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The phytohormone auxin plays crucial roles in nearly every aspect of plant growth and development. The auxin response factor (ARF) transcription factor family regulates auxin-responsive gene expression and exhibits nuclear localization in regions of high auxin responsiveness. Here we show that the ARF7 and ARF19 proteins accumulate in micron-sized assemblies within the cytoplasm of tissues with attenuated auxin responsiveness. We found that the intrinsically disordered middle region and the folded PB1 interaction domain of ARFs drive protein assembly formation. Mutation of a single lysine within the PB1 domain abrogates cytoplasmic assemblies, promotes ARF nuclear localization, and results in an altered transcriptome and morphological defects. Our data suggest a model in which ARF nucleo-cytoplasmic partitioning regulates auxin responsiveness, providing a mechanism for cellular competence for auxin signaling. • Condensation of transcription factors to attenuate hormone response • Mechanism for nucleo-cytoplasmic partitioning of transcription factors ARF transcription factors mediate the activity of the phytohormone auxin, regulating every aspect of plant development. Powers et al. determine that some ARFs undergo phase transition to form large-order cytoplasmic protein assemblies that limit auxin responsiveness in a developmentally relevant context, illustrating a strong link between condensate formation and biological function. [ABSTRACT FROM AUTHOR]

• Phosphoethanolamine methyltransferases (PMT) are critical for the growth and development of nematodes. • Analysis of PMT1 from the pine wilt nematode (Bursaphelenchus xylophilus) confirms biochemical function. • The crystal structure of BxPMT1 reveals how the enzyme functions to methylate substrates. • Site-directed mutagenesis identifies key residues on two surface loops as critical for phosphobase methylation.. In free-living and parasitic nematodes, the methylation of phosphoethanolamine to phosphocholine provides a key metabolite to sustain phospholipid biosynthesis for growth and development. Because the phosphoethanolamine methyltransferases (PMT) of nematodes are essential for normal growth and development, these enzymes are potential targets of inhibitor design. The pine wilt nematode (Bursaphelenchus xylophilus) causes extensive damage to trees used for lumber and paper in Asia. As a first step toward testing BxPMT1 as a potential nematicide target, we determined the 2.05 Å resolution x-ray crystal structure of the enzyme as a dead-end complex with phosphoethanolamine and S-adenosylhomocysteine. The three-dimensional structure of BxPMT1 served as a template for site-directed mutagenesis to probe the contribution of active site residues to catalysis and phosphoethanolamine binding using steady-state kinetic analysis. Biochemical analysis of the mutants identifies key residues on the β1d-α6 loop (W123F, M126I, and Y127F) and β1e-α7 loop (S155A, S160A, H170A, T178V, and Y180F) that form the phosphobase binding site and suggest that Tyr127 facilitates the methylation reaction in BxPMT1. [ABSTRACT FROM AUTHOR]

Developmental processes that control root system architecture are critical for soil exploration by plants, allowing for uptake of water and nutrients. Conversion of the auxin precursor indole-3-butyric acid (IBA) to active auxin (indole-3-acetic acid; IAA) modulates lateral root formation. However, mechanisms governing IBA-to-IAA conversion have yet to be elucidated. We identified TRANSPORTER OF IBA1 (TOB1) as a vacuolar IBA transporter that limits lateral root formation. Moreover, TOB1 , which is transcriptionally regulated by the phytohormone cytokinin, is necessary for the ability of cytokinin to exert inhibitory effects on lateral root production. The increased production of lateral roots in tob1 mutants, TOB1 transport of IBA into the vacuole, and cytokinin-regulated TOB1 expression provide a mechanism linking cytokinin signaling and IBA contribution to the auxin pool to tune root system architecture. • TRANSPORTER OF IBA1 (TOB1) identified as transporter of the auxin precursor IBA • TOB1 localizes to the vacuolar membrane • TOB1 regulates root system architecture (RSA) • TOB1 integrates cytokinin response and auxin homeostasis to regulate RSA Michniewicz and colleagues identify a vacuolar transporter of the auxin precursor indole-3-butyric acid (IBA) that regulates lateral root branching, which they named TOB1. TOB1 is transcriptionally regulated by the phytohormone cytokinin to alter IBA contributions to the auxin pool, thus providing a mechanism for cytokinin to regulate lateral root development. [ABSTRACT FROM AUTHOR]