Reprogramming of Stem Cell Activity to Convert Thorns into Branches

Curr Biol. 2020 Aug 3;30(15):2951-2961.e5. doi: 10.1016/j.cub.2020.05.068. Epub 2020 Jun 18.


Thorns arise from axillary shoot apical meristems that proliferate for a time and then terminally differentiate into a sharp tip. Like other meristems, thorn meristems contain stem cells but, in the case of thorns, these stem cells undergo a programmed cessation of proliferative activity. Using Citrus, we characterize a gene network necessary for thorn development. We identify two Citrus genes, THORN IDENTITY1 (TI1) and THORN IDENTITY2 (TI2), encoding TCP transcription factors, as necessary for stem cell quiescence and thorn identity. Disruption of TI1 and TI2 function results in reactivation of stem cells and concomitant conversion of thorns to branches. Expression of WUSCHEL (WUS) defines the shoot stem cell niche in the apical meristems of many angiosperm species; we show that TI1 binds to the Citrus WUS promoter and negatively regulates its expression to terminate stem cell proliferation. We propose that shifts in the timing and function of components of this gene network can account for the evolution of Citrus thorn identity. Modulating this pathway can significantly alter plant architecture and could be leveraged to improve crop yields.

Keywords: BRANCHED1; TCP transcription factor; WUSCHEL; cell proliferation; citrus; differentiation; meristem; stem cell; thorn; tree.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Cell Differentiation / genetics*
  • Cell Differentiation / physiology*
  • Cell Proliferation / genetics
  • Cell Proliferation / physiology
  • Citrus / anatomy & histology
  • Citrus / cytology
  • Citrus / genetics*
  • Citrus / physiology*
  • Gene Regulatory Networks / genetics*
  • Gene Regulatory Networks / physiology*
  • Genes, Plant / genetics*
  • Genes, Plant / physiology*
  • Meristem / cytology
  • Meristem / genetics*
  • Meristem / physiology*
  • Mutation*
  • Plant Physiological Phenomena / genetics*
  • Plant Proteins / genetics*
  • Plant Proteins / physiology*
  • Stem Cells / physiology*


  • Plant Proteins