提出问题：Like other meristems, thorn meristems contain stem cells but, in the case of thorns, these stem cells undergo a programmed cessation of proliferative activity.

主要发现：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.

试验结果：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.

结论：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.