Cellular trajectory analysis links tissue maturation to cellular specialization in the plant meristem II

The mechanisms that allow cells in the plant meristem to coordinate tissue-wide maturation gradients with specialized cell networks are critical for indeterminate growth. Here, we combine long-term imaging, single-cell transcriptomics, genetic analysis, and network modeling to reconstruct the differentiation trajectory of the 20 protophloem cells in the Arabidopsis root. We found that lineage bifurcation and cellular specification are mediated near the stem cell niche by PEAR regulators. However, many PEAR downstream effects are repressed by a broad gradient of PLETHORA transcription factors, which directly inhibit PEARs' own direct target APL until dissipating seven cells from the stem cell niche. Modeling and molecular epistasis suggest a so-called “seesaw” cascade of sharp transitions in which transcription factors regulating consecutive steps mutually inhibit each other's targets, ending in programmed enucleation. This analysis provides a mechanistic understanding of how morphogen-like maturation gradients interface with cell-type specific transcriptional regulators to stage cellular differentiation Overall design: RNA seq profiling of mutant and overexpressor lines