Developmental Responses of Root Hairs to Mg Deficiency

ABSTRACT

Magnesium (Mg), an essential element for plants is easily leached in acidic and sandy soils. Magnesium deficiency induces the initiation and elongation of root hairs, which allows the plant roots to acquire more Mg under Mg-limited conditions. However, the signals involved in the regulatory cascade leading to the induction of root hair development under Mg deficiency are largely unknown to date. Recent studies have revealed that many chemical signal molecules such as ethylene, nitric oxide, auxin, reactive oxygen, and calcium regulate the root hair development induced owing to Mg deficiency. This mini-review intends to briefly discuss the role of these chemical signals in the induction of root hair development under Mg-deficient conditions.

Magnesium is fundamental to many biochemical processes, including chlorophyll synthesis, photosynthetic carbon fixation, protein and nucleic acid synthesis.^1–3 However, plants frequently suffer from Mg deficiency in acidic and sandy soils, for Mg is easily leached and antagonized by other cations in these soils.⁴ Fortunately, plants have evolved adaptive strategies to overcome Mg deficiency.^5,6 Roots define the spatial exploration capacity in soil, promote the absorption of nutrients, especially in scanty and patchy soils.^7,8 Root hairs enhance the root surface area and enable access to farther areas in the soil for nutrient uptake. Root hairs originate from epidermal cells located between the cortical cells (“H” position), whereas the epidermal cells that are in contact with only one cortical cell (“N” position) do not develop root hair.^9,10 In Arabidopsis, Mg deficiency induces root hair initiation and elongation, which facilitates the acquisition of Mg under the Mg-limiting conditions¹¹. Root hair development stimulated owing to Mg deficiency is related to the initiation of new trichoblasts. Transcription data revealed that over 95% hair (“H”) genes are down-regulated by high Mg, which further demonstrates the importance of Mg in determining the fate of root hair cells at a later stage.¹¹ Moreover, the data from Arabidopsis suggest that the genes related to root hair elongation are affected by Mg supply.

How Mg deficiency regulates the root hair development remains a key question. Many studies show auxin as an indispensable signal for controlling root hair development. Under normal growth conditions, auxin not only promotes root hair growth but also induces trichoblasts to develop root hair.^12,13 The auxin-transport defective mutant aux1 and auxin response mutants axr1 and axr2 exhibit impaired root hair phenotype when compared with that of the wild-type plants.^14,15 In addition, pharmacological studies have also shown that the root hair growth significantly increases after treatment with the auxin analog, NAA. Interestingly, the elevated levels of endogenous auxin are also involved in the regulation of root hair development in response to nutrient deficiency.^16–18 Since Mg is involved in transforming auxin from an active to inactive state,^19,20 it is possible that higher Mg concentration affects the proportion of active auxin and thus inhibits root hair growth. Our observations are consistent with these results. We found that Mg deficiency elevates the levels of auxin in the roots, which then regulate the initiation and elongation of root hairs.²¹ Furthermore, the root hair development induced by Mg deficiency appeared to be controlled by a relatively specific auxin transport pathway. Root hair development induced by Mg deficiency was strongly or completely inhibited in the auxin transport mutants pin1-1, aux1-7 and pin2, but not in pin3-5, pin4-3 and pin7-2.²² Consistently, Mg deficiency also upregulates the expression of AUX1, PIN1 and PIN2 genes, but not PIN3, PIN4 and PIN7.²²

Ethylene and NO are also critical signals controlling root hair morphogenesis. Ethylene not only stimulates root hair outgrowth but also induces non-hair cell to form root hairs.^23,24 Similarly, NO is involved in the root hair development.²⁵ Moreover, nutrient deficiency stress, such as deficiency of either iron, phosphorus, or potassium, induces ethylene and NO production in plants. Under Mg deficiency, ethylene and NO production are induced in the roots, which then regulate root hair morphogenesis.²² Both auxin and ethylene/NO modulate root hair growth under Mg deficiency. The key question is whether coordination exists between auxin and ethylene/NO in the root hair development induced by Mg deficiency. Previous studies showed that auxin and ethylene/NO coordinately regulate several physiological processes in plants. Ethylene frequently acts upstream of auxin in modulating root cell elongation and lateral root growth in Arabidopsis. NO acts downstream of auxin to induce root development. In the model proposed by Liu et al. (2018), ethylene and NO functions upstream of auxin in modulating root hair morphogenesis in Arabidopsis.²¹ Ethylene also promotes the activities of nitrate reductase and nitric oxide synthase-like enzyme to stimulate NO production.In turn, NO elevates ethylene levels by stimulating 1-aminocyclopropane-1-carboxylate (ACC) oxidase and ACC synthase. Ethylene and NO promote auxin accumulation by promoting the expression of AUX1, PIN1, and PIN2 genes and proteins in the roots and acts upstream of auxin to promote root hair morphogenesis under Mg deficiency stress.

Increasing evidence suggests that calcium (Ca²⁺) signaling is involved in the root hair initiation and elongation.^13,26 The cytosolic Ca²⁺ gradient formed in the growing apex is closely correlated with the growth rate of root hair tip cells. The disruption of Ca²⁺ concentration gradient by either Ca²⁺ ionophores or channel blockers inhibit root hair tip growth. Ca²⁺ exerts effects on cytoskeletal components, actin, and vesicle movement to regulate root hair tip growth.²⁷ The competition by Ca²⁺ and Mg²⁺ for the same binding site on the plasma membrane in plants and the increase in Mg reduces the concentration of either Ca²⁺ or cytosolic Ca²⁺ or both. Hence, the increase in Mg²⁺ supply likely affects root hair growth by disrupting the Ca²⁺ gradient in the root hair cells. Niu et al. (2014) suggested that the higher Mg supply restricted the growth of root hair by reducing the concentration gradient of Ca²⁺ in the root hair tip.¹¹ Moreover, the addition of Ca²⁺ to high Mg solution increased cytosolic Ca²⁺ concentration in the tip of root hairs and restored root hair growth.

Similar to Ca²⁺, ROS has been suggested to promote root hair elongation.^28,29 Exogenous application of ROS promotes the growth of root hair, while the ROS scavenging distorts the growth of root hairs, and in some cases completely stopped the growth.^30,31 Evidence from genetic studies also suggests that the rhd2-1 mutant defective in NADPH oxidase, the main enzyme involved in ROS production reduces root hair development.^32,33 A previous study found that root hair development under Mg deficiency is closely correlated with ROS accumulation in plants¹¹. Importantly, the NADPH oxidase inhibitor DPI significantly inhibited root hair growth, while the ROS donor PMS recovered the root hair growth under Mg sufficiency. Furthermore, Mg deficiency activated the NADPH oxidase, which led to ROS production in the roots of Arabidopsis.¹¹ Transcriptional data suggest that Mg deficiency upregulates the expression of FRO5, FRO4, and FSD1 genes that are related to NADPH oxidase than that seen during Mg sufficiency¹¹.

Many studies have suggested interactions occurring between ROS and Ca²⁺ in plants.^34,35 ROS regulates the activity of Ca²⁺ channels and activates the Ca²⁺ pump in the plasma membrane and the cytosolic Ca²⁺ concentration.³⁶ Ca²⁺ also promotes the phosphorylation of NADPH oxidase to induce ROS accumulation, which in turn promotes Ca²⁺ influx in the cytoplasm.^37,38 Consistently, Niu et al. (2014) also found that Mg deficiency promotes ROS accumulation in the tips of root hair, which activates the Ca²⁺ channel in the plasma membrane resulting in Ca²⁺ influx in the cytoplasm.¹¹ The inhibition of Ca²⁺ absorption due to high Mg²⁺ limits the activity of NADPH oxidase and reduces ROS accumulation in the roots. At the same time, the high Mg supply also reduces ROS in the roots, which is unfavorable for Ca²⁺ channel activity.¹¹ Therefore, ROS and Ca²⁺ form a positive feedback loop to regulate root hair growth induced by Mg deficiency.

Studies suggest the existence of two signaling transduction pathways involving Mg-deficiency-induced root hair development (Figure 1). The first one is evoked by ethylene/NO and auxin, while the other is manipulated by ROS and Ca²⁺ in Arabidopsis. A question may be proposed as to whether a linkage between the above two exists in the signaling pathway in Mg deficiency-induced root hair development. It is suggested that the auxin application triggers the Ca²⁺ signal cascade, induces a rapid increase in the cytosolic Ca²⁺ concentration and modulates the root curvature.³¹ In addition, Ca²⁺ transduces auxin signals to the Ca²⁺ sensors and regulates PIN1-mediated auxin transport in the post-embryonic development process.³⁹ A relatively high Ca²⁺ supply enhances the auxin-induced reaction and affects cell sensitivity to auxin. The importance of Ca²⁺ in auxin-dependent reactions was also found in root hairs of tomato.⁴⁰ Additionally, auxin and ROS are involved in the regulation of root hair growth.²⁹ It is tempting to hypothesize that auxin and Ca²⁺/ROS probably interactively regulate root hair development induced by Mg deficiency. Further work is needed to dissect the possible linkage of auxin response to other signal molecules under Mg deficiency.

Figure 1.

Schematic model for the regulation of root hair development via chemical signals induced under Mg deficiency.^11,21,22.

In addition to the above chemical signals, there are some other signal molecules involved in the mediation of the adaptive response to Mg deficiency. Abscisic acid (ABA) plays a major role in the adaptive responses to Mg deficiency, since the ABA signaling is massively triggered by Mg deficiency stress.^4,41 The transcriptomic data suggest that Mg deficiency rapidly induced HAB1 expression, a negative regulator in the ABA-mediated signaling cascade leading to ABA-mediated pathway deregulation. Mg deficiency also induces genes related to protein response, and gibberellin and cytokinin^4,41 biosynthesis. However, it is not clear whether these hormone signaling cascades are affected by Mg absorption or by the induction of root hair development under limited Mg availability. This requires further exploration.

Taken together, signal molecules such as ROS and plant hormone could function in concert or upstream/downstream in the signaling transduction of root hair development induced by Mg deficiency for the better adaptation of plants to Mg limitations.

Funding Statement

This work was financially supported by the Natural Key R&D Program of China [2016YFD0200103], the Natural Science Foundation of China [31622051], and the Fundamental Research Funds for the Central Universities [2017XZZX002-06].

Disclosure of potential conflicts of interest

No potential conflicts of interest are disclosed.