title: Strigolactone- and Karrikin-Independent SMXL Proteins Are Central Regulators of Phloem Formation.
abstract: Plant stem cell niches, the meristems, require long-distance transport of energy metabolites and signaling molecules along the phloem tissue. However, currently it is unclear how specification of phloem cells is controlled. Here we show that the genes SUPPRESSOR OF MAX2 1-LIKE3 (SMXL3), SMXL4, and SMXL5 act as cell-autonomous key regulators of phloem formation in Arabidopsis thaliana. The three genes form an uncharacterized subclade of the SMXL gene family that mediates hormonal strigolactone and karrikin signaling. Strigolactones are endogenous signaling molecules regulating shoot and root branching [1] whereas exogenous karrikin molecules induce germination after wildfires [2]. Both activities depend on the F-box protein and SCF (Skp, Cullin, F-box) complex component MORE AXILLARY GROWTH2 (MAX2) [3-5]. Strigolactone and karrikin perception leads to MAX2-dependent degradation of distinct SMXL protein family members, which is key for mediating hormonal effects [6-12]. However, the nature of events immediately downstream of SMXL protein degradation and whether all SMXL proteins mediate strigolactone or karrikin signaling is unknown. In this study we demonstrate that, within the SMXL gene family, specifically SMXL3/4/5 deficiency results in strong defects in phloem formation, altered sugar accumulation, and seedling lethality. By comparing protein stabilities, we show that SMXL3/4/5 proteins function differently to canonical strigolactone and karrikin signaling mediators, although being functionally interchangeable with those under low strigolactone/karrikin signaling conditions. Our observations reveal a fundamental mechanism of phloem formation and indicate that diversity of SMXL protein functions is essential for a steady fuelling of plant meristems.
authors.affiliation: Centre for Organismal Studies (COS), Heidelberg University, Im Neuenheimer Feld 230, 69120 Heidelberg, Germany.
authors.forename: Eva-Sophie
authors.initials: ES
authors.lastname: Wallner
authors.affiliation: Centre for Organismal Studies (COS), Heidelberg University, Im Neuenheimer Feld 230, 69120 Heidelberg, Germany.
authors.forename: Vadir
authors.initials: V
authors.lastname: López-Salmerón
authors.affiliation: Institute of Biotechnology, University of Helsinki, Viikinkaari 5d, 00014 Helsinki, Finland.
authors.forename: Ilya
authors.initials: I
authors.lastname: Belevich
authors.affiliation: Centre for Organismal Studies (COS), Heidelberg University, Im Neuenheimer Feld 230, 69120 Heidelberg, Germany.
authors.forename: Gernot
authors.initials: G
authors.lastname: Poschet
authors.affiliation: Centre for Organismal Studies (COS), Heidelberg University, Im Neuenheimer Feld 230, 69120 Heidelberg, Germany.
authors.forename: Ilona
authors.initials: I
authors.lastname: Jung
authors.affiliation: Gregor Mendel Institute (GMI), Austrian Academy of Sciences, Vienna Biocenter (VBC), Dr. Bohr-Gasse 3, 1030 Vienna, Austria.
authors.forename: Karin
authors.initials: K
authors.lastname: Grünwald
authors.affiliation: Institute of Biotechnology, University of Helsinki, Viikinkaari 5d, 00014 Helsinki, Finland; Department of Biological and Environmental Sciences, University of Helsinki, 00014 Helsinki, Finland.
authors.forename: Iris
authors.initials: I
authors.lastname: Sevilem
authors.affiliation: Institute of Biotechnology, University of Helsinki, Viikinkaari 5d, 00014 Helsinki, Finland.
authors.forename: Eija
authors.initials: E
authors.lastname: Jokitalo
authors.affiliation: Centre for Organismal Studies (COS), Heidelberg University, Im Neuenheimer Feld 230, 69120 Heidelberg, Germany.
authors.forename: Rüdiger
authors.initials: R
authors.lastname: Hell
authors.affiliation: Institute of Biotechnology, University of Helsinki, Viikinkaari 5d, 00014 Helsinki, Finland; Department of Biological and Environmental Sciences, University of Helsinki, 00014 Helsinki, Finland; Sainsbury Laboratory, University of Cambridge, Bateman Street, Cambridge CB2 1LR, UK.
authors.forename: Yrjö
authors.initials: Y
authors.lastname: Helariutta
authors.affiliation: Gregor Mendel Institute (GMI), Austrian Academy of Sciences, Vienna Biocenter (VBC), Dr. Bohr-Gasse 3, 1030 Vienna, Austria.
authors.forename: Javier
authors.initials: J
authors.lastname: Agustí
authors.affiliation: Centre for Organismal Studies (COS), Heidelberg University, Im Neuenheimer Feld 230, 69120 Heidelberg, Germany.
authors.forename: Ivan
authors.initials: I
authors.lastname: Lebovka
authors.affiliation: Centre for Organismal Studies (COS), Heidelberg University, Im Neuenheimer Feld 230, 69120 Heidelberg, Germany. Electronic address: thomas.greb@cos.uni-heidelberg.de.
authors.forename: Thomas
authors.initials: T
authors.lastname: Greb
mesh_terms: D017360:Arabidopsis / Q000187:drug effects / Q000254:growth & development* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant / Q000187:drug effects*; D007783:Lactones / Q000494:pharmacology*; D052585:Phloem / Q000187:drug effects / Q000254:growth & development* / Q000378:metabolism; D010937:Plant Growth Regulators / Q000494:pharmacology*; D015398:Signal Transduction / Q000187:drug effects
chemical_list: D029681:Arabidopsis Proteins; D007783:Lactones; D010937:Plant Growth Regulators
keywords: D53; MAX2; SMXL; karrikin; meristem; phloem; root; sieve elements; strigolactone