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Title: Root Suberin Forms an Extracellular Barrier That Affects Water Relations and Mineral Nutrition in Arabidopsis}
Article:
Abstract:
Though central to our understanding of how roots perform their vital function of scavenging water and solutes from the soil, no direct genetic evidence currently exists to support the foundational model that suberin acts to form a chemical barrier limiting the extracellular, or apoplastic, transport of water and solutes in plant roots. Using the newly characterized enhanced suberin1( esb1) mutant, we established a connection in Arabidopsis thaliana between suberin in the root and both water movement through the plant and solute accumulation in the shoot. Esb1 mutants, characterized by increased root suberin, were found to have reduced day time transpiration rates and increased water-use efficiency during their vegetative growth period. Furthermore, these changes in suberin and water transport were associated with decreases in the accumulation of Ca, Mn, and Zn and increases in the accumulation of Na, S, K, As, Se, and Mo in the shoot. Here, we present direct genetic evidence establishing that suberin in the roots plays a critical role in controlling both water and mineral ion uptake and transport to the leaves. The changes observed in the elemental accumulation in leaves are also interpreted as evidence that a significant component of the radial root transport of Ca, Mn, and Zn occurs in the apoplast.
Introduction:
The plant root is a specialized organ that allows uptake of water and selective uptake of solutes from the soil environment, to support normal plant growth and development. To accomplish this function roots take up water and solutes at the root surface and transport them across the root to the xylem vessels in the central vascular tissue in the stele, where they are transported to the shoot. Transport across the root to the central xylem vessels can occur through the extracellular space( apoplastic) or via cell-to-cell transport( symplastic). Specificity of both water and solute transport is generally thought to be provided by transport proteins at the plasma membrane of root cells. However, to achieve such specificity, the non-specific apoplastic transport pathway between cells needs to be regulated. A long held model to explain this regulation includes the function of the extracellular biopolymer suberin acting as a barrier to limit the apoplastic transport of water and solutes. Plant roots contain two such suberin barriers, at the exodermis and the endodermis. Endodermal suberin is thought to prevent the apoplastic movement of water and solutes into the stele, whereas exodermal suberin blocks apoplastic transport at the root surface. Though central to our understanding of how roots function, a careful review of the literature reveals a lack of genetic evidence to support this foundational model. The effects of altered root suberin content on water relations and ion transport at both the exodermis and endodermis have been tested experimentally by modifying the cultivation conditions or utilizing roots at different developmental stages[reviewed in 1 & 2]. For example, roots of maize grown hydroponically lack a suberized exodermis when compared to aeroponically grown plants[2]. Such differences have been utilized to identify negative correlations between suberin content and root hydraulic conductivity, and radial transport of ABA[1],[3],[4]. Higher suberin content within mature endodermis also results in decreased Ca translocation at different developmental stages along the root of Cucurbita pepo[5]. Further, in species with an exodermis and endodermis( Zea mays, Allium cepa, and Helianthus annuus), or just an endodermis( Vicia faba and Pisum sativum), higher root suberin, in mature regions of the root, correlates with lower apoplastic transport of fluorochromes[6] and root water loss[7]. In concluding that quantitative differences in suberin content, in either the exodermis or endodermis of the root, determines the permeability of the apoplast to both water and solutes the implicit assumption is that differences observed in these physiological parameters are related to the altered suberin contents observed. It has previously been suggested that qualitative differences in the compositions of suberin also need to be considered[8]. Furthermore, given the varied growth conditions, developmental stage or species used in these studies it is quite feasible that differences other than suberin are either directly or indirectly responsible for the observed effects on water relations and ion transport. Direct evidence for a role of suberin in regulating apoplastic radial transport requires a plant harboring a mutation that specifically alters the amount of suberin deposited in the root. Currently, there are only three reports of Arabidopsis thaliana mutants with altered suberin. As it is generally accepted that A. thaliana roots do not contain an exodermis[9], suberin in these mutants is likely altered in the endodermis. Knockout alleles of GPAT5( encoding an acyl-CoA: glycerol-3-phosphate acetyltransferase) in A. thaliana were recently shown to contain 20–50% less C20–C24 aliphatic monomers in root suberin[10]. However, the effect of loss-of-function of GPAT5 on either water relations or ion-transport was not directly measured. The recently isolated A. thaliana mutants horst-1 and horst-2 containing knockout alleles of the cytochrome P450 fatty acid ω-hydroxylase CYP86A1 gene contain 60% less total aliphatic suberin monomers in roots than wild-type plants[11],[12]. Knockout alleles of the DAISY locus, which encodes a β-ketoacyl-CoA synthase involved in the formation of docosanoic acid, also have altered aliphatic suberin monomer composition, including a decrease in C22 and C24 fatty acid derivatives and an increase in C16, C18 and C20 fatty acid derivatives[13]. No evidence for impacts on water relations or ion-transport for either the horst or DAISY mutants was reported. Given the assumed role of suberin in regulating radial transport of solutes in the root, we would predict that perturbations in suberin content and/or its composition would affect the elemental composition, or ionome, of the shoot. High-throughput screening of the shoot ionome may therefore identify mutants with altered root suberin. We have identified numerous ionomics mutants in a previously reported screen[14], and here we report on the cloning and characterization of one of these ionomics mutants, originally reported as mutant 14501[14]. Here we report that 14501 results in a doubling of all the aliphatic monomer components of root suberin, and we renamed this mutant enhanced suberin1( esb1-1). This elevated suberin, most likely in the endodermis given that A. thaliana roots lack an exodermis, results in a reduction in transpiration, reduced wilting after water withdrawal, and a perturbation of the shoot ionome, including increases in Na, S, K, As, Se and Mo, and decreases in Ca, Mn and Zn. Grafting experiments confirm that these phenotypes are the result of elevated root suberin. This report provides direct evidence that suberin acts in the root as a barrier to the extracellular transport of both water and mineral ions.
Discussion:
Radial rather than axial transport is known to limit water loading by roots[23]. The force driving water across the root is provided by tension( negative pressure) created by transpiration, and flow is thought to be primarily apoplastic. However, during periods of reduced transpiration, at night or in dry or saline soils, it is root pressure, caused by osmotic water flow driven by the uptake of nutrients into the xylem that drives water transport to the shoots. This osmotic pathway for water is primarily cell-to-cell. It has been proposed that this “Composite Model” for water uptake allows plant roots to take up water under various conditions and balance it with demands[24]. However, because the Casparian strip is thought to be somewhat permeable to water, but not to mineral ions, transport of water and solutes are differentiated in the endodermal apoplast, with the Casparian strip forming the main barrier to radial ion movement[25]. However, to date no mutants with elevated root suberin have been described to help elucidate this model further. The esb1 mutant described here has significantly higher root suberin compared to wild type plants, and grafting experiments establish that this increase in suberin is responsible for the observed changes in the shoot ionome. The final accumulation of mineral elements in the shoot would be expected to be a balance between the negative effect of suberin reducing radial apoplastic solute transport, and the positive effect of the reduced transpiration of esb1 on the xylem concentration of solutes. The reductions in total shoot Ca, Mn and Zn observed in esb1 suggest that a significant component of the radial root transport of these elements is via the apoplast. This evidence confirms the earlier conclusion of White[26], based on modeling of physiological and biochemical parameters, that a significant root apoplastic bypass pathway exists for Ca. It is also consistent with the observation that increased endodermal cytoplasmic Ca during cooling is inversely proportional to the level of suberin deposition[27]. Further, the conclusion by White and coworkers[28] that Zn may also reach the xylem via an apoplastic pathway is also supported by our data on esb1. The increase in shoot S, K, As, Se and Mo in esb1 suggests that these elements may be primarily transported radially in the root via a cell-to-cell pathway, making them resistant to changes in endodermal suberin, and more prone to the influence of increasing xylem concentrations of solutes due to the reduced transpiration driven water flow in esb1. We also observe Na concentration increasing in the shoot of esb1, suggesting the absence of an apoplastic bypass pathway for this element at the low external Na concentrations present in the potting mix used in these studies. However, elevated suberin has been implicated in reducing Na accumulation and increasing Na tolerance in rice[29],[30]. Such evidence suggests that at low external Na apoplastic bypass flow is limited. However, at elevated external Na concentrations apoplastic bypass flow becomes significant, and reduction in this flow can lead to elevated Na tolerance. It has previously been reported that Ca and Mg shoot concentrations are positively correlated across taxa[31], within-species and genetically[32]. We also observe a strong correlation between Ca and Mg in both wild-type and esb1( R2 = 0. 75 and 0. 68, respectively. Data taken from trays 533, 534 and 535, n = 30 for each genotype; see www. ionomicshub. org). Such correlations suggest that the mechanisms driving Ca and Mg accumulation are related. However, the fact that in esb1 Ca is reduced without significantly changing Mg suggests that the regulatory mechanisms controlling Ca and Mg can be separated. Elevated root suberin in esb1 is also associated with a root-dependent increase in time to wilting during water stress. Resistance to wilting can be achieved via a reduction in water loss to the environment, from both the root and leaves, or through an increase in the ability to take up water from the soil. Delayed wilting in esb1 appears to be related to the reduced stomatal aperture and reduced daytime transpiration rates in this mutant. However, grafting experiments establish that this delayed wilting in esb1 is a root-dependent phenomenon, suggesting signaling from root to shoot. A possible explanation for this signal is that roots of esb1 are constitutively responding to water stress, due to increased suberin causing enhanced hydraulic resistance to radial transport of water. This stress is transmitted to the shoot via ABA, or directly as a hydraulic signal[33]. Such a model remains to be tested. Reduced transpiration is often correlated with reduced biomass accumulation, as observed here for esb1. However, biomass reduction in esb1 is less than the reduction in transpiration, leading to an overall increase in water use efficiency of esb1. Increased root suberin may therefore present new opportunities for developing enhancing drought resistance in crop plants. Here we establish that loss-of-function of the endodermally expressed gene At2g28670 leads to a doubling of all the aliphatic components of root suberin. The enzymatic function of the protein encoded by At2g28670, and how this function affects suberin, remains an open question. Importantly, the elevated root suberin observed in two independent At2g28670 loss-of-function mutants( esb1-1 and esb1-2) directly affects the shoot ionome, causing several changes, including a 50% reduction in Ca and a 40% increase in Na. These changes are also associated with decreased transpiration and increased wilting resistance. Overall, these observations provide strong experimental support for the standard model that suberin acts as an extracellular transport barrier limiting apoplastic radial transport of water and solutes. Our observations suggest that elevation of root suberin may represent another approach to the development of drought resistant crops with improved WUE. Furthermore, manipulation of suberin may also provide new opportunities for the development of plant-based foods with altered mineral nutrient contents.
The root system is a highly specialized plant organ that works to get both water and essential mineral nutrients from the changing chemically and physically complex environment of the soil . Roots do this by both controlling the uptake of water and essential mineral ions , as well as regulating their movement to the central vascular system of the plant for long distance transport to the shoot . To allow the cellular control of water and mineral ion uptake and transport via specialized transport proteins , plant roots contain a waxy layer of suberin that acts to seal connections between cells , preventing uncontrolled leakage of water and mineral ions between cells . By screening thousands of mutant A . thaliana plants , we were able to identify a plant with elevated levels of suberin in the root . Using this mutant , we were able to uncover the importance of suberin in sealing connections between root cells to regulate water movement through the plant and accumulation of various essential and nonessential minerals in leaves , including sodium , sulfur , potassium , calcium , manganese , zinc , arsenic , selenium , and molybdenum .