Function, Expression and Applications of AtGluR Genes

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This entry is adapted from the peer-reviewed paper 10.3390/plants11192580

Arabidopsis thaliana contains 20 glutamate receptor genes (AtGluR) comparable to the human ionotropic glutamate (iGluRs) receptor. Many studies have proved that AtGluR genes are involved in a number of plant growth and physiological activities, such as in the germination of seeds, roots, abiotic and biotic stress, and cell signaling, which clarify the place of these genes in plant biology.

fluorescence resonance energy transfer glutamate receptors ligand binding domain

1. Clade I

1.1. AtGluR-1.1

The glutamate receptor genes (AtGluR)-1.1 gene has been reported to play a role in potential activity by regulating and signaling abscisic acid (ABA) biosynthesis in A. thaliana ^[1]. AtGluR-1.1 also regulates C, N, and water balance, which are essential for plant growth. The GUS expression was first observed in stipules and the collette region of 7-day-old Arabidopsis plants. Later, its expression was found in the leaf margins and in the cells of the lateral roots (Figure 1). A low expression of GluR-1.1 was also detected in the flowers, siliques, and reproductive organs ^[2].

Figure 1. Flowchart showing the expression and function of the AtGluR-1.1 gene.

1.2. AtGluR-1.2 and 1.3

Zheng et al. ^[3] demonstrated that GluR-1.2 and -1.3 regulate cold tolerance in Arabidopsis owing to cold stress by stimulating endogenous jasmonate synthesis, and these genes also play a crucial role in the downstream CBF/DREB1pathway during cold stress ^[3]^[4] (Figure 2). A fluorescent tag was ligated with GluR-1.2 and -1.3 to detect its expression, and an increased fluorescence was observed in the plasma membrane of Nicotiana benthamiana ^[3].

Figure 2. Flowchart of AtGluR-1.2/-1.3.

1.3. AtGluR-1.4

The expression of GFP-tagged AtGluR-1.4 in the plasma membrane of wild-type Arabidopsis plants was studied. StREM1.3, a plasma membrane marker tagged with red fluorescent protein (RFP), was also employed for co-localization ^[5]. However, Roy et al. ^[6] detected a varying expression of AtGluR-1.4 in different cells of the same plant, as well as in distinct plants. In addition, Ca²⁺ depolarization in leaf cells is perhaps caused by amino acid signaling ^[5] (Figure 3).

Figure 3. Expression and effect of the agonist and antagonist on AtGluR-1.4.

2. Clade II

2.1. AtGluR-2.1

On the basis of the phylogenetic and expression analysis of the glutamate receptor, Chiu et al. ^[2] reported that expression of a clade-II gene (GluR-2.1) was observed in the shoot of a 5-day-old plant that showed similarity with GluR-1.1. In addition, GluR-2.1 was also detected in the radical of emerging seedling, and it was expended in all cells of the root, except in the root apex of 3-day-old seedling. However, a slight expression was detected in the reproductive organs and no such expression was shown in the siliques or flowers ^[2]. Roy et al. ^[6] found that AtGluR-2.1 was expressed in 4-week-old leaf tissue. The pretreatment of glutamate in plants enhanced the upregulation of the gene expression and the accumulation of amino acids ^[7] (Figure 4).

Figure 4. Diagrammatic representation of the presence, absence, and lower expression of Clade II genes in Arabidopsis. + (Expression), (no expression), ~ (low expression).

2.2. AtGluR-2.2

The expression of AtGluR-2.2 was reported in the root, whereas it was lost in reproductive organs and the leaves ^[2]. A similar observation was reported by Roy et at. ^[6], where AtGluR-2.2 was expressed in the root but was absent in leaf, stem, and petiole part of the plant.

2.3. AtGluR-2.3

The expression of AtGluR-2.3 was quite different, as it was restricted in 8-week-old plants; after that, it was seen in the root tissue ^[2]. AtGluR-2.3 was not expressed in the flowers and silique, while a deficient expression was detected in the leaves of the developing plant ^[6].

2.4. AtGluR-2.4

In addition, AtGluR-2.4 was expressed in the root and silique, but the expression was completely stopped in stem, petioles, and leaf tissues ^[2]^[6]^[8].

2.5. AtGluR-2.5

Based on the mRNA expression analysis, Chiu et al. ^[2] reported that AtGluR-2.5 was expressed in the whole plant tissues.

2.6. AtGluR-2.6

It was expressed in the root ^[2], and was absent in the leaf, stem, and petioles ^[6].

2.7. AtGluR-2.7

All of the plant parts showed a higher expression of AtGluR-2.7 instead of the flowers ^[2]^[9].

2.8. AtGluR-2.8

Along with the root and shoot, a good expression was also observed in the leaf mesophyll cells around the vascular bundles with a GUS straining and expression became higher in the leaves during the senescence stage ^[8]^[9] (Figure 4).

2.9. AtGluR-2.9

Similar to the other Clade II receptors, the AtGluR-2.9 gene was also expressed in the root ^[8]; however, Roy et al. ^[6] observed that gene 2.9 either showed a lower expression in the leaves or was completely stopped (Figure 4).

3. Clade III

3.1. AtGluR-3.1

In order to comprehend the expression and function of AtGluR-3.1, Kong et al. ^[10] investigated AtGluR-3.1 in depth and fused a green fluorescent protein (GFP) tag with GluR-3.1 and 3.5, determining that these proteins were situated in the plasma membrane and were expressed in the guard cells. Furthermore, the study found that these receptors were expressed in the seedling cells, other than the guard cells. Cho et al. ^[11], on the other hand, suggested that an increased expression of AtGluR-3.1 was seen in the guard cells, followed by the mesophyll cells. Furthermore, the deregulated expression of AtGluR-3.1 altered the stomatal closure, without impairing cytosolic Ca²⁺.

3.2. AtGluR-3.2

The expression of AtGluR-3.2 was confirmed in all parts of the plant and was highly expressed in the root cells ^[12]. The exogenous supply of Ca²⁺ demonstrated the function of AtGluR-3.2 to overcome the Ca²⁺ deprived condition, which was induced through overexpressed AtGluR-3.2 in transgenic plants ^[13].

3.3. AtGluR-3.3

According to Li et al. ^[14], supplementation of exogenous GSH activates the AtGluR-3.3 and participates in the early transcriptional process of the leaves. However, the genetic process underlying GSH and the independent expression of the gene is not known. Furthermore, root gravitropism in AtGluR-3.3 mutant lines is likely to diminish amino acid regulated Ca²⁺ signaling ^[15].

3.4. AtGluR-3.4

Among the other AtGluRs, AtGluR-3.4 is strongly expressed in guard cells, vascular bundles, roots, and mesophyll and stem. AtGluR-3.4 responds under abiotic stimuli such as cold and touch in a Ca²⁺ dependent manner. Along with the different expression, a weak expression was also observed in the cortex, root epidermis, and hairs. However, the activity of the cold expression was halted by adding lanthanum (Ca²⁺ channel blocker). Moreover, a mutation in AtGluR-3.4 enhanced sensitivity towards ABA, which caused an impact on seed germination ^[16].

3.5. AtGluR-3.5

The expression of AtGluR-3.5 predominantly appeared in Arabidopsis germinating seeds, where this receptor enhances the Ca²⁺ concentration. No significant expression was seen in the dry and mature seeds. However, repression in AtGluR-3.5 impacted Ca²⁺ signaling and was more sensitive to ABA, which exhibited a delay in seed germination. In contrast, a higher expression of AtGluR-3.5 was less sensitive to ABA and led to early germination. The site of expression of AtGluR-3.5 was confirmed by creating a transgenic plant with the GUS gene and it was identified that AtGluR-3.5 was expressed in the cotyledons of the germinating embryo ^[17].

3.6. AtGluR-3.6

The study of AtGluR-3.6 demonstrated that the expression of AtGluR-3.6 is based in the developmental stages of the roots. Singh et al. ^[18] reported a higher expression level in the early stages of root tissues than the mature root tissues. The overexpression of AtGluR-3.6 promoted root growth. Furthermore, KRP4 (kip related protein) played a crucial role in maintaining AtGluR-3.1-related root meristem ^[18]. Cell cycle regulation also influenced the formation of the main and lateral roots ^[18]^[19]. Furthermore, it has been suggested that AtGluR-rec-3.6 may play a role in leaf wound signaling via the jasmonate pathway ^[20]. Other research indicated that AtGluR-3.6/3.3 played a vital role to induce Ca²⁺ elevation on the aphid feeding sites ^[21].

3.7. AtGluR-3.7

Considerably, research on AtGluR-3.7 has found that this receptor is likely expressed in every plant cell and serves an important function as an ion transporter in Arabidopsis ^[6]. Further research has demonstrated that AtGluR-3.7 responds to salt stress in Arabidopsis via calcium signaling and is involved in seed germination and regulation ^[22]^[23]. The overexpression of AtGluR-3.7 provoked a marked increase in root growth.

4. AtGluRs Gene Respond to Environmental Stress

4.1. Stress Due to Salt

Salt stress alters plant behavior and adaption mechanisms, which are usually mediated by the Ca²⁺ signaling network ^[24]^[25]^[26]^[27]^[28]. The study of Cheng et al. ^[29] demonstrated that AtGluR-3.7 played a significant role during seed germination under salt stress conditions. Furthermore, Cheng et al. ^[30] confirmed that salt stress treatment on Arabidopsis wild and mutant plants revealed that AtGluR-(3.4-1/2) of mutants were more sensitive in seed germination when compared with the wild type. An expansion study showed NaCl caused a rise in Ca²⁺ wave in the wild variety of plants, which was blocked by inserting an animal glutamate receptor antagonist (DNQX (6,7-dinitroquinoxaline-2,3-dione)). However, an increase in Ca²⁺, triggered by NaCl, was impaired in mutated AtGluRs (3.4-1/2). In addition, more Na⁺ was accumulated in the mutant during salt stress than in the wild-type seeds. Overexpressed AtGluR-3.4 was highly resistant to abscisic acid ^[30]. Furthermore, Wang et al. ^[22] reported in their study that AtGluR-3.7, in conjunction with other proteins (14-3-3), plays a potential role in Arabidopsis under salt stress by influencing the Ca²⁺ signaling pathway. As a result of these findings, seed germination under salt conditions was regulated by Ca²⁺ inflow, which was modulated by AtGluRs-3.4/3.7. Therefore, the Clade III gene, GluR-3.7, plays a significant role in salt stress in A. thaliana ^[31].

4.2. Stress Due to Cold

In addition to salt stress, cold stress is also another factor that harms plants. In 1999, Thomashow ^[4] reported that temperate region originating plants such as spinach and Arabidopsis showed cold tolerance upon exposure to low temperatures. This phenomenon is known as cold acclimation. In addition to the plasma membrane, chloroplast also plays a sensing role at low temperatures. Earlier, Meyerhoff et al. ^[24] suggested that because of the osmotic stress in Arabidopsis, AtGluR-3.4 was expressed in an ABA-independent fashion and provides strength in fast signaling via Ca²⁺ ion.

In transgenic plants, cold and glutamate (Glu) were able to induce Ca²⁺ ion channels, which was blocked by the antagonist (DNQX/6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)). The study of Hu et al. ^[32] indicated that during cold stress, phytohormones altered their activity and response towards plants. Further research revealed that the membrane protein sensed cold stress and perhaps activated Ca²⁺ signal transduction pathways. These signals may lead to activating the downstream transcriptional regulatory cascade and allow them to cope and survive in cold stress ^[33]^[34]^[35]^[36]^[37]. Thus, exposure to cold temperature plants induces tolerance in cold conditions, as well as a massive alteration in gene expression and Ca²⁺ signal transduction pathways. In addition, Zheng et al. ^[3] unveiled that AtGluRs-1.2 and -1.3 mutants performed a positive role towards cold stress in Arabidopsis, whereas with the influence of the plant hormone jasmonate, the activity of AtGluRs-1.2/-1.3 mutants to cold stress was reduced. Furthermore, under a freezing environment, the expression of these mutants was lower in the C-repeat binding factor/DRE binding factor 1 (CBF/DREB1) transcriptional regulatory pathway compared with the wild-type plant. In brief, Zheng et al. ^[3] suggested that AtGluRs-1.2/1.3 could increase cold tolerance by trigging endogenous jasmonate accumulation (Figure 5).

Figure 5. Glutamate receptors mediated by Ca²⁺ ion due to environmental stress, glutamate (Glu), abscisic acid (ABA), reactive oxygen species (ROS), nitric oxide (NO), jasmonate (Jas).

4.3. Stress Due to Drought

Drought stress is a severe environmental barrier to plant productivity. Drought stress impacts different physiological processes and reduces crop yield ^[38]^[39]. Water scarcity has been observed at a morphological to molecular level in plants. Drought stress severely impacts on the photosynthetic abilities of plants, which imbalances metabolic activities. When faced with stress, plants modify their strategies to allow them to be capable of resisting and adapting to drought conditions ^[38]^[39]. After many debates on the closure of the stomata, its impact on drought stress or metabolic impairment was found. It has been proven that stomatal closure is one of the main factors for controlling water deficits as well as gas exchange during stress, which is regulated by the co-ordination of different signaling molecules, including Ca²⁺, H₂S, reactive oxygen species, ABA, and nitric oxide NO ^[7]^[38]^[39]. The study of Yoshida et al. ^[39] demonstrated that glutamate (Glu), a signaling molecule that increases Ca²⁺ in guard cells, is responsible for closing the stomata and preventing light from opening up the stomata in Arabidopsis, as well as in Vicia faba. Furthermore, to discover the complete relevance of glutamate receptors with stomatal closure due to glutamate, different Ca²⁺ chelators such as EGTA (extracellular Ca²⁺ chelator) and BAPTA-AM (intracellular chelator) were used, and it was found that these chelators prevent the stomata from being closed, which is induced by glutamate, and show that the influx of Ca²⁺ into the cytosol is required. They also observed that Glu-dependent Ca²⁺ flow triggers Ca²⁺ dependent protein kinase and endorse the SLAC activity to close the stomata ^[39]. Other than Arabidopsis, Glu treatment in Brassica responds by triggering Ca²⁺ signaling, which increases proline accumulation due to drought stress and manages drought tolerance ^[40]. In addition, Philippe et al. ^[41] used animal glutamate receptor antagonist-AP-5, DNQX, in Medicago truncatula and reported that these antagonists were responsible for reducing NO accumulation, which showed the least effect on stomata closure. Furthermore, two glutamate receptors obtained from rice (GLR1 and GLR2) exhibited drought tolerance in Arabidopsis ^[42].

5. Role of AtGluRs in Plant Defense Signaling

Plants have defended themselves by evolving various defense tactics against the opponents, which take the shape of insects, pests, and other herbivores. Plants use both chemical and mechanical means to detect herbivores. Many recent studies have focused on monitoring early defense signaling to protect them from herbivore attacks in the form of fluorescent-based genetically encoded nanosensors to detect in real time signaling and modulate the ion channels that are commonly associated with plant−herbivore interactions ^[43]. Furthermore, a number of studies have shown that Glu receptors protect plants from insect injury. Because of insect activity on plants, Glu receptors increase Ca²⁺ flow, causing defensive signals to change in order for the plants to survive ^[44]^[45].

In an experiment when A. thaliana was wounded by S. littoralis, it was found that AtGluR-3.3 induced Ca²⁺ ion flow to reduce the surface potential caused by the larvae and showed a potential role in altering the defense signaling in plants ^[20]. Glu receptors are involved in plant defense signaling as they are activated by the wound ^[19]^[20]. In addition, recent research suggested that GluRs-3.3/3.6 modulates Ca²⁺ ion flux to defend the plants against wound-creating organisms ^[43]^[46]. Initially, the research of Kang et al. ^[47] showed that in transgenic Arabidopsis, overexpressed radish GluR intermingles with the increased expression of some defense-linked genes and accelerates the resistance against fungal pathogens. Afterwards, pharmacological studies showed that ionotropic glutamate receptors (iGluRs) antagonists were found to take part in the immune response in seedlings of A. thaliana ^[23] and in the suspension culture of tobacco ^[45]. In AtGluR-3.3 mutants, because of the attack of a bacterial pathogen (Pseudomonas syringae), susceptibility increased with regard to immune response, which exhibited the inadequacy in the activity of defense-related gene expression against infection ^[14]. Although almost all AtGluR genes take part in the defense mechanism, AtGluR-3.3 was found to be more responsible, as in P syringae, activation of the defense gene in response to enhancing immunity against infection depends on AtGluR-3.3 ^[48]. A study suggested that the glutamate receptors associated with Ca²⁺ influx are essential for initiating downstream signaling processes towards the plant defense. Further involvement of AtGluR-3.3 is important for resistance infected by Hyaloperonospora arabidopsis (oomycete pathogen) ^[49]. It has been found that the Clade III gene fully participates in the defensive mechanism against mechanical wounding caused by feeding insects and pests ^[20]^[50].

6. Developed FRET Based Glutamate Receptors Nanosensors

Modern biosensors based on fluorescence resonance energy transfer (FRET) provide a potential tool. FRET has become an advanced phenomenon for monitoring the interactions, such as molecule to molecule or protein to protein, or conformational changes (Figure 6). The biggest advantage is that the distances are obviously shorter than the diffraction limits of other microscopies. Fluorescent proteins (FP) are used as an essential component in the creation of biosensors. This method is based on the excitation of the donor fluorophore, followed by the non-radiative transfer of the excited energy to the related other fluorophore protein, known as the acceptor molecule ^[51]^[52]. Various FRET-based nanosensors have been constructed, but the contents here concentrated on FRET-based approaches to glutamate receptors. Glutamate receptors are not only for animal neurons and glial cells ^[53]. These receptors exist in the entire world, as well as eukaryotes and prokaryotes ^[54]^[55]^[56]. Okumoto et al. ^[57] were the first to construct a glutamate ratiometric sensor by sandwiching a truncated binding protein (gltl) of glutamate/aspartate between a fluorophore protein pair (enhanced cyan fluorescent protein (ECFP), mVenus). Hires et al. ^[58] introduced EYFP (enhanced yellow fluorescent protein) in lieu of m-Venus and conducted systematic monitoring of the glutamate affinities. Next, Marvin et al. ^[59]^[60] proposed that single-wavelength fluorescence receptors (FR) could detect neurotransmitters with a great spatial and temporal resolution. They created a number of glutamate iGluSnFR sensor versions employing blue, green, cyan, and yellow color emissions and circular permutated cp-GFP, with activities ranging from sub-micromolecular to millimolar.

Figure 6. FRET phenomenon, a potential tool.

The plant glutamate receptor protein family emerged as a binding cassette for neurotransmitters with the introduction of animal glutamate nanosensors. In a similar way to how glutamate acts in neurons, it also aids in the flow of impulses in plants ^[19]^[61]. Plants use the fluorescent indicator protein for glutamate (FLIPE) ^[61] (Figure 7). According to Forde and Roberts ^[54], glutamate receptor proteins have been found to bind glutamate/4-aminobutanoic acid (GABA) and amino acids such as glutamate/glycine. The high-affinity GABA transporter (AtGAT1) in A. thaliana may be required for GABA sensor function, as found by Meyer et al. ^[62]. To change the spectral range of the glutamate sensor, Wu et al. ^[63] created circular permutated (cp) R-iGluSnFR1 using red Glu sensing FR and noncircular permutated (ncp) Rncp-iGluSnFR1.

Figure 7. Expression of FRET in the form of nanosensor in a plant cell; PM—plasma membrane; ER—endoplasmic reticulum; Chl—chloroplast; Vac—vacuole; Nuc—nucleus; Cyt—cytoplasm.

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Badrah S. Alghamdi

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