Fig. 1: Antagonistic auxin efflux impairments trigger similar non-random protophloem differentiation failures.
From: Local auxin competition explains fragmented differentiation patterns
a Schematic overview of a developing protophloem sieve element (PPSE) cell file in the Arabidopsis root meristem. b Confocal microscopy image of a 7-day-old wild type (Col-0) root meristem, propidium iodide (PI) cell wall staining (protophloem cell file marked by an asterisk, as hereafter). c, d Phenotypic range of brx or pax mutant root meristems. Brackets point out protophloem “gaps”, i.e., PPSE precursors that fail to differentiate. Arrowheads highlight an isolated differentiated PPSE. e, f Quantification of gap cell frequency (e) and gap size (f) in indicated genotypes. g Comparison of experimentally observed simulated gap-size distributions. Simulation of y-axis values indicates differentiation failure probability of an individual cell, total or split, as a function of differentiation failure in the preceding cell. h Overview of the models developed in this study. Left: idealized PPSE strand (SC stem cells, MZ meristematic zone, DZ differentiation zone). Cellular PIN and AUX1 levels dictate auxin transport dynamics (shoot-derived auxin supplied to the differentiation zone via bulk phloem sap). The model incorporates cellular growth, division, early expansion, and differentiation dynamics, causing individual cells to move from the meristematic to the differentiation zone. Stem cells undergo slow, meristematic cells rapid divisions; differentiation zone cells undergo early phases of elongation. Right, top: individual model cells contain a regulatory network governing BRX membrane occupancy, PAX and PIN phosphorylation, and auxin efflux dynamics (black). This model network is incrementally augmented with auxin-dependent AUX1 expression, then differentiation, and finally differentiation-dependent YUCCA expression (gray). Right, bottom: individual model cells have a polar PIN pattern, and an apolar AUX1 pattern. i Steady-state auxin profiles in wild type, brx, and pax mutant settings in the initial PSSE model. Dark red indicates the meristematic zone, blue the differentiation zone. Discrete jumps in auxin levels reflect the transition between distinct cells. Within cells, more graded auxin changes occur. Plots display individual values (dots) and their density distribution. See Source Data for raw measurements and statistical test details.
