Positional signaling and expression of ENHANCER OF TRY AND CPC1 are tuned to increase root hair density in response to phosphate deficiency in Arabidopsis thaliana

Abstract

Phosphate (Pi) deficiency induces a multitude of responses aimed at improving the acquisition of Pi, including an increased density of root hairs. To understand the mechanisms involved in Pi deficiency-induced alterations of the root hair phenotype in Arabidopsis (Arabidopsis thaliana), we analyzed the patterning and length of root epidermal cells under control and Pi-deficient conditions in wild-type plants and in four mutants defective in the expression of master regulators of cell fate, CAPRICE (CPC), ENHANCER OF TRY AND CPC 1 (ETC1), WEREWOLF (WER) and SCRAMBLED (SCM). From this analysis we deduced that the longitudinal cell length of root epidermal cells is dependent on the correct perception of a positional signal ('cortical bias') in both control and Pi-deficient plants; mutants defective in the receptor of the signal, SCM, produced short cells characteristic of root hair-forming cells (trichoblasts). Simulating the effect of cortical bias on the time-evolving probability of cell fate supports a scenario in which a compromised positional signal delays the time point at which non-hair cells opt out the default trichoblast pathway, resulting in short, trichoblast-like non-hair cells. Collectively, our data show that Pi-deficient plants increase root hair density by the formation of shorter cells, resulting in a higher frequency of hairs per unit root length, and additional trichoblast cell fate assignment via increased expression of ETC1.

Figure 1. Compiled confocal micrographs of roots from wild-type (wt) plants and mutant lines with defects in the expression of genes involved in cell fate acquisition grown under control and Pi-deficient conditions.

Scale bar = 150 µm.

Figure 2. Increase in root hair density in wild-type, cpc, scm and wer mutant plants.

A, Position-independent hair capture probability distributions. Boxplots drawn using MATLAB, 50% of the data points fall within the box, non-overlapping notches indicate the medians are significantly different with a P value less than or equal to 0.05. B, Position-dependent hair capture probability distributions. C, Strength of cortical bias under control (blue) and phosphate deficient (red) conditions.

Figure 3. Cell length linked to cortical bias.

Distributions of epidermal cell lengths for wild-type, cpc, scm and wer mutant plants grown under control conditions.

Figure 4. Delayed exit from default trichoblast pathway reduces atrichoblast elongation.

A, Cell fate determination schematics for wild type and scm. B, The absence of cortical bias results in increased cell fate resolution times for simulated epidermal cells (n = 5,000) for all parameter sets tested (see Table S1). C, WER/MYB23-complex expression probability time course data for 100 cells from WT and scm simulations (parameter set F, Table S1). When the expression probability is 1 (green), the cell has left the default trichoblast pathway.

Figure 5. Reduction in cell length is not sufficient to account for increased hair density under Pi-deficient conditions.

A, Distributions of epidermal cell lengths for wild-type, cpc, scm and wer mutant plants grown under control and Pi-deficient conditions. B, Root hair capture probability distributions for plants grown under control conditions (+Pi) compared with normalized capture probabilities for plants grown under Pi-deficient conditions (|−Pi|, dark blue box plots). The un-normalized capture probabilities for plants grown under Pi-deficient conditions are shown in light blue for comparison.

Figure 6. ETC1 provides an additional mechanism for increasing hair density under Pi-deficient conditions.

A, (Left) Root hair capture probability for etc1 plants grown under control and Pi-deficient conditions. A, (Middle) Cell length distributions for etc1 plants. A, (Right) Capture probability for etc1 plants grown under control conditions compared with the normalized capture probability of etc1–Pi plants. B, Root hair capture probability for cpc etc1 plants grown under control and Pi-deficient conditions.