The clinical and developmental applications of this new measure are discussed.

SummaryThe colonization of the land by plants, sometime before 470 million years ago, was accompanied by the evolution tissue systems [1, 2, 3]. Specialized structures with diverse functions—from nutrient acquisition to reproduction—derived from single cells in the outermost layer (epidermis) were important sources of morphological innovation at this time [2, 4, 5]. In extant plants, these structures may be unicellular extensions, such as root hairs or rhizoids [6, 7, 8, 9], or multicellular structures, such as asexual propagules or secretory hairs (papillae) [10, 11, 12]. Here, we show that a ROOTHAIR DEFECTIVE SIX-LIKE (RSL) class I basic helix-loop-helix transcription factor positively regulates the development of the unicellular and multicellular structures that develop from individual cells that expand out of the epidermal plane of the liverwort Marchantia polymorpha; mutants that lack MpRSL1 function do not develop rhizoids, slime papillae, mucilage papillae, or gemmae. Furthermore, we discovered that RSL class I genes are also required for the development of multicellular axillary hairs on the gametophyte of the moss Physcomitrella patens. Because class I RSL proteins also control the development of rhizoids in mosses and root hairs in angiosperms [13, 14], these data demonstrate that the function of RSL class I genes was to control the development of structures derived from single epidermal cells in the common ancestor of the land plants. Class I RSL genes therefore controlled the generation of adaptive morphological diversity as plants colonized the land from the water.

Cell size is determined by the duration and rate of growth and plays a central role in cell function. Root hairs are tip-growing cellular projections that emerge from the root epidermis and explore the soil to acquire nutrients and water(1). Previously we demonstrated that the basic helix-loop-helix transcription factor root hair defective 6-like 4 (RSL4) is necessary and sufficient for root hair growth(2). Here we show that RSL4 is synthesized in a 4-h pulse at the initiation of hair elongation and is gradually degraded by the 26S proteasome. The amount of RSL4 synthesis during this pulse is modulated as part of a root hair growth response to low phosphate. RSL4 synthesis increases in low phosphate and this increase prolongs the growth phase, resulting in the development of long root hairs. Our data demonstrate that the amount of RSL4 synthesized during the pulse directly determines the final size of the differentiated root hair cell. We propose that the modulation of growth-promoting transcription factors by external cues could be a general mechanism for the regulation of cell growth by environmental factors during development.

SummaryTo discover mechanisms that controlled the growth of the rooting system in the earliest land plants, we identified genes that control the development of rhizoids in the liverwort Marchantia polymorpha. 336,000 T-DNA transformed lines were screened for mutants with defects in rhizoid growth, and a de novo genome assembly was generated to identify the mutant genes. We report the identification of 33 genes required for rhizoid growth, of which 6 had not previously been functionally characterized in green plants. We demonstrate that members of the same orthogroup are active in cell wall synthesis, cell wall integrity sensing, and vesicle trafficking during M. polymorpha rhizoid and Arabidopsis thaliana root hair growth. This indicates that the mechanism for constructing the cell surface of tip-growing rooting cells is conserved among land plants and was active in the earliest land plants that existed sometime more than 470 million years ago [1, 2].

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