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Plant survival depends on vascular tissues, which originate in a self-organizing manner as strands of cells co-directionally transporting the plant hormone auxin. The latter phenomenon (also known as auxin canalization) is classically hypothesized to be regulated by auxin itself via the effect of this hormone on the polarity of its own intercellular transport. Correlative observations supported this concept, but molecular insights remain limited. In the current study, we established an experimental system based on the model Arabidopsis thaliana, which exhibits auxin transport channels and formation of vasculature strands in response to local auxin application. Our methodology permits the genetic analysis of auxin canalization under controllable experimental conditions. By utilizing this opportunity, we confirmed the dependence of auxin canalization on a PIN-dependent auxin transport and nuclear, TIR1/AFB-mediated auxin signaling. We also show that leaf venation and auxin-mediated PIN repolarization in the root require TIR1/AFB signaling. Further studies based on this experimental system are likely to yield better understanding of the mechanisms underlying auxin transport polarization in other developmental contexts.

International audience; Plant growth and development is coordinated by complex networks of interacting hormones, and cross‐talk between ethylene and auxin signaling is essential for a wide range of plant developmental processes. Nevertheless, the molecular links underlying the interaction between the two hormones remain poorly understood. In order to decipher the cross‐talk between the Ethylene Response Factor Sl‐ERF.B3 and Sl‐IAA27, mediating ethylene and auxin signaling, respectively, we combined reverse genetic approaches, physiological methods, transactivation experiments and electrophoretic mobility shift assays. Sl‐ERF.B3 is responsive to both ethylene and auxin and ectopic expression of its dominant repressor version (ERF.B3‐SRDX) results in impaired sensitivity to auxin with phenotypes recalling those previously reported for Sl‐IAA27 downregulated tomato lines. The expression of Sl‐IAA27 is dramatically reduced in the ERF.B3‐SRDX lines and Sl‐ERF.B3 is shown to regulate the expression of Sl‐IAA27 via direct binding to its promoter. The data support a model in which the ethylene‐responsive Sl‐ERF.B3 integrates ethylene and auxin signaling via regulation of the expression of the auxin signaling component Sl‐IAA27. The study uncovers a molecular mechanism that links ethylene and auxin signaling in tomato.

The root meristem is organized around a quiescent center (QC) surrounded by stem cells that generate all cell types of the root. In the transit-amplifying compartment, progeny of stem cells further divides prior to differentiation. Auxin controls the size of this transit-amplifying compartment via auxin response factors (ARFs) that interact with auxin response elements (AuxREs) in the promoter of their targets. The microRNA miR390 regulates abundance of ARF2, ARF3, and ARF4 by triggering the production of trans-acting (ta)-siRNA from the TAS3 precursor. This miR390/TAS3/ARF regulatory module confers sensitivity and robustness to auxin responses in diverse developmental contexts and organisms. Here, we show that miR390 is expressed in the transit-amplifying compartment of the root meristem where it modulates response to exogenous auxin. We show that a single AuxRE located in miR390 promoter is necessary for miR390 expression in this compartment and identify that ARF5/MONOPTEROS (MP) binds miR390 promoter via the AuxRE. We show that interfering with ARF5/MP-dependent auxin signaling attenuates miR390 expression in the transit-amplifying compartment of the root meristem. Our results show that ARF5/MP regulates directly the expression of miR390 in the root meristem. We propose that ARF5, miR390, and the ta-siRNAs-regulated ARFs modulate the response of the transit-amplifying region of the meristem to exogenous auxin.

Key message: Extensive modulation of numerous ARF transcripts in the embryogenic culture of Arabidopsis indicates a substantial role of auxin signaling in the mechanism of somatic embryogenesis induction. Abstract: Somatic embryogenesis (SE) is induced by auxin in plants and auxin signaling is considered to play a key role in the molecular mechanism that controls the embryogenic transition of plant somatic cells. Accordingly, the expression of AUXIN RESPONSE FACTOR (ARF) genes in embryogenic culture of Arabidopsis was analyzed. The study revealed that 14 of the 22 ARFs were transcribed during SE in Arabidopsis. RT-qPCR analysis indicated that the expression of six ARFs (ARF5, ARF6, ARF8, ARF10, ARF16, and ARF17) was significantly up-regulated, whereas five other genes (ARF1, ARF2, ARF3, ARF11, and ARF18) were substantially down-regulated in the SE-induced explants. The activity of ARFs during SE was also monitored with GFP reporter lines and the ARFs that were expressed in areas of the explants engaged in SE induction were detected. A functional test of ARFs transcribed during SE was performed and the embryogenic potential of the arf mutants and overexpressor lines was evaluated. ARFs with a significantly modulated expression during SE coupled with an impaired embryogenic response of the relevant mutant and/or overexpressor line, including ARF1, ARF2, ARF3, ARF5, ARF6, ARF8, and ARF11 were indicated as possibly being involved in SE induction. The study provides evidence that embryogenic induction strongly depends on ARFs, which are key regulators of the auxin signaling. Some clues on the possible functions of the candidate ARFs, especially ARF5, in the mechanism of embryogenic transition are discussed. The results provide guidelines for further research on the auxin-related functional genomics of SE and the developmental plasticity of somatic cells., NCN

Since Darwins' pioneering experiments, monocot coleoptiles have been used to investigate indole-3-acetic acid (IAA) production and polar transport. In a recent study, using maize coleoptiles, we first showed that the asymmetric IAA flow from the tip in response to gravistimulus directly affects the TIR/AFBs-mediated auxin signaling pathway, which results in tropic curvature. In this work, we also showed that IAA is synthesized from tryptophan (Trp) in the apical 1 mm region, and from there the synthesized IAA moves to the basal part via polar transport by ZmPIN1(s). These results clearly show the importance of the tip region in perception of gravistimulus and in transmitting the perceived information to the lower region using IAA as a messenger signal. Thus, it is concluded that IAA production and transport from the tip are key factors controlling the cell elongation rate in the lower part of the coleoptiles, by making a regulated and dynamic IAA flow net work in the coleoptiles.

(Current Biology 21, 520–525; March 22, 2011)In this article, we reported that the AFB4 protein is a negative regulator of auxin response based on the auxin hypersensitivity of the afb4-2 mutant line. Recently we obtained a second afb4 allele and confirmed that it is an RNA null. This mutant is not auxin hypersensitive, indicating that our conclusion that AFB4 is a negative regulator of auxin response is incorrect. For this reason, we wish to retract the article. Other conclusions from the study, including the demonstration that AFB4 and AFB5 are auxin receptors and are the major targets of the synthetic auxin picloram, remain valid. The corresponding author wishes to apologize to the scientific community for any confusion that might have arisen from the study.