The effects of auxins on plant growth and development have been known for more than 100 years, yet our understanding of how plants synthesize this essential plant hormone is still fragmentary at best. Gene loss-and gain-of-function studies have conclusively implicated three gene families, CYTOCHROME P450 79B2/B3 (CYP79B2/B3), YUCCA (YUC), and TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1/TRYPTOPHAN AMINOTRANSFERASE-RELATED (TAA1/TAR), in the production of this hormone in the reference plant Arabidopsis thaliana. Each of these three gene families is believed to represent independent routes of auxin biosynthesis. Using a combination of pharmacological, genetic, and biochemical approaches, we examined the possible relationships between the auxin biosynthetic pathways defined by these three gene families. Our findings clearly indicate that TAA1/TARs and YUCs function in a common linear biosynthetic pathway that is genetically distinct from the CYP79B2/B3 route. In the redefined TAA1-YUC auxin biosynthetic pathway, TAA1/TARs are required for the production of indole-3-pyruvic acid (IPyA) from Trp, whereas YUCs are likely to function downstream. These results, together with the extensive genetic analysis of four pyruvate decarboxylases, the putative downstream components of the TAA1 pathway, strongly suggest that the enzymatic reactions involved in indole-3-acetic acid (IAA) production via IPyA are different than those previously postulated, and a new and testable model for how IAA is produced in plants is needed.

Establishment of the embryonic axis foreshadows the main body axis of adults both in plants and in animals, but underlying mechanisms are considered distinct. Plants utilize directional, cell-to-cell transport of the growth hormone auxin to generate an asymmetric auxin response that specifies the embryonic apical-basal axis. The auxin flow directionality depends on the polarized subcellular localization of PIN-FORMED (PIN) auxin transporters. It remains unknown which mechanisms and spatial cues guide cell polarization and axis orientation in early embryos. Herein, we provide conceptually novel insights into the formation of embryonic axis in Arabidopsis by identifying a crucial role of localized tryptophan-dependent auxin biosynthesis. Local auxin production at the base of young embryos and the accompanying PIN7-mediated auxin flow toward the proembryo are required for the apical auxin response maximum and the specification of apical embryonic structures. Later in embryogenesis, the precisely timed onset of localized apical auxin biosynthesis mediates PIN1 polarization, basal auxin response maximum, and specification of the root pole. Thus, the tight spatiotemporal control of distinct local auxin sources provides a necessary, non-cell-autonomous trigger for the coordinated cell polarization and subsequent apical-basal axis orientation during embryogenesis and, presumably, also for other polarization events during postembryonic plant life.

Graphical AbstractHighlights d Local auxin production in roots is required for maintaining functional root meristems d Local biosynthesis and transport of auxin cooperate at generating robust auxin maxima d Auxin produced in the root quiescent center is sufficient for root meristem viability

). † These authors contributed equally to this work. SummaryAn Arabidopsis mutant, eer5-1, which has an enhanced ethylene response in etiolated seedlings, including hypersensitivity and extreme exaggeration of response to ethylene, was isolated and characterized. As with other identified eer mutants, the enhanced response phenotype of eer5-1 was correlated with failure to induce appropriately a subset of ethylene-regulated genes, suggesting that proper ethylene-responsive gene expression is necessary for resetting the ethylene response pathway. eer5-1 represents a mutation that causes an amino acid substitution in a previously uncharacterized gene, which encodes a protein with a PAM [proteasome COP9 initiation factor (PCI/PINT)-associated module] domain similar to those found in components of the COP9 signalosome (CSN). Genetic analysis shows that manifestation of the eer5 mutant phenotype is solely dependent on ethylene signaling, as the ein2-5 eer5-1 double mutant was indistinguishable from ein2-5 in the presence of saturating ethylene concentrations. In contrast, the ein3-1 eer5-1 double mutant displayed characteristics of an enhanced ethylene response, and this suggests that EER5 regulates ethylene signaling independently of EIN3. Analysis of the EER5 protein indicates that it interacts with the Cterminus of EIN2 and with the CSN, suggesting that EER5 serves as a bridge between EIN2 and the modification or degradation of target proteins, including a proposed group of transcriptional repressors, as part of a resetting mechanism during or following ethylene signaling.

eer4 was isolated as an Arabidopsis mutant with an extreme response to ethylene in dark-grown seedlings that was also found to have partial ethylene insensitivity at the level of ethylene-dependent gene expression, including ERF1. Subsequent cloning of eer4 revealed an inappropriate stop codon in a previously uncharacterized TFIID-interacting transcription factor homologous to human TAF12 and yeast TAF61. Genetic and pharmacological analysis demonstrated that the eer4 phenotype is strictly ethylene dependent in seedlings, yet a double mutant with the partially ethylene-insensitive Arabidopsis mutant, ein3-1, had restored ethylene responsiveness, indicating that eer4 also regulates a previously unknown resetting or dampening mechanism for the ethylene signalling pathway. Consistent with the absolute requirement of EER4 for ERF1 expression, biochemical analysis showed that EER4 is localized to the nucleus where it probably recruits EIN3 and probably other transcription factors along with components of the TFIID complex for expression of a subset of genes required for either manifestation or subsequent dampening of the response to ethylene.