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.
Highlights
Higher plants are composed of more than 40 different cell types, each equipped with a distinct set of proteins, transcripts and metabolites, enabling them to fulfill various specialized functions
The decision to enter the hair cell fate appears to be made at different developmental stages among these three mechanisms, all root hairs derive from specialized epidermal cells named trichoblasts
A higher frequency of root hairs per unit area can derive from a decreased longitudinal length of epidermal cells, an increase in the number of root hair-forming cells leading to the formation of root hairs in non-hair positions, or a combination of the two
Summary
Higher plants are composed of more than 40 different cell types, each equipped with a distinct set of proteins, transcripts and metabolites, enabling them to fulfill various specialized functions. Almost all differentiated cells in plants posses ‘hidden’ totipotency and can, after perceiving the permissive array of signals, transdifferentiate into any cell type. This quality compensates for the sessile lifestyle of plants and allows for rapid adjustment of developmental programs after wounding, infection or in response to environmental signals. In Arabidopsis, root hairs develop from epidermal cells that are located in the H position. The decision to enter the hair cell fate appears to be made at different developmental stages among these three mechanisms (positionally cued, random, and asymmetric cell division), all root hairs derive from specialized epidermal cells named trichoblasts. The term trichoblast was introduced by Leavitt [4] to describe a group of specialized cells that are typically characterized by shorter cell length, a higher rate of cell division, higher cytoplasmic density, and a smaller degree of vacuolation, characteristics that are associated with higher metabolic activity [5]
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