Auxins mediate various processes that are involved in plant growth and development in response to specific environmental conditions. Its proper spatio-temporal distribution that is driven by polar auxin transport machinery plays a crucial role in the wide range of auxins physiological effects. Numbers of approaches have been developed to either directly or indirectly monitor auxin distribution in vivo in order to elucidate the basis of its precise regulation. Herein, we provide an updated list of valuable techniques used for monitoring auxins in plants, with their utilities and limitations. Because the spatial and temporal resolutions of the presented approaches are different, their combination may provide a comprehensive outcome of auxin distribution in diverse developmental processes.

• Keywords
• auxin, auxin distribution, auxin signalling, auxin transport, direct visualization, indirect visualization, receptor, sensor,
• MeSH
• Arabidopsis metabolism   MeSH
• Indoleacetic Acids metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Plant Development physiology   MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• Indoleacetic Acids MeSH

Auxin steers numerous physiological processes in plants, making the tight control of its endogenous levels and spatiotemporal distribution a necessity. This regulation is achieved by different mechanisms, including auxin biosynthesis, metabolic conversions, degradation, and transport. Here, we introduce cis-cinnamic acid (c-CA) as a novel and unique addition to a small group of endogenous molecules affecting in planta auxin concentrations. c-CA is the photo-isomerization product of the phenylpropanoid pathway intermediate trans-CA (t-CA). When grown on c-CA-containing medium, an evolutionary diverse set of plant species were shown to exhibit phenotypes characteristic for high auxin levels, including inhibition of primary root growth, induction of root hairs, and promotion of adventitious and lateral rooting. By molecular docking and receptor binding assays, we showed that c-CA itself is neither an auxin nor an anti-auxin, and auxin profiling data revealed that c-CA does not significantly interfere with auxin biosynthesis. Single cell-based auxin accumulation assays showed that c-CA, and not t-CA, is a potent inhibitor of auxin efflux. Auxin signaling reporters detected changes in spatiotemporal distribution of the auxin response along the root of c-CA-treated plants, and long-distance auxin transport assays showed no inhibition of rootward auxin transport. Overall, these results suggest that the phenotypes of c-CA-treated plants are the consequence of a local change in auxin accumulation, induced by the inhibition of auxin efflux. This work reveals a novel mechanism how plants may regulate auxin levels and adds a novel, naturally occurring molecule to the chemical toolbox for the studies of auxin homeostasis.

• MeSH
• Arabidopsis drug effects   growth & development   MeSH
• Cinnamates chemistry   metabolism   pharmacology   MeSH
• Cyclin B genetics   metabolism   MeSH
• Plants, Genetically Modified MeSH
• Isomerism MeSH
• Plant Roots drug effects   growth & development   metabolism   MeSH
• Indoleacetic Acids metabolism   MeSH
• Bryopsida drug effects   growth & development   MeSH
• Arabidopsis Proteins genetics   metabolism   MeSH
• Qa-SNARE Proteins genetics   metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Selaginellaceae drug effects   growth & development   MeSH
• Signal Transduction MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Cinnamates MeSH
• cinnamic acid MeSH Browser
• CycB1 protein, Arabidopsis MeSH Browser
• Cyclin B MeSH
• KNOLLE protein, Arabidopsis MeSH Browser
• Indoleacetic Acids MeSH
• Arabidopsis Proteins MeSH
• Qa-SNARE Proteins MeSH

The phenylpropanoid 3,4-(methylenedioxy)cinnamic acid (MDCA) is a plant-derived compound first extracted from roots of Asparagus officinalis and further characterized as an allelochemical. Later on, MDCA was identified as an efficient inhibitor of 4-COUMARATE-CoA LIGASE (4CL), a key enzyme of the general phenylpropanoid pathway. By blocking 4CL, MDCA affects the biosynthesis of many important metabolites, which might explain its phytotoxicity. To decipher the molecular basis of the allelochemical activity of MDCA, we evaluated the effect of this compound on Arabidopsis thaliana seedlings. Metabolic profiling revealed that MDCA is converted in planta into piperonylic acid (PA), an inhibitor of CINNAMATE-4-HYDROXYLASE (C4H), the enzyme directly upstream of 4CL. The inhibition of C4H was also reflected in the phenolic profile of MDCA-treated plants. Treatment of in vitro grown plants resulted in an inhibition of primary root growth and a proliferation of lateral and adventitious roots. These observed growth defects were not the consequence of lignin perturbation, but rather the result of disturbing auxin homeostasis. Based on DII-VENUS quantification and direct measurement of cellular auxin transport, we concluded that MDCA disturbs auxin gradients by interfering with auxin efflux. In addition, mass spectrometry was used to show that MDCA triggers auxin biosynthesis, conjugation, and catabolism. A similar shift in auxin homeostasis was found in the c4h mutant ref3-2, indicating that MDCA triggers a cross talk between the phenylpropanoid and auxin biosynthetic pathways independent from the observed auxin efflux inhibition. Altogether, our data provide, to our knowledge, a novel molecular explanation for the phytotoxic properties of MDCA.

• MeSH
• Trans-Cinnamate 4-Monooxygenase antagonists & inhibitors   metabolism   MeSH
• Arabidopsis drug effects   genetics   metabolism   MeSH
• Benzoates metabolism   pharmacology   MeSH
• Biosynthetic Pathways drug effects   MeSH
• Cinnamates chemistry   metabolism   pharmacology   MeSH
• Phenylpropionates chemistry   metabolism   pharmacology   MeSH
• Plants, Genetically Modified MeSH
• Mass Spectrometry MeSH
• Homeostasis drug effects   MeSH
• Coenzyme A Ligases antagonists & inhibitors   metabolism   MeSH
• Microscopy, Confocal MeSH
• Plant Roots drug effects   genetics   metabolism   MeSH
• Indoleacetic Acids metabolism   MeSH
• Lignin biosynthesis   MeSH
• Seedlings drug effects   genetics   growth & development   metabolism   MeSH
• Dose-Response Relationship, Drug MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• 4-coumarate-CoA ligase MeSH Browser
• Trans-Cinnamate 4-Monooxygenase MeSH
• Benzoates MeSH
• Cinnamates MeSH
• cinnamic acid MeSH Browser
• Phenylpropionates MeSH
• Coenzyme A Ligases MeSH
• Indoleacetic Acids MeSH
• Lignin MeSH
• phenylpropanoid 3,4-(methylenedioxy)cinnamic acid MeSH Browser
• piperonylic acid MeSH Browser