Umeå University's logo

Apical hook development is an ideal model for studying differential growth in plants and is controlled by complex phytohormonal crosstalk, with auxin being the major player. Here, we identified a bioactive small molecule that decelerates apical hook opening in Arabidopsis thaliana. Our genetic studies suggest that this molecule enhances or maintains the auxin maximum found in the inner hook side and requires certain auxin signaling components to modulate apical hook opening. Using biochemical approaches, we then revealed the WD40 repeat scaffold protein RECEPTOR FOR ACTIVATED C KINASE 1A (RACK1A) as a direct target of this compound. We present data in support of RACK1A playing a positive role in apical hook opening by activating specific auxin signaling mechanisms and negatively regulating the differential auxin response gradient across the hook, thereby adjusting differential cell growth, an essential process for organ structure and function in plants.

Distribution of auxin within plant tissues is of great importance for developmental plasticity, including root gravitropic growth. Auxin flow is directed by the subcellular polar distribution and dynamic relocalisation of auxin transporters such as the PIN‐FORMED (PIN) efflux carriers, which can be influenced by the main natural plant auxin indole‐3‐acetic acid (IAA). Anthranilic acid (AA) is an important early precursor of IAA and previously published studies with AA analogues have suggested that AA may also regulate PIN localisation.

Using Arabidopsis thaliana as a model species, we studied an AA‐deficient mutant displaying agravitropic root growth, treated seedlings with AA and AA analogues and transformed lines to over‐produce AA while inhibiting its conversion to downstream IAA precursors.

We showed that AA rescues root gravitropic growth in the AA‐deficient mutant at concentrations that do not rescue IAA levels. Overproduction of AA affects root gravitropism without affecting IAA levels. Treatments with, or deficiency in, AA result in defects in PIN polarity and gravistimulus‐induced PIN relocalisation in root cells.

Our results revealed a previously unknown role for AA in the regulation of PIN subcellular localisation and dynamics involved in root gravitropism, which is independent of its better known role in IAA biosynthesis.

The plant hormone auxin coordinates almost all aspects of plant development. Throughout plant life, the expression of hundreds of genes involved in auxin regulation is orchestrated via several combinatorial and cell-specific auxin perception systems. An effective approach to dissect these complex pathways is the use of synthetic molecules that target specific processes of auxin activity. Here, we describe synthetic auxins, RubNeddins (RNs), which act as selective auxin agonists. The RN with the greatest potential for dissecting auxin perception was RN4, which we used to reveal a role for the chromatin remodeling ATPase BRAHMA in apical hook development. Therefore, the understanding of RN mode of action paves the way to dissecting specific molecular components involved in auxin-regulated developmental processes.Auxin phytohormones control most aspects of plant development through a complex and interconnected signaling network. In the presence of auxin, AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) transcriptional repressors are targeted for degradation by the SKP1-CULLIN1-F-BOX (SCF) ubiquitin-protein ligases containing TRANSPORT INHIBITOR RESISTANT 1/AUXIN SIGNALING F-BOX (TIR1/AFB). CULLIN1-neddylation is required for SCFTIR1/AFB functionality, as exemplified by mutants deficient in the NEDD8-activating enzyme subunit AUXIN-RESISTANT 1 (AXR1). Here, we report a chemical biology screen that identifies small molecules requiring AXR1 to modulate plant development. We selected four molecules of interest, RubNeddin 1 to 4 (RN1 to -4), among which RN3 and RN4 trigger selective auxin responses at transcriptional, biochemical, and morphological levels. This selective activity is explained by their ability to consistently promote the interaction between TIR1 and a specific subset of AUX/IAA proteins, stimulating the degradation of particular AUX/IAA combinations. Finally, we performed a genetic screen using RN4, the RN with the greatest potential for dissecting auxin perception, which revealed that the chromatin remodeling ATPase BRAHMA is implicated in auxin-mediated apical hook development. These results demonstrate the power of selective auxin agonists to dissect auxin perception for plant developmental functions, as well as offering opportunities to discover new molecular players involved in auxin responses.

Regio- and stereoselective addition of alkyl Grignard reagents to pyridine-N-oxides gave C2-alkylated N-hydroxy-1,2,5,6-tetrahydropyridines and trans-2,3-disubstituted N-hydroxy-1,2,5,6-tetrahydropyridines in good to excellent yields. These intermediates were aromatized or alternatively reduced in one-pot methodologies for efficient syntheses of alkylpyridines or piperidines, respectively. These reactions have a broad substrate scope and short reaction times.