Outer apoplastic barriers in roots: prospects for abiotic stress tolerance

Introduction

Climate change is increasing the frequency and intensity of extreme weather events limiting crop production and reducing food supply worldwide (IPCC 2019). Of these, floods, drought and soil salinity are considered to be major ones and account for ~US$130 billion p.a. losses in crop and pasture productivity (Razzaq et al. 2021). Moreover, water supply for agricultural uses becomes increasingly scarce with those mainly affected being in the Global South (Rijsberman 2006) and the quality of irrigation water is gradually deteriorating (Liu et al. 2020), leading to land salinisation. Increasing the crops’ resilience to abiotic stresses is therefore utterly important (Lynch 2007; Palmgren et al. 2015). Is there a root trait that can be a ‘silver bullet’ for these abiotic stresses? The present paper focuses on the outer apoplastic barriers, which have been shown to be a ‘jack of all trades’ as these provide tolerance to: (1) soil flooding, (2) water deficit (as under drought and saline conditions) and (3) protects roots from soil phytotoxins (e.g. in contaminated soils). This new broad term proposed here encompasses the various apoplastic barriers formed in the outer parts of the root, such as the exodermis and sclerenchyma layers, to respond to different environmental cues. The term ‘outer’ implies the location of these apoplastic barriers in the periphery of the root tissues rather than in the central parts (endodermis). We describe the environmental signals related to the barrier formation, present difficulties to unravel its induction pathway and discuss whether this root trait could be the single solution to the multiple abiotic stresses challenging crop production. Flooding literature constitutes the backbone of our review since most research on the outer apoplastic barriers is based on the so called ‘barrier to radial O₂ loss’ (ROL) formed as a response to soil flooding. Lastly, we suggest that the term ‘outer apoplastic barriers’ is a more fitting new name rather than barrier to radial O₂ loss, given all the additional functions these provide besides preventing radial O₂ loss.

Contrasting water availability impact key soil properties

Excessive water results in profound changes in physical, chemical, and biological properties of the soil causing acclimation in key root anatomical traits. The exchange of gases between the soil and atmosphere is extremely slow in flooded soils because gas diffusion in water is 10 000-fold slower than in air (Armstrong 1979). Flooded soils are therefore typically anoxic or severely hypoxic with a low redox potential, since O₂ consumption by roots and soil microorganisms is faster than resupply of O₂ from the atmosphere (Ponnamperuma 1972). Lack of O₂ leads to impaired root respiration, nutrient uptake and root growth due to a significant decline in ATP production (Gibbs and Greenway 2003; Colmer et al. 2015). Tissue anoxia in the roots not only halts root elongation (Armstrong and Webb 1985) but can also cause root apex death (Armstrong et al. 1991). In addition, anaerobic microorganisms use alternative electron acceptors such as Fe₂O₂ or SO₄²⁻ in the absence of O₂ for their metabolism (Laanbroek 1990), leading to the production of reduced iron (Fe²⁺) or sulfides, which can reach toxic concentrations for plant growth (Armstrong et al. 1991).

On the other end of the spectrum from flooding is water deficit. Low soil water potential occurs under drought conditions and is also observed in the presence of high amounts of dissolved salts in the rhizosphere (hence, prompting salinity being called a ‘physiological drought’) (Ma et al. 2020). Low soil water potential increases the tensile forces that retain water with the soil particles, limiting water and nutrient uptake by roots. Soil desiccation also reduces soil porosity, as the soil particles are pulled closer together, making the soil harder and restricting root elongation (Bengough et al. 2006). Since compacted soils, i.e. low porosity soils, restrict root elongation, the volume of soil explored is reduced and thus access to potential resources is impaired (Bengough et al. 2011). As the soils become drier, water uptake can be further restricted due to the loss of root–soil contact caused by root shrinkage, when the cortical tissue contracts (Carminati et al. 2009). At the physiological level, low soil water potential triggers stomata closure in leaves, thus reducing their ability to assimilate CO₂ (hence, causing yield penalties). Nutrients delivery to the root surface is compromised (due to reduction in both mass flow and diffusion) and reduced transpiration rates caused by stomatal closure affect nutrient transport to the shoot. In addition, low soil water potential severely reduces the mineralisation of nitrogen and carbon, leading to changes in the soil microbial communities as these nutrients become limited (Yahdjian et al. 2006; Zhang et al. 2019). Consequently, the microbial activity of the soil changes, causing soil nutrient deficiencies since the recycling of nitrogen, carbon and phosphorous is diminished (Sardans and Peñuelas 2005).

Root acclimations to contrasting water availability

Aerenchyma formation and changes in root thickness

Root acclimation to drought or flooding share some key anatomical features. Both flooding and water deficit can induce the formation of large gas filled spaces (aerenchyma) as a result of programmed cell death (Fig. 1). In such scenarios, aerenchyma formation reduces: (1) the metabolic maintenance cost of the root, (2) the amount of water in root tissues and (3) the amount of C, N and P allocated to roots (Zhu et al. 2010; Yamauchi et al. 2021). While in flooded conditions, aerenchyma formation enables effective O₂ diffusion from the above water tissues down to the roots and saves water and organic carbon during drought conditions, i.e. longer roots can be formed with the same investment in water and carbon.

Fig. 1.

Root acclimations to contrasting water availability. Root cross sections before (left side) and after (right side) acclimating to either soil flooding or water deficit. The enhancement of aerenchyma formation (right side) is the result of programmed cell death in order to reduce the maintenance cost (water, energy, nutrients) of the root and to improve gas diffusion longitudinally through the tissues. In this conceptual diagram, the increased root thickness is driven by the enlargement of the cortex to allow for more space to form aerenchyma and to increase the mechanical strength of the root. Ex, exodermis; Scl, sclerenchyma (black arrows); Co, cortex; En, endodermis; Ae, aerenchyma (gas-filled spaces); St, stele. Cross-section images from Peralta Ogorek (2022).

In addition to aerenchyma, root thickness also increases during flooding or drought conditions (Fig. 1). Soil flooding can induce the enlargement of the cortex to enable larger cross-sectional aerenchymatous tissues, which further facilitates diffusional supply of molecular O₂ to the root (Yamauchi et al. 2019). Thicker roots also have a reduced radial O₂ loss to the anoxic soils due to the lower surface area to volume ratio, improving the oxygen status of root tissues (Pedersen et al. 2021). Interestingly, root thickening can also be induced during drought to increase the mechanical strength of the roots and the capability to penetrate harder soils (Yu et al. 1995; Bengough et al. 2011).

Suberised and lignified outer apoplastic barriers: the barrier to radial O₂ loss

Root responses to flooding include the formation of an apoplastic barrier in the outer part of the root to impede radial O₂ loss (Fig. 2). In flooded soils, the formation of such barriers impedes radial O₂ loss from roots to the rhizosphere, thereby enhancing internal, longitudinal O₂ diffusion from the shoot to the roots, and thus improving root respiration, nutrient uptake and root penetration into anoxic soils (Colmer 2003a). Interestingly, the barrier to radial O₂ loss has also been shown to greatly restrict radial loss of water from roots, indicating a role in drought tolerance for this root trait (Peralta Ogorek et al. 2021; Song et al. 2023). While the formation of such barrier can limit water uptake to root regions where the barrier is not formed, it also prevents the backflow of water from the root to the soil (Ranathunge et al. 2011a) thus limiting water loss (Brunner et al. 2015). In some species (e.g. rice (Oryza spp.) and Hordeum marinum), the barrier can be inducible (Colmer 2003b), but in others (e.g. in wild rice, the genera of Echinochloa and Urochloa), the barrier is constitutively expressed (Ejiri and Shiono 2019; Jiménez et al. 2019). In the following sections, we refer to the barrier to radial O₂ loss as an ‘outer apoplastic barrier’.

Fig. 2.

Functions of outer apoplastic barriers. The loss of O₂ (blue arrows) occurs closer to the root–shoot junction in the absence (left side) of barrier to radial O₂ loss (outer apoplastic barrier). Once the barrier is formed (right side), radial O₂ loss takes place closer to the root apex as the barrier enhances longitudinal diffusion of O₂ in combination with enhanced aerenchyma formation. The figure summarises several alternative functions of the barrier (restricted Fe²⁺, H₂S, Na⁺ and Cl⁻ intrusion, restricted radial water loss). Question marks indicate that the barrier might play a role against organic acids intrusion or pathogen invasion, but these functions are currently untested. Figure adapted from Yamauchi et al. (2018).

Alternative functions of outer apoplastic barriers

The outer apoplastic barriers not only restricts O₂ diffusion, but also restricts intrusion of phytotoxins into the roots that are produced by facultative or obligate anaerobes during soil flooding. It has been widely hypothesised that the barriers can prevent intrusion of H₂S (Armstrong and Armstrong 2005; Soukup et al. 2007; Nishiuchi et al. 2012) and this has only recently been experimentally tested. Experimental setups using H₂S microsensors revealed that the apparent permeance to H₂S of the outer parts of the root was reduced by almost 99% by the outer apoplastic barriers (Peralta Ogorek et al. 2023). The protective role was demonstrated at the physiological level; root tissue respiration was unaffected in roots with outer apoplastic barriers by external H₂S concentrations up to 500 μM (cytochrome c oxidase is inhibited already in the range of 1–10 μM H₂S; Fenchel and Finlay 1995). Moreover, H₂S intrusion at the root–shoot junction was greatly restricted by the presence of the outer apoplastic barriers, indicating that H₂S diffusion to the shoot of intact plants is also restricted, demonstrating the importance of this trait not only for the roots, but also at the whole plant level (Peralta Ogorek et al. 2023). Besides H₂S, the outer apoplastic barriers also restrict the diffusion of water vapour and hydrogen gas (Peralta Ogorek et al. 2021), suggesting that suberised and lignified apoplastic barriers might be capable of restricting the diffusion of gases in general.

Very recently, the outer apoplastic barriers have been proposed to restrict venting of greenhouse gases from rice (Jimenez and Pedersen 2023), where methane (CH₄) and nitrous oxide (N₂O) emissions from paddy rice contribute with 8 and 10%, respectively, to global anthropogenic emissions (Saunois et al. 2020; Wang et al. 2020). The idea is that tight outer apoplastic barriers would restrict radial diffusion of greenhouse gases produced in the soil into the roots, and thereby reduce plant-mediated diffusion with subsequent reduction in emission via the shoot (Jimenez and Pedersen 2023).

Besides gases, intrusion of toxic reduced iron (Fe²⁺) from the anoxic soil is also prevented in areas where fully suberised lamellae have developed in the root exodermis (Jiménez et al. 2021). Interestingly, a 1.5-fold increase in the amount of hypodermal suberin and a 2-fold increase in lignin content were reported in Cd-treated roots (Schreiber et al. 1999), suggesting a role of these cell wall components in root adaptation to heavy metal toxicity. Even stronger evidence exists for the essential role of apoplastic barriers in salinity tolerance, and reinforcement of apoplastic barriers (present in the outer part of the root or endodermis) has been nominated as key targets for breeding of salt tolerant crops (Cui et al. 2021). Such a breeding strategy is justified by a significant reduction in radial apoplastic movements of Na⁺ and Cl⁻ by the outer apoplastic barriers and a subsequent reduction in translocation to the shoot of rice (Ranathunge et al. 2011b).

The biochemical composition of the exodermal cell walls significantly restricts radial loss of water from the roots. Species with a root exodermis showed reduced radial water loss when compared to species without exodermis (Taleisnik et al. 1999). In addition, Iris germanica roots can form a multiseriated exodermis under dry conditions restricting water permeability (Meyer et al. 2011). Rice roots possess an exodermis that restricts radial water loss to a certain extent. However, if suberised and lignified outer apoplastic barriers are formed, radial water loss from roots is further restricted by 93%, indicating a potential role in drought tolerance for this root trait (Song et al. 2023). In rice, the outer apoplastic barriers can limit water uptake to root regions where the barriers are not formed, i.e. short lateral roots (Noorrohmah et al. 2020), and prevent the backflow of water from the root to the soil (Ranathunge et al. 2011a), thus limiting water loss (Brunner et al. 2015). Accordingly, root hydraulic conductance in rice is only positively correlated to the number of short laterals and not other types of roots (Watanabe et al. 2020). In a drought situation, the topsoil will be significantly drier than the deeper soil, and in such conditions suberised and lignified outer barriers in proximal roots would restrict radial loss to the topsoil of water taken up from deeper layers of the soil.

Suberin, a key component of these apoplastic barriers, has been shown to serve an important role against pathogen invasion. Roots with suberin depositions containing higher aliphatic domains provided resistance to fungal invasion, while those with higher aromatic domains were relevant against bacterial infection (Lulai and Corsini 1998; Ranathunge et al. 2011a). In addition, increased amounts of constitutive suberin depositions in roots of soybean were correlated with higher resistance to Phytophthora sojae causing stem and root rot, compared to roots with less suberin (Thomas et al. 2007). Thus, there is growing evidence that suberised outer apoplastic barriers also provide benefits against soil phytotoxins and pathogens.

Lignin, another key component of apoplastic barriers, has been shown to increase the mechanical strength for root penetration in hard soils. Roots with multiseriate sclerenchyma had higher lignin concentrations and improved root penetration in hard soils (Schneider et al. 2021). These studies suggest that the presence of outer apoplastic barriers, by enhancing tissue suberisation and lignification, may provide tolerance to pathogens or even mechanical strength to penetrate hard soils.

In summary, the outer apoplastic barriers and/or their components provide a wide variety of benefits against several abiotic and potentially biotic stressors (Fig. 2), making these a strong focal point in the process of improving crop resilience in a changing environment.

Environmental signals induce the formation of outer apoplastic barriers

The formation of outer apoplastic barriers can be induced by several environmental cues. As mentioned in the previous sections, soil flooding triggers the formation of outer apoplastic barriers functioning as a barrier to radial O₂ loss (Colmer 2003a). Interestingly, it is not the lack of O₂ nor the accumulation of CO₂ or the gaseous hormone ethylene that induces the formation of such barriers (Colmer et al. 2006). In rice, the outer apoplastic barriers restrict radial O₂ loss and can be induced in stagnant deoxygenated nutrient solutions (Colmer et al. 1998), but the chemical component(s) acting as a signal for barrier formation in hydroponic culture has not yet been identified.

Several studies have instead focused on soil phytotoxins produced in anoxic soils as chemical candidates involved in signalling for barrier formation (Fig. 2). Phytotoxins such as low-molecular mass organic acids, which are produced by anaerobic bacteria, have been shown to induce the barrier at low, sub-toxic concentrations (Kotula et al. 2014; Colmer et al. 2019) and at comparatively higher concentrations where these compounds are already toxic (Armstrong and Armstrong 2001). Similarly for reduced iron, which is also toxic to roots (Becker and Asch 2005), supplying high concentrations of Fe²⁺ to aerated nutrient solutions that mimic well-oxygenated soils induced outer apoplastic barriers in rice (Mongon et al. 2014). Outer apoplastic barriers can also be induced in rice by sub-toxic concentrations of H₂S (Peralta Ogorek et al. 2023), which is produced by sulfate-reducing bacteria and is highly toxic to plants (Raven and Scrimgeour 1997). Moreover, outer apoplastic barriers can be further enhanced in rice using higher and severely toxic concentrations of H₂S (Armstrong and Armstrong 2005). Therefore, the formation of outer apoplastic barriers seems tightly linked to the presence of phytotoxins, but, surprisingly, also to conditions of water-deficit, where such phytotoxins would not be found in the soil. At present, environmental signals for the formation of outer apoplastic barriers under water deficit have not been identified (Song et al. 2023).

Interestingly, some studies demonstrated that the extent of suberisation and lignification of root tissues is strongly influenced by soil nutrient stress. Accelerated biosynthesis of lignin was reported in rice under conditions of high ammonium concentrations (Ranathunge et al. 2016), and lignin biosynthesis genes were also highly upregulated in apple (Malus spp.) rootstock under low N conditions (Wang et al. 2023).

Genetic regulation of the formation of outer apoplastic barriers

The development of outer apoplastic barriers have been shown to be beneficial for plants growing in flooded or drought conditions. However, the molecular mechanisms and genetic regulations controlling this root trait remain largely unknown, and only studied on the barrier to radial O₂ loss. Conventional hybridisation of wild-type flood tolerant species with their flood sensitive relatives and subsequent pyramidisation of quantitative trait loci (QTL) for formation of an outer apoplastic barrier have been a key strategy for gene discovery and development of plants with barriers (Mano and Omori 2013). Successful attempts to hybridise wild-type flood tolerant species with their agriculturally important crop relatives have been done for maize (Zea spp.) and wheat (Triticum aestivum). QTL mapping from flood sensitive maize × flood tolerant Zea nicaraguensis progenies revealed that a segment of the short-arm of chromosome 3 of the Z. nicaraguensis conferred barrier formation to the maize inbred line (Watanabe et al. 2017). In addition, amphiploids developed through wide hybridisation of flood sensitive wheat × flood tolerant Hordeum marinum, showed an outer apoplastic barrier when grown in low O₂ conditions, in comparison to no barrier formation of the wheat donor lines (Malik et al. 2011).

The use of laser capture microdissection technologies and cell type specific gene expression tools are fundamental to achieve the resolution needed to understand the genetic signature encoding the development of outer apoplastic barriers (cf. Nakazono et al. 2003; Reynoso et al. 2022). Gene expression profiles from isolated hypodermal/exodermal roots of rice suggest that WRKY, AP2, MYB and NAC transcription factors regulate suberin biosynthesis in rice roots grown in anoxic conditions (Shiono et al. 2014), whereas MYB and NAC were key transcription factors modulating an upregulated pattern for suberin biosynthesis in exodermal cells of rice roots grown under water deficit conditions (Reynoso et al. 2022). Gene expression profiles indicated that 128 genes were significantly up- or downregulated in the hypodermal/exodermal layers of rice roots during formation of an outer apoplastic barrier, where several genes associated with suberin biosynthesis were strongly upregulated (Shiono et al. 2014). Most of those genes that were upregulated during barrier formation were also highly enhanced by exogenous abscisic acid (ABA), suggesting that ABA is an inducer of exodermal suberin lamellae in rice roots (Shiono et al. 2022). However, the signal(s) that triggers an ABA, or other signalling network, to form an outer apoplastic barrier in roots is not known. Interestingly, gene expression analyses during barrier formation induced by low concentrations of H₂S revealed a significant upregulation of genes related to lignin (Peralta Ogorek et al. 2023). During exposure to comparatively high concentrations of organic acids, target genes also related to lignin biosynthesis (cinnamyl alcohol dehydrogenase, Os01g0528800 (CADL) and cinnamoyl CoA reductase, Os02g0811800 (OsCCR10), respectively) were significantly upregulated (Colmer et al. 2019). The prevalence of either suberin or lignin during the formation of the outer apoplastic barriers suggest that the environmental conditions, which the roots are subjected to, might influence synthesis of these biopolymers.

Progress in understanding the regulatory factors and genes associated with suberisation and/or lignification of hypodermal/exodermal cells in roots forming barriers remains slow. Major difficulties to achieve such progress include: (1) the formation of such barriers is developmentally complex and cell-type specific, (2) inter- and intraspecific differences in suberin and/or lignin monomeric composition of the exodermis cell walls, (3) outer apoplastic barriers can be a constitutive or inducible, (4) outer apoplastic barriers can be triggered by different environmental conditions and (5) formation of outer apoplastic barriers is dependent of root tissue age/development.

Trade-offs of outer apoplastic barriers

Can the outer apoplastic barriers, including the barrier to radial O₂ loss, be a ‘silver bullet’ for abiotic stress tolerance in crops – at least, for flood, drought, and salinity tolerance? Will the increased suberisation/lignification of plant roots come with potential penalties for plant nutrient acquisition? What are the possible trade-offs, and can they be accepted?

Radial uptake of essential nutrients in plants occurs via two parallel pathways: apoplastic and symplastic. In broad terms, the apoplastic transport will be affected by the presence of two barriers: an external one (in the exodermis); and an internal one (in the endodermis) (Schreiber et al. 1999; Barberon and Geldner 2014). Both suberin and lignin can play a major role in establishing an apoplastic transport barrier in exodermis as well as in endodermis (Schreiber et al. 1999; Naseer et al. 2012; Reyt et al. 2021). Also, while most of the knowledge comes from studies on the endodermis, Casparian strips can form in the hypodermis (Reinhardt and Rost 1995; Enstone and Peterson 1998). Hence, some analogies between deposition of suberin and/or lignin in exo- and endodermis can likely be made.

An increase in exodermal and endodermal suberin deposition affected root radial transport properties, with a 2–3 -fold decrease in root hydraulic conductivity reported in Zea mays (Zimmermann and Steudle 1998). Similarly, in Arabidopsis thaliana, horst mutants, which have delayed suberin lamellae formation and lower suberin content in the endodermis had higher hydraulic conductivity compared with the wild-type (Höfer et al. 2008). However, the esb1 mutant characterised by enhanced root suberisation likely in the endodermis as Arabidopsis roots lack an exodermis, displayed increased water use efficiency (Baxter et al. 2009; Hosmani et al. 2013). As for essential nutrients, modulation of suberin-degrading enzymes in Arabidopsis demonstrated that ectopic suberisation in endodermal cells limits Ca transport and delivery to the shoot (Kamiya et al. 2015). However, while an ectopic suberisation in esb1 mutant was associated with a decreased accumulation of Ca, Mn and Zn, it also increased content of K and Mo, revealing the complex relationship between the extent of root suberisation and selective nutrient uptake/transport. Similarly, the sgn3 Arabidopsis mutant with a defective Casparian band in the endodermis caused by alterations in lignin biosynthesis shows increased Mg but reduced Zn and K phenotype (Pfister et al. 2014). In barley, less suberin in the endodermis favoured the inwards radial transport of Ca but negatively affected K concentration in the stele (Chen et al. 2019).

Of specific interest is an impact of suberisation on plant adaptive responses to salinity. In Pongamia pinnata roots, increased exodermal suberin deposition was associated with decreased Na⁺ uptake (Marriboina and Attipalli 2020), and in Arabidopsis reducing the extent of endodermal suberisation by overexpressing cutinase (CDEF1) led to increased Na⁺ accumulation and salt-sensitive phenotype (Barberon et al. 2016). Moreover, Na⁺ and Cl⁻ permeabilities were restricted in rice roots that developed outer apoplastic barriers in conditions mimicking soil flooding, but such barriers did not reduce the root hydraulic conductivity (Ranathunge et al. 2011b). Nevertheless, some plants use Na⁺ as a ‘cheap osmotic’ solute. This phenomenon is observed not only for halophytic species but also for many salt tolerant crops (Chen et al. 2007; Shabala 2013; Puniran-Hartley et al. 2014; Rawat et al. 2022). Hence, the effectiveness of the mechanisms preventing sodium flow into the plants will likely depend on the importance of sodium exclusion for salt tolerance of different plant species. Sodium accumulation in vacuoles is used for osmotic adjustment, thereby maintaining the gradient of water potential between roots and external solution. Preventing Na⁺ uptake by apoplastic barriers may therefore potentially increase the carbon cost for plant osmotic adjustment by forcing plants to rely on de novo synthesis of organic osmolytes, which comes with high energy cost (Munns et al. 2020).

One of the anatomical hallmarks of rice is the presence of so-called ‘bypass flow’, originating from the breakage of the integrity of Casparian band by lateral roots (Flam-Shepherd et al. 2018; Ishikawa and Shabala 2019). As a result, the SOS-mediated mechanism for Na⁺ exclusion that is considered to be the major mechanism for preventing Na⁺ toxicity (Zhao et al. 2020) is not efficient in rice, as the presence of the apoplastic bypass flow may result in a futile cycle, depleting metabolic energy but not achieving a significant reduction in Na⁺ influx. Tight outer apoplastic barriers may, however, overcome this limitation. Indeed, the salt tolerant rice cultivar Pokkali is capable of forming strong suberised barriers in both exo- and endodermis, and it is correlated with reduced Na⁺ accumulation in the shoot (Krishnamurthy et al. 2009). In addition, when exposed to high Na⁺ concentrations, the lateral root density increases without creating a significant bypass flow for Na⁺ (Krishnamurthy et al. 2011). It would be interesting to test whether the enhanced suberisation caused by Na⁺ would also restrict radial O₂ loss, as salt stress can often occur together with soil flooding (Colmer and Flowers 2008), although it would likely be the case since the barrier to radial O₂ loss reduced the root permeability to Na⁺ and Cl⁻ (Ranathunge et al. 2011b). Regardless, these studies clearly indicate that despite the potential breakage of the apoplastic barrier integrity caused by lateral root emergence creating a window for Na⁺ intrusion, this effect is compensated by enhanced suberisation of exo- and endodermis.

So, what about other species? Will the benefits from reducing Na⁺ uptake by forming outer apoplastic barriers overcome possible issues with uptake of essential nutrients? Can potentially detrimental effects of suberisation of the apoplast be compensated for by increased activity of transporters of these elements? Or will the hydraulic conductivity in other species be negatively affected by the presence of outer apoplastic barriers, ultimately impacting nutrient uptake and shoot growth? While the explicit answers to these questions are yet to come, a possible solution could be in turning our attention towards the symplastic pathway of nutrient acquisition and, specifically, using tissue-specific promoters for preferential expression of key nutrient transporters in the root hairs. For example, accumulation of mRNA for the LeNrtl-2 nitrate transporter was restricted exclusively to root hairs, while LeNrtl-1 transcripts were detected in both root hairs and other root tissues (Lauter et al. 1996). Thus, the possibility of engineering plants with a dominant symplastic pathways of nutrient uptake is certainly plausible.

Conclusions and perspectives

The term ‘barrier to radial O₂ loss’ was coined several decades ago, but an extensive array of functions beyond that of restricting radial O₂ loss have been discovered since then. Therefore, a more appropriate name for this apoplastic and gas-tight barrier in the periphery of the root is ‘outer apoplastic barriers’.

The outer apoplastic barriers are indeed a ‘jack of all trades’ as these provide tolerance to soil flooding, drought, and salinity stress, and protect roots from various soil phytotoxins as well. However, more attention needs to be placed on the possible trade-offs of the barriers, particularly across species, before this trait can be used to improve abiotic stress tolerance in crops sensitive to soil flooding, salinity, or drought. Moreover, we also propose to further unravel the genetic network involved during the barrier formation in the outer parts of the root with particular emphasis on species forming constitutive outer apoplastic barriers such as in Urochloa humidicola (Jiménez et al. 2019) or wild rice relatives (Ejiri et al. 2020). This would help reducing the complexity of studying this root trait present in species with inducible barriers.