Auxin, indole-3-acetic acid (IAA), is a key phytohormone with diverse morphogenic roles in land plants, but its function and transport mechanisms in algae remain poorly understood. We therefore aimed to explore the role of IAA in a complex, streptophyte algae Chara braunii. Here, we described novel responses of C. braunii to IAA and characterized two homologs of PIN auxin efflux carriers: CbPINa and CbPINc. We determined their localization in C. braunii using epitope-specific antibodies and tested their function in heterologous land plant models. Further, using phosphoproteomic analysis, we identified IAA-induced phosphorylation events. The thallus regeneration assay showed that IAA promotes thallus elongation and side branch development. Immunolocalization of CbPINa and CbPINc confirmed their presence on the plasma membrane of vegetative and generative cells of C. braunii. However, functional assays in tobacco BY-2 cells demonstrated that CbPINa affects auxin transport, whereas CbPINc does not. The IAA is effective in the acceleration of cytoplasmic streaming and the phosphorylation of evolutionary conserved targets such as homolog of RAF-like kinase. These findings suggest that, although canonical PIN-mediated auxin transport mechanisms might not be fully conserved in Chara, IAA is involved in morphogenesis and fast signaling processes.

• Keywords
• Chara, auxin transport, indole‐3‐acetic acid, plant evolution, streptophytes,
• MeSH
• Biological Transport drug effects   MeSH
• Cell Membrane metabolism   drug effects   MeSH
• Chara * metabolism   drug effects   MeSH
• Phosphorylation drug effects   MeSH
• Indoleacetic Acids * metabolism   pharmacology   MeSH
• Membrane Transport Proteins * metabolism   MeSH
• Plant Proteins * metabolism   MeSH
• Nicotiana metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• indoleacetic acid MeSH Browser
• Indoleacetic Acids * MeSH
• Membrane Transport Proteins * MeSH
• Plant Proteins * MeSH

The genomes of charophyte green algae, close relatives of land plants, typically do not show signs of developmental regulation by phytohormones. However, scattered reports of endogenous phytohormone production in these organisms exist. We performed a comprehensive analysis of multiple phytohormones in Viridiplantae, focusing mainly on charophytes. We show that auxin, salicylic acid, ethylene and tRNA-derived cytokinins including cis-zeatin are found ubiquitously in Viridiplantae. By contrast, land plants but not green algae contain the trans-zeatin type cytokinins as well as auxin and cytokinin conjugates. Charophytes occasionally produce jasmonates and abscisic acid, whereas the latter is detected consistently in land plants. Several phytohormones are excreted into the culture medium, including auxin by charophytes and cytokinins and salicylic acid by Viridiplantae in general. We note that the conservation of phytohormone biosynthesis and signaling pathways known from angiosperms does not match the capacity for phytohormone biosynthesis in Viridiplantae. Our phylogenetically guided analysis of established algal cultures provides an important insight into phytohormone biosynthesis and metabolism across Streptophyta.

• MeSH
• Biological Evolution MeSH
• Chlorophyta metabolism   genetics   MeSH
• Cyclopentanes metabolism   MeSH
• Cytokinins * metabolism   MeSH
• Ethylenes metabolism   MeSH
• Phylogeny * MeSH
• Abscisic Acid metabolism   MeSH
• Salicylic Acid metabolism   MeSH
• Indoleacetic Acids * metabolism   MeSH
• Oxylipins metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Plant Growth Regulators * metabolism   MeSH
• Signal Transduction MeSH
• Viridiplantae metabolism   genetics   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Cyclopentanes MeSH
• Cytokinins * MeSH
• ethylene MeSH Browser
• Ethylenes MeSH
• jasmonic acid MeSH Browser
• Abscisic Acid MeSH
• Salicylic Acid MeSH
• Indoleacetic Acids * MeSH
• Oxylipins MeSH
• Plant Growth Regulators * MeSH

The proper distribution of the hormone auxin is essential for plant development. It is channeled by auxin efflux carriers of the PIN family, typically asymmetrically located on the plasma membrane (PM). Several studies demonstrated that some PIN transporters are also located at the endoplasmic reticulum (ER). From the PM-PINs, they differ in a shorter internal hydrophilic loop, which carries the most important structural features required for their subcellular localization, but their biological role is otherwise relatively poorly known. We discuss how ER-PINs take part in maintaining intracellular auxin homeostasis, possibly by modulating the internal levels of IAA; it seems that the exact identity of the metabolites downstream of ER-PINs is not entirely clear as well. We further review the current knowledge about their predicted structure, evolution and localization. Finally, we also summarize their role in plant development.

• Keywords
• ER-PINs, PIN proteins, PIN5, PIN8, auxin metabolism, auxin transport,
• Publication type
• Journal Article MeSH
• Review MeSH