatments showed that the B4 clade of RAF-like kinases formed a hub. These kinases are activated as soon as 30 seconds after auxin treatment, and mutants show much less hyperphosphorylation in response to auxin in both Arabidopsis and Marchantia. Moreover, some differentially phosphorylated targets are shared in both species, suggesting a conserved role. Indeed, in both Arabidopsis and Marchantia the accelerating effect of auxin on cytoplasmic streaming disappears in the raf mutants. Phosphorylation of these RAF-like kinases is disturbed in abp1 and tmk1 mutants, showing that they act downstream in the fast auxin signalling response. Interestingly, the differentially phosphorylated targets in raf mutants entail myosin binding and actin bundling proteins, which are good candidates to further study how RAF-like kinases regulate cytoplasmic streaming.
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