title: PHABULOSA Mediates an Auxin Signaling Loop to Regulate Vascular Patterning in Arabidopsis.
abstract: Plant vascular tissues, xylem and phloem, differentiate in distinct patterns from procambial cells as an integral transport system for water, sugars, and signaling molecules. Procambium formation is promoted by high auxin levels activating class III homeodomain leucine zipper (HD-ZIP III) transcription factors (TFs). In the root of Arabidopsis (Arabidopsis thaliana), HD-ZIP III TFs dose-dependently govern the patterning of the xylem axis, with higher levels promoting metaxylem cell identity in the central axis and lower levels promoting protoxylem at its flanks. It is unclear, however, by what mechanisms the HD-ZIP III TFs control xylem axis patterning. Here, we present data suggesting that an important mechanism is their ability to moderate the auxin response. We found that changes in HD-ZIP III TF levels affect the expression of genes encoding core auxin response molecules. We show that one of the HD-ZIP III TFs, PHABULOSA, directly binds the promoter of both MONOPTEROS (MP)/AUXIN RESPONSE FACTOR5, a key factor in vascular formation, and IAA20, encoding an auxin/indole acetic acid protein that is stable in the presence of auxin and able to interact with and repress MP activity. The double mutant of IAA20 and its closest homolog IAA30 forms ectopic protoxylem, while overexpression of IAA30 causes discontinuous protoxylem and occasional ectopic metaxylem, similar to a weak loss-of-function mp mutant. Our results provide evidence that HD-ZIP III TFs directly affect the auxin response and mediate a feed-forward loop formed by MP and IAA20 that may focus and stabilize the auxin response during vascular patterning and the differentiation of xylem cell types.
authors.affiliation: Physiological Botany, Department of Organismal Biology and Linnean Centre for Plant Biology in Uppsala, Uppsala University, SE-756 51 Uppsala, Sweden.
authors.forename: Christina Joy
authors.identifier: http://orcid.org/0000-0002-8116-0705
authors.initials: CJ
authors.lastname: Müller
authors.affiliation: Physiological Botany, Department of Organismal Biology and Linnean Centre for Plant Biology in Uppsala, Uppsala University, SE-756 51 Uppsala, Sweden.
authors.forename: Ana Elisa
authors.initials: AE
authors.lastname: Valdés
authors.affiliation: Physiological Botany, Department of Organismal Biology and Linnean Centre for Plant Biology in Uppsala, Uppsala University, SE-756 51 Uppsala, Sweden.
authors.forename: Guodong
authors.initials: G
authors.lastname: Wang
authors.affiliation: Physiological Botany, Department of Organismal Biology and Linnean Centre for Plant Biology in Uppsala, Uppsala University, SE-756 51 Uppsala, Sweden.
authors.forename: Prashanth
authors.identifier: http://orcid.org/0000-0001-8447-1691
authors.initials: P
authors.lastname: Ramachandran
authors.affiliation: Physiological Botany, Department of Organismal Biology and Linnean Centre for Plant Biology in Uppsala, Uppsala University, SE-756 51 Uppsala, Sweden.
authors.forename: Lisa
authors.identifier: http://orcid.org/0000-0001-6437-3322
authors.initials: L
authors.lastname: Beste
authors.affiliation: Physiological Botany, Department of Organismal Biology and Linnean Centre for Plant Biology in Uppsala, Uppsala University, SE-756 51 Uppsala, Sweden.
authors.forename: Daniel
authors.identifier: http://orcid.org/0000-0002-4584-6478
authors.initials: D
authors.lastname: Uddenberg
authors.affiliation: Physiological Botany, Department of Organismal Biology and Linnean Centre for Plant Biology in Uppsala, Uppsala University, SE-756 51 Uppsala, Sweden annelie.carlsbecker@ebc.uu.se.
authors.forename: Annelie
authors.identifier: http://orcid.org/0000-0002-8450-3718
authors.initials: A
authors.lastname: Carlsbecker
mesh_terms: D017360:Arabidopsis / Q000196:embryology* / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D019521:Body Patterning; D018507:Gene Expression Regulation, Developmental; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D018745:Genome, Plant; D018398:Homeodomain Proteins; D007210:Indoleacetic Acids / Q000378:metabolism*; D018519:Meristem / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D058526:Plant Vascular Bundle / Q000196:embryology* / Q000235:genetics; D015398:Signal Transduction; D052584:Xylem / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D018398:Homeodomain Proteins; D007210:Indoleacetic Acids; C114647:PHB protein, Arabidopsis