Characteristics of a root hair-less line of Arabidopsis thaliana under physiological stresses

Elucidating molecular links between cell-fate regulatory networks and dynamic patterning modules is a key for understanding development. Auxin is important for plant patterning, particularly in roots, where it establishes positional information for cell-fate decisions. PIN genes encode plasma membrane proteins that serve as auxin efflux transporters; mutations in members of this gene family exhibit smaller roots with altered root meristems and stem-cell patterning. Direct regulators of PIN transcription have remained elusive. Here, we establish that a MADS-box gene (XAANTAL2, XAL2/AGL14) controls auxin transport via PIN transcriptional regulation during Arabidopsis root development; mutations in this gene exhibit altered stem-cell patterning, root meristem size, and root growth. XAL2 is necessary for normal shootward and rootward auxin transport, as well as for maintaining normal auxin distribution within the root. Furthermore, this MADS-domain transcription factor upregulates PIN1 and PIN4 by direct binding to regulatory regions and it is required for PIN4-dependent auxin response. In turn, XAL2 expression is regulated by auxin levels thus establishing a positive feedback loop between auxin levels and PIN regulation that is likely to be important for robust root patterning.

Peer reviewed

Root hairs develop from specialised root epidermis cells, called trichoblasts. The geometrical arrangement of root hairs on roots makes them very suitable for intercepting and absorbing diffusion limited nutrients such as phosphorus (P) and potassium. Our experiments, where only root hairs were allowed to penetrate P-32 labelled soil, have confirmed the significant role of root hairs in P uptake. The significant variation in root hairs of cereal cultivars in the laboratory and under field conditions suggests that it may be possible to upgrade P efficiency of cereals by genetic manipulation of root hairs. We discovered and characterised a spontaneous barley mutant (Hordeum vulgare L.) that completely lacks the ability to develop root hairs. Amplified Fragment Length Polymorphisms (AFLP) analysis of the genomes of the mutant and spring barley cultivar Pallas confirmed it to be a mutant of spring barley cultivar Pallas (wild type). While detailed root anatomical and genetic studies of the mutation are underway, we have studied the ability of the mutant (lacking root hairs) and the wild type (producing normal root hairs) to absorb phosphorus from soil in the vicinity of their roots.The variation in P depletion profiles in the rhizosphere showed that the wild type absorbed nearly two times more phosphorus than the mutant. The P depletion profile of Pallas extended to 0.8 mm from root surface, which is equal to its root hair length. A mathematical model based on the theory of diffusion and mass flow explained the observed P depletion profile of the mutant, but not that of wild type. This further confirmed the role of root hairs in phosphorus uptake from soil near root surface.

The role of root hairs in cadmium acquisition by barley

Enhancement of root hair development in response to phosphate (Pi) deficit has been reported extensively. Root hairs are involved in major root functions such as the absorption of water, acquisition of nutrients and secretion of organic acids and enzymes. Individual root hair cells maintain these functions and appropriate structure under various physiological conditions. We carried out a study to identify protein(s) which maintain the structure and function of root hairs, and identified a protein (SEED AND ROOT HAIR PROTECTIVE PROTEIN, SRPP) that was induced in root hairs under Pi-deficient conditions. Promoter assay and mRNA quantification revealed that SRPP was expressed in root hairs and seeds. A knockout mutant, srpp-1, consistently displayed defects in root hairs and seeds. Root hairs in srpp-1 were short and the phenotypes observed under Pi-deficient conditions were also detected in ethylene-treated srpp-1 plants. Propidium iodide stained most root hairs of srpp-1 grown under Pi-deficient conditions, suggesting cell death. In addition to root hairs, most srpp-1 seeds were withered and their embryos were dead. SRPP tagged with green fluorescent protein was detected in the cell wall. Electron microscopy showed abnormal morphology of the cell wall. Wild-type phenotypes were restored when the SRPP gene was expressed in srpp-1. These data strongly suggest that SRPP contributes to the construction of robust cell walls, whereby it plays a key role in the development of root hairs and seeds.

Root hairs play an important role in the acquisition of soil resources by increasing the absorptive surface area. Yet, key factors driving the variation of root hair traits across different species along biogeographic gradients are still poorly quantified, limiting our understanding of the functional relevance of root hairs. We measured root hair length, diameter and areal density of 75 xerophytic species across a 1000‐km latitudinal gradient in a dry valley system. The influences of phylogeny, environment and fine‐root morpho‐anatomical traits on root hairs were quantified. We found that 95% of sampled species had root hairs, total absorptive root area increased by roughly 0.3% for species with very few root hairs, and up to a 180% for species with large and dense root hairs. Phylogeny did not appear to be a significant factor influencing root hair traits. Root hair diameter and length were positively related to root diameter and cortex thickness, while root hair densities were negatively related to cortex thickness and root diameter. Across latitudes, mean annual precipitation (MAP) was the main factor driving the variations in root hairs as hair density generally increased with higher MAP. Synthesis. Root hairs were prevalent in native species in these arid ecosystems. Variation in root hair traits was more directly dependent on root traits rather than phylogeny across species. Climate was the main driver of biogeographic patterns of root hair density. These results provide a more comprehensive understanding of the fine‐root foraging strategies and the role of root hairs within the root economics space. Read the free Plain Language Summary for this article on the Journal blog.

Root hairs substantially extend root surface for ion uptake. Although many reports suggest a relationship between root hairs and phosphorus (P) uptake of plants, the role of root hairs in phosphorus uptake from soils is still debated. We measured uptake of phosphorus from soil directly via root hairs. Root hairs only were allowed to penetrate through a tightly stretched nylon screen (53 µm) glued to the bottom of a PVC tube. The penetrating root hairs grew for 2 and 4 days in soil labelled with radioisotope phosphorus (P) tracer 32P (185 kBq g-1 dry soil) filled in another PVC tube. Transparent plastic rings of thickness ranging from 0.25 mm to 2.0 mm were inserted between the two PVC tubes. This provided slit width for microscopic observations in situ, which confirmed that only root hairs were growing into the 32P labelled soil. In some cases no rings were inserted (slit width = 0) where both root hairs and root surface were in contact with the labelled soil (total 32P uptake). The uptake of32 P from soil via the root hairs only was quantified by measuring activity of 32P in the plant shoot (32P uptake only via root hairs). The results showed that when 70 percent of the root hairs grew into the labelled soil, they contributed to 63 percent of the total P uptake. With decreasing number of root hairs growing into the 32P labelled soil, the quantity of 32P in the plant shoot decreased. In this study, P uptake via root hairs was measured in a soil-based system, where root hairs were the only pathway of 32P from soil to the plant shoot. Therefore, this study provides a strong evidence on the substantial participation of root hairs in uptake of phosphorus from soil.

Plants' roots promote changes in soil structure, forming a strongly-bound soil layer in the surroundings of the root, which is named as rhizosheath. Rhizosheath formation is attributed mainly to the root hairs' presence, that favors the enmeshment of the soil particles around the roots, and the release of mucilage and exudates, which acts as gluing agents of those soil particles. In the present work, we studied the rhizosheath aggregate formation of two Zea mays L. genotypes with contrasting root hair development: a mutant with root hair defective elongation (rth3) and a corresponding wild type (WT). We also tracked the fate of recently-deposited C in the rhizosheath aggregates using two 13CO2 pulse labeling approaches (single vs. multiple pulse labeling). The sampled rhizosheath aggregates were further separated using dry-sieving fractionation into three aggregate size classes: primary small particles and smaller microaggregates (<53 µm), larger microaggregates (53-250 µm) and macroaggregates (>250 µm). We observed that the aggregate size distribution followed the same pattern in both genotypes. This result reinforces the assumption that other soil properties are more important for rhizosheath aggregation than root hair elongation. We observed that the higher potion of the recently-deposited root-derived C (57%) was accumulated in the macroaggregates. Moreover, the multiple pulse labeling approach proportioned a higher 13C enrichment of the rhizosheath aggregates fractions than applying a single pulse. Despite both single and multiple labeling approaches have resulted in a similar distribution of 13C in the rhizosheath aggregates, multiple pulse labeling provided a higher enrichment in the rhizosheath aggregates, which allowed a better separation of significant differences between the genotypes.

Root hairs, tubular protrusions of epidermal root cells, are considered a key rhizosphere feature: by substantially increasing the contact area between roots and soil, they enhance the ability of plants to capture soil resources. Hence, they are considered a breeding target for improving drought tolerance and yield stability of crops. While their pivotal role in the uptake of immobile nutrients such as phosphorus is well accepted, their effect on root water uptake remains controversial as it varies across plant species. By means of image-based modelling, our objective was to identify environmental conditions (e.g. soil water content) and hair traits (e.g. root hair length and density) that determine the effectiveness of root hairs in root water uptake. Furthermore, we investigated the effect of drought stress-induced root hair shrinkage on root water uptake.We scanned root compartments of 8 days old maize seedlings (Zea Mays L.) grown in a loamy soil using synchrotron radiation X-ray CT. Based on the collected image-data, we implemented a 3D root water uptake model. By solving Richards equation numerically, we computed the propagation of water potential gradients across the root-soil continuum which allowed to quantify root water uptake. The high spatial resolution of the acquired images enabled us to explicitly take rhizosphere features, such as root hairs and root-soil matrix contact into account. We determined the key parameters governing the effectiveness of root hairs in water uptake by comparing a set of six maize root compartments before and after digitally removing their hairs. The quantification of root hair turgor-loss in response to progressive soil drying allowed us to implement hair shrinkage within our model.We found that the effect of root hairs in root water uptake is governed by 1) the root hair induced increase in root soil contact and 2) root hair length. Furthermore, our results suggest that root hairs potentially facilitate root water uptake under dry soil conditions (< -0.1MPa). However, in the dry range, root hair shrinkage severely reduces the effect of hairs. Depending on their turgor-loss curve, root hairs may still provide a positive effect on root water uptake in a narrow range of soil matric potential. In summary, the effect of root hairs on root water uptake depends on soil water content, root-soil contact, root hair length and the turgor-loss point of hairs.

Sufficient water is essential for plant growth and production. Root hairs connect roots to the soil, extend the effective root radius, and greatly enlarge the absorbing surface area. Although the efficacy of root hairs in nutrient uptake, especially phosphorus, has been well recognized, their role in water uptake remains contentious. Here we review recent advances in this field, discuss the factors affecting the role of root hairs in water uptake, and propose future directions. We argue that root hair length and shrinkage, in response to soil drying, explain the apparently contradictory evidence currently available. Our analysis revealed that shorter and vulnerable root hairs (i.e. rice and maize) made little, if any, contribution to root water uptake. In contrast, relatively longer root hairs (i.e. barley) had a clear influence on root water uptake, transpiration, and hence plant response to soil drying. We conclude that the role of root hairs in water uptake is species (and probably soil) specific. We propose that a holistic understanding of the efficacy of root hairs in water uptake will require detailed studies of root hair length, turnover, and shrinkage in different species and contrasting soil textures.

Root hairs are tubular extensions of epidermal cells found on roots of most vascular plant species (Cormack 1962; Hofer 1991; Peterson and Farquhar 1996). There has been considerable debate as to the role of root hairs in nutrient uptake and observations of the normal growth of shoots of two of three root hair mutant lines of Zea mays L. (maize) suggest that they may not always play a significant role (Wen and Schnable 1994). Root hairs when present, however, extend the absorbing surface of roots and therefore presumably affect the uptake of nutrients in the vicinity of the root cylinder (Jungk 1991). Clarkson (1991) argues that if the density of root hairs is great enough, the depletion zones of adjacent hairs will overlap thus most available ions in the soil nutrient solution between adjacent root hairs will be available for uptake. The distance away from the depletion zone surrounding the root cylinder to which root hairs elongate extends the region of the rhizosphere from which nutrients can be absorbed (Nye 1966; and Fig. 1). There has been some interest, therefore, in using genotypes of agricultural species with long root hairs to enhance nutrient uptake. This characteristic, as well as the ability of root hairs to grow into small soil pores and into soil particles, could positively affect nutrient uptake. This would be of particular importance in terms of ions such as phosphate that are bound to soil fractions and are essentially immobile (Jungk 1991).

A Reservoir of Pluripotent Phloem Cells Safeguards the Linear Developmental Trajectory of Protophloem Sieve Elements.

Rapid alkalinization factor (RALF) is a 49-amino-acid peptide that rapidly alkalinizes cultivated tobacco cell cultures. In the native tobacco Nicotiana attenuata, NaRALF occurs as a single-copy gene and is highly expressed in roots and petioles. Silencing the NaRALF transcript by transforming N. attenuata with an inverted-repeat construct generated plants (irRALF) with normal wild-type (WT) above-ground parts, but with roots that grew longer and produced trichoblasts that developed into abnormal root hairs. Most trichoblasts produced a localized 'bulge' without commencing root hair tip growth; fewer trichoblasts grew, but were only 10% as long as those of WT plants. The root hair phenotype was associated with slowed apoplastic pH oscillations, increased pH at the tips of trichoblasts and decreased accumulation of reactive oxygen species in the root hair initiation zone. The root hair growth phenotype was partially restored when irRALF lines were grown in a low-pH-buffered medium, and reproduced in WT plants grown in a high-pH-buffered medium. When irRALF plants were grown in pH 5.6, 6.7 and 8.1 soils together with WT plants in glasshouse experiments, they were out-competed by WT plants in basic, but not acidic, soils. When WT and irRALF lines were planted into the basic soils of the native habitat of N. attenuata in the Great Basin Desert, irRALF plants had smaller leaves, shorter stalks, and produced fewer flowers and seed capsules than did WT plants. We conclude that NaRALF is required for regulating root hair extracellular pH, the transition from root hair initiation to tip growth and plant growth in basic soils.

Root proliferation is a response to a heterogeneous nutrient distribution. However, the growth of root hairs in response to heterogeneous nutrients and the relationship between root hairs and lateral roots remain unclear. This study aims to understand the effects of heterogeneous nutrients on root hair growth and the trade-off between root hairs and lateral roots in phosphorus (P) acquisition. Near-isogenic maize lines, the B73 wild type (WT) and the rth3 root hairless mutant, were grown in rhizoboxes with uniform or localized supply of 40 (low) or 140 (high) mg P kg-1 soil. Both WT and rth3 had nearly two-fold greater shoot biomass and P content under local than uniform treatment at low P. Significant root proliferation was observed in both WT and rth3 in the nutrient patch, with the WT accompanied by an obvious increase (from 0.7 to 1.2 mm) in root hair length. The root response ratio of rth3 was greater than that of WT at low P, but could not completely compensate for the loss of root hairs. This suggests that plants enhanced P acquisition through complementarity between lateral roots and root hairs, and thus regulated nutrient foraging and shoot growth. The disappearance of WT and rth3 root response differences at high P indicated that the P application reduced the dependence of the plants on specific root traits to obtain nutrients. In addition to root proliferation, the root response to a nutrient-rich patch was also accompanied by root hair elongation. The genotypes without root hairs increased their investment in lateral roots in a nutrient-rich patch to compensate for the absence of root hairs, suggesting that plants enhanced nutrient acquisition by regulating the trade-off of complementary root traits.

Ubiquitin-mediated protein modification plays a key role in many cellular signal transduction pathways. The Arabidopsis gene XBAT32 encodes a protein containing an ankyrin repeat domain at the N-terminal half and a RING finger motif. The XBAT32 protein is capable of ubiquitinating itself. Mutation in XBAT32 causes a number of phenotypes including severe defects in lateral root production and in the expression of the cell division marker CYCB1;1::GUS. The XBAT32 gene is expressed abundantly in the vascular system of the primary root, but not in newly formed lateral root primordia. Treatment with auxin increases the expression of XBAT32 in the primary root and partially rescues the lateral root defect in xbat32-1 mutant plants. Thus, XBAT32 is a novel ubiquitin ligase required for lateral root initiation.