Wood is formed through the activity of a specialized lateral meristem, the so called cambium. Although the cambium evolved after the occurrence of apical growth, regulators of stem cell activity and maintenance are partially conserved across the different types of meristems. Since different types of meristems form different organs and tissues, divergence of meristematic regulators can be expected at the level of cell differentiation. In order to unravel mechanisms specific to the differentiation of woody cells we are studying cambial functions of homologs of transcription factors with known roles during organogenesis in the shoot apical meristem. To this end we make use of the simple wood anatomy of the Arabidopsis hypocotyl and analyze differentiation of cambial derivatives in loss-of-function mutants. We are currently focusing on a pair of transcription factors, which is expressed across all the meristems. Interestingly, these transcription factors repress differentiation in the shoot apical meristem but promote the formation of secondary xylem in the hypocotyl.

Urs Fischer 1150In both the shoot apical meristem and the cambium, the plant hormone auxin is thought to be an important regulator of cell differentiation. A radial auxin gradient spanning wood-forming tissues has been suggested to be instructive for cell division, expansion and terminal differentiation of cambial derivatives. We recently discovered a first component of the auxin transport machinery, WAT1, which is directly involved in the differentiation of secondary cell walls. Defective wall differentiation in wat1 mutants can be completely rescued by local applications of auxin. The WAT1 protein, which we localized to the tonoplast, can facilitate the export of auxin from the vacuole to the cytoplasm. We are now studying how WAT1 interacts with other auxin transporters and to which extent it is involved in the generation of auxin gradients.
figure1 wood formationFigure 1. Wood formation in the Arabidopsis hypocotyl. Various degrees of xylem formation and differentiation in different ecotypes.

While for metazoan model systems a plethora of markers for consecutive steps of differentiation and cell types are available plant researchers have interpreted differentiation processes mainly on the basis of the position of a cell in a tissue/organ context, histologic features and activity of reporter genes. We think that chemical properties of the cell wall are typical for distinct stages of differentiation and for specific cell types. Therefore, we are currently building up a library of monoclonal antibodies against cell wall epitopes in order to obtain stable markers for cell differentiation in the Arabidopsis hypocotyl. Along with this initiative we implement state-of-the-art imaging analysis in order to obtain quantitative data of morphometric parameters and fluorescence signal.

Together with an industrial partner we are trying to transfer our knowledge about the differentiation of secondary cell walls into economically important poplar trees with the goal of tailoring wood properties to industrial needs.

figure2 image analysisFigure 2. Image analysis pipeline. Vascular bundle. A) Immunohistochemistry with a primary antibody against a specific pectin epitope. B) Watershedding. C) Binary image. D) Automatic object recognition. Fluorescence and morphometric parameters for each object can be determined.

Publications list

  1. A Local Auxin Gradient Regulates Root Cap Self-Renewal and Size Homeostasis
    Current Biology 2018, 28(16):2581-+
  2. The plant hormone auxin directs timing of xylem development by inhibition of secondary cell wall deposition through repression of secondary wall NAC-domain transcription factors
    Physiol Plant. 2018 May 28 [Epub ahead of print]
  3. AspWood: High-spatial-resolution transcriptome profiles reveal uncharacterized modularity of wood formation in Populus tremula
    Plant Cell. 2017, 29 (7):1585-1604
  4. The ERECTA and ERECTA-like genes control a developmental shift during xylem formation in Arabidopsis
    New Phytol. 2017 Mar;213(4):1562-1563
  5. Environmental and hormonal control of cambial stem cell dynamics
    J Exp Bot. 2017, 68(1):79-87
  6. Precision Automation of Cell Type Classification and Sub-Cellular Fluorescence Quantification from Laser Scanning Confocal Images
    Front Plant Sci. 2016 Feb 9;7:119.eCollection 2016
  7. Conservation of the abscission signaling peptide IDA during Angiosperm evolution: withstanding genome duplications and gain and loss of the receptors HAE/HSL2
    Front Plant Sci. 2015 Oct 30;6:931. eCollection 2015
  8. Auxin is a long-range signal that acts independently of ethylene signaling on leaf abscission in Populus
    Front Plant Sci. 2015, 6:634
  9. Class I KNOX transcription factors promote differentiation of cambial derivatives into xylem fibers in the Arabidopsis hypocotyl
    Development 2014, 141(22):4311-4319
  10. Auxin gradients across wood - instructive or incidental?
    Physiol Plant. 2014, 151(1):43-51
  11. Arabidopsis WAT1 is a vacuolar auxin transport facilitator required for auxin homoeostasis
    Nat Commun. 2013; 4:2625
  12. Ikeda Y, Men S, Fischer U, Stepanova AN, Alonso JM, Ljung K, Grebe M
    Local auxin biosynthesis modulates gradient-directed planar polarity in Arabidopsis
    Nature Cell Biology: 2009 11:731-738
  13. Singh SK, Fischer U, Kumar M, Grebe M, Marchant A
    Insight into the early steps of root hair formation revealed by the procuste1 cellulose synthase mutant of Arabidopsis thaliana
    BMC Plant Biology: 2008 8:57
  14. Fischer U, Ikeda Y, Grebe M
    Planar polarity of root hair positioning in Arabidopsis
    Biochemical Society Transaction: 2007 35:149-151
  15. Fischer U, Ikeda Y, Ljung K, Serralbo O, Singh M, Heidstra R, Palme K, Scheres B, Grebe M
    Vectorial information for Arabidopsis planar polarity is mediated by combined AUX1, EIN2, and GNOM activity
    Current Biology: 2006 16:2143-2149
  16. Fischer U, Men S, Grebe M
    Lipid function in plant cell polarity
    Curr Opin Plant Biol: 2004 7:670-676