Recently, the molecular basis of ABA transport has been defined. Two models of transmembrane ABA translocation, based on diffusion and the presence of primary and secondary transporters, are now widely accepted. ABA, as a weak acid, exists in an anionic (ABA^-), and in a protonated (ABAH) form. The latter, when uncharged, is able to diffuse through the plasma membrane (PM). The diffusion of weak acids has its limitations. An increase in pH (ABA pKa 4.7) due to drought stress can drastically decrease the pool of apoplastic-diffusible ABAH. This limiting step highlights the necessity of ABA transporters, which increase the amount of ABA accessible to intracellular receptors upon stress conditions (Boursiac et al., 2013).

It has been proposed that ABA-GE conjugates are actively transported into the vacuoles of mesophyll cells by ABC transporters (see Figure 1) or proton gradient-driven transporters (Burla et al., 2013). Two members of the A. thaliana ABCC/MRP (multidrug resistance-associated protein) subfamily, AtABCC1/MRP1 and AtABCC2/MRP2, are localized in tonoplast and exhibit ABA-GE transport activity in a yeast heterologous expression system. According to microarray analysis, the expression of ABCC2 is induced upon drought. In addition to ABCCs, secondary transporters also participate in the vacuolar sequestration of ABA conjugates; these transporters are presumably members of the MATE family (Burla et al., 2013).

The reliable and fast adjustment of stomatal aperture is necessary for effective drought avoidance in plants. A rapid response reduces the amount of water lost, increases water use efficiency, and allows plants to survive (Farooq et al., 2009). There are two pathways leading to stomatal closure, passive, and active. In the passive pathway, turgor loss occurs without a reduction of the solute content of the guard cells (McAdam and Brodribb, 2014). Because the passive pathway cannot sufficiently protect the plant against drought, active solute loss is necessary. The movement of osmoregulatory ions (K⁺, Cl^-, and malate^2-) across guard cell membranes and the gluconeogenic conversion of malate into starch, accompanied by water export through aquaporins (AQPs), results in a decrease in cell volume. Turgor is lost, and the stomata finally close. Mature guard cells have no plasmodesmata; the existence of channels, transporters, and pumps in the PM are required (Lawson and Blatt, 2014).

In the active solute loss scenario, ABA is imported into guard cells by ABCG40 in response to drought. This triggers the production of reactive oxygen species (ROS) such as hydrogen peroxide (H₂O₂) and nitric oxide (NO) and cytoplasmic Ca²⁺ ([Ca²⁺]_cyt) increases (Kim et al., 2010). The level of oxidative stress is controlled by a group of low molecular antioxidant compounds known as flavonols (Watkins et al., 2014). The molecular nature of flavonol transporters in these particular cells remains to be elucidated. However, the action of such proteins indirectly influences the aperture of guard cells. Members of the MATE and ABC protein families are involved in the transport of secondary plant metabolites, including flavonoids (Zhao and Dixon, 2010). The accumulation of [Ca²⁺]_cyt, is possible due to Ca²⁺-permeable channels located in the tonoplast and the PM (see Figure 2). These proteins are encoded by genes belonging to the TPC1 (two-pore channel 1), CNGC (cyclic nucleotide-gated channel), and GLR (glutamate receptor-like) families. TPC1 has been classified as slow vacuolar (SV) non-selective cation channel that is regulated by luminal Ca²⁺ (Islam et al., 2010). In A. thaliana, members of the CNGC and GLR families, namely AtCNGC5, AtCNGC6, and AtGLR3.1, are PM-located Ca²⁺ channels (Cho et al., 2009; Wang et al., 2013). The distinguishing feature of CNGC proteins is cGMP-dependent activation (Wang et al., 2013), which is also required for an increase in [Ca²⁺]_cyt (Dubovskaya et al., 2011). The long-term Ca²⁺-programmed stomatal response that prevents guard cell reopening is established by AtGLR3.1 (Cho et al., 2009).

The ABA-induced signal cascade results in the activation of S-type (slow-activating sustained) and R-type (rapid-transient) anion channels, which cause PM depolarization. Simultaneously, the inhibition of H⁺-ATPase and inward K⁺ channels (KAT1/KAT2) prevents the hyperpolarization of the PM (Osakabe et al., 2014). Recent studies have identified the genes encoding S and R-type anion channels. The A. thaliana SLAC1 (slow anion channel-associated 1) protein is a PM-localized (Negi et al., 2008; Vahisalu et al., 2008) S-type anion channel with a preference for Cl^- and NO₃^- (see Figure 2; Geiger et al., 2009; Lee et al., 2009b). This protein operates mainly in response to ABA, CO₂, Ca²⁺, NO, and H₂O₂ (Vahisalu et al., 2008). Mutations in SLAC1 result in the excessive accumulation of anions, such as Cl^-, in cell protoplasts and the disruption of stomatal closure (Negi et al., 2008; Vahisalu et al., 2008). Phosphorylation at distinct sites enables SLAC1 activation by both calcium-dependent and calcium-independent pathways downstream of ABA (Maierhofer et al., 2014). A second S-type anion channel, SLAH3 (SLAC homolog 3), is expressed in A. thaliana guard cells (Geiger et al., 2011). In contrast to its homolog, SLAH3 has higher permeability toward NO₃^- than Cl₂^- (see Figure 2; Geiger et al., 2009, 2011; Lee et al., 2009b). AtMRP5/ABCC5, a vacuolar inositol hexakisphosphate (InsP6) transporter from the ABC transporter family, may be a central regulator of Ca²⁺-permeable and S-type anion channels in the PM of guard cells. This regulation is possible due to the complexing of divalent cations by InsP6 or the triggering a continuous Efflux of Ca²⁺ into the cytosol by an InsP6-regulated channel (see Figure 2; Nagy et al., 2009). A member of the aluminum-activated malate transporter family in A. thaliana, QUAC1/ALMT12 (quick anion channel 1/aluminum-activated anion channel 12), has been identified and classified as malate-sensitive R-type anion channel (see Figure 2; Meyer et al., 2010) or a chloride and nitrate current facilitator (Sasaki et al., 2010). Loss of QUAC1 function results in impairment of stimulus-induced stomatal closure (owing to factors such as ABA, CO₂, Ca²⁺; Meyer et al., 2010; Sasaki et al., 2010). Compared to our understanding of anion influx across the tonoplast, the molecular mechanisms of Efflux from the vacuole are poorly understood.

Plasma membrane depolarization activates the Efflux of potassium. This transport is facilitated by GORK (guard cell outward rectifying K⁺) channels, which belong to the Shaker family of voltage-gated ion channels (see Figure 2). GORK transcripts have been widely observed in A. thaliana, especially in guard cells (Becker et al., 2003; Hosy et al., 2003), where they represent the only outward-rectifying K⁺ channel (Eisenach et al., 2014). The inhibition of GORK activity in guard cells results in defects in K⁺ Efflux and a lower rate of stomatal closure (Hosy et al., 2003). Osakabe et al. (2013) identified stress-responsive K⁺ uptake permeases (KUPs), which share redundant functions with GORK channels (see Figure 2). The KUP6 and KUP8 proteins in A. thaliana function as K⁺/H⁺ symporters. KUP6, which is highly up-regulated in response to the ABA treatment and water deficit, is localized to the PMs of guard cells, root tip cells and vascular tissues. KUP6-overexpressing lines were more tolerant to drought. A kup68g double mutant line (with combined kup6 and kup8 mutations in addition to mutation in the gork potassium channel) had impaired stomatal closure and ABA sensitivity; this mutant also had greater stomatal conductance and water loss rates (Osakabe et al., 2013). An isoform of the ZIFL1 (zinc-induced facilitator-like1) transporter, which belongs to the major facilitator superfamily (MFS), proved to be a modulator of stomatal aperture and conferred drought stress tolerance. The PM-localized AtZIFL1.3 protein possesses H⁺-coupled K⁺ transport activity, and its overexpression resulted in more efficient stomatal closure (see Figure 2; Remy et al., 2013). The main cellular depository of ions, including potassium, is the vacuole. The release of K⁺ from the vacuole is mediated by channels and transporters localized to the tonoplast. TPK1, a member of the two pore K⁺ channel (TPK) family, is responsible for vacuolar K⁺ translocation in guard cells (see Figure 2). The removal of TPK1 was associated with a weak phenotype; most often the stomatal closure was slower, but the opening kinetics were normal (Gobert et al., 2007).

The outflow of water results in a reduction in guard cell turgor and stomatal closure; this is mediated by AQPs, which are membrane channels that belong to the MIP (major intrinsic protein) family (see Figure 2; Heinen et al., 2009). AQPs may form homo- and heterotetramers, with each monomer defining a single pore. Based on amino acid similarity and membrane localization, AQPs are divided into five subgroups; for a review, see (Maurel et al., 2008; Chaumont and Tyerman, 2014). The expression of several genes encoding AQPs belonging to the PIP (PM intrinsic proteins) and TIP (tonoplast intrinsic proteins) subgroups was up-regulated in leaves, mainly in the guard cells, in drought stress conditions (Heinen et al., 2009). Heinen et al. (2014) have identified Zea mays PIPs that are localized to stomatal complexes (mainly ZmPIP1;1, ZmPIP1;3, ZmPIP2;2) and presumably participate in stomatal movement (Heinen et al., 2014). The expression of PIP1 from Vicia faba and Brassica juncea improved drought resistance in A. thaliana and Nicotiana tabacum plants, respectively, through the promotion of stomatal closure (Cui et al., 2008; Zhang et al., 2008).

Proteins that regulate stomatal movements by supporting swelling belong to a separate group of transporters. The disruption of Cl^- (AtCLCc, AtALMT9), K⁺ (AtNHX1, AtNHX2, AtCHX20), and NO₃^- (AtNRT1.1, AtCLCa) channels/transporters, as well as the AtMRP4/AtABCC4 protein, results in a delay in stomatal opening (Guo et al., 2003; Klein et al., 2004; Padmanaban et al., 2007; Jossier et al., 2010; De Angeli et al., 2013; Andres et al., 2014; Wege et al., 2014). Phosphorylation-dependent changes in the activity of AtCLCa enable the protein to perform dual roles, in anion sequestration in the vacuole during stomatal opening and in anion release during stomatal closure, in response to ABA (Wege et al., 2014). Mutations in a malate importer protein (AtABCB14) result in accelerated stomatal closure (Lee et al., 2008). Mutations in AtALMT9, AtNRT1.1, and AtMRP4 transporters result in the impairment of stomatal responses and are associated with increased drought tolerance (Guo et al., 2003; Klein et al., 2004; De Angeli et al., 2013).

To date, the intracellular mechanism of wax and cutin precursor trafficking remains unclear. Several hypothetical pathways to the PM have been suggested (see Figure 3). These precursors might reach the PM via the following routes: (i) direct relocation at ER-PM contact sites (Samuels and McFarlane, 2012); (ii) transport by cytosolic carrier proteins (e.g., Acyl-CoA binding proteins – ACBPs; Li-Beisson et al., 2010); (iii) vesicular translocation in oleosome bodies coated by oleosin-like proteins; (iv) translocation in uncoated vesicles that sequestrate the cuticular lipids into lipid rafts (Schulz and Frommer, 2004); or (v) Golgi mediated exocytosis (Samuels et al., 2008). Through mutant analysis combined with live cell imaging, electron microscopy and chemical phenotyping, McFarlane et al. (2014) demonstrated that the movement of intracellular wax requires vesicle traffic mediated by Golgi- and trans-Golgi networks. However, AtACBP1 has been shown to contribute to the formation of the stem cuticle; dysfunction in this protein resulted in a reduction in wax and cutin monomer composition. Moreover, AtACBP1 exhibits biding activity to wax precursors (Xue et al., 2014). The cytosol-localized protein LjLTP3, a member of the LTP (lipid transfer protein) family, is able to bind lipids and confers drought tolerance in Lotus japonicus.

Members of the ABC transporter family mediate the flux of cuticular components through the PM. Two half-size ABC transporters, named AtABCG12/WBC12/CER5 (Pighin et al., 2004) and AtABCG11/WBC11/DSO/COF1 (Bird et al., 2007; Luo et al., 2007; Panikashvili et al., 2007; Ukitsu et al., 2007), are involved in the secretion of cuticle precursors into the apoplastic environment (see Figure 3). Mutation of both genes results in a significant reduction in the stem wax load (approx. 50%); a major depletion in C29 alkane, the major component of cuticular waxes in stems, is also observed (Pighin et al., 2004; Bird et al., 2007). The total amount of wax in the epidermis remains unchanged, indicating that the absence of these two proteins impedes wax transport rather than biosynthesis. AtABCG11 is also likely involved in the translocation of cutin monomers in vegetative and reproductive organs and the transport of suberin in roots (Panikashvili et al., 2007, 2010). The evidence that AtABCG11 forms homo- and heterodimers with AtABCG12 comes from bimolecular fluorescence complementation and protein trafficking assays (McFarlane et al., 2010). AtABCG11 also plays a dominant role in the created complex; ABCG12 could not traffic to the PM in the absence of ABCG11, whereas ABCG11 trafficking was independent of ABCG12 (McFarlane et al., 2010). The half-size ABCG transporter from Solanum tuberosum (StABCG1) was shown to contribute to the export of suberin components across the PM (Landgraf et al., 2014). Silencing of the StABCG1 gene, which is expressed mainly in the roots and tuber periderm, results in a reduction in esterified suberin monomers; this is accompanied by an accumulation of the presumed suberin precursors. Root and tuber morphology was altered, and tubers were more susceptible to drought (Landgraf et al., 2014). The putative ortholog of AtABCG12 found in the moss species Physcomitrella patens is involved in the transport of a wax precursor. This finding suggests that the role of ABCG transporters, particularly half-size transporters, in the movement of cuticle components is evolutionarily conserved among land plants (Buda et al., 2013). Loss-of-function in AtABCG13/WBC13 (another half-size ABCG transporter that belongs to the ABCG12 clade) results in a significant depletion in petal cutin monomers; this result indicates that the protein plays a role in cutin deposition in flowers (see Figure 3; Panikashvili et al., 2011). The full-size ABCG transporter (AtABCG32) has been shown to contribute to the formation of functional cuticles (see Figure 3; Bessire et al., 2011). Knockout mutations in AtABCG32 result in a significant increase in cuticle permeability in leaves and flowers, as well as in water loss from rosettes and herbicide sensitivity. The ultrastructural changes in the cuticle were associated with reduced amounts of minor aliphatic cutin monomers; these monomers are most likely exported from epidermal cells by AtABCG32. The AtABCG32 monocot orthologs OsABCG31 1 and HvABCG31 may also be involved in cuticle deposition. Loss-of-function mutations in these genes, both in Oryza sativa and in Hordeum vulgare, result in water retention deficiency phenotypes (Chen et al., 2011).

Members of the ABCG subfamily in A. thaliana are also required for proper pollen coat (extracellular matrix) and exine (outer wall) formation. These structures protect gametophytes in unfavorable environmental conditions. Dysfunction in AtABCG9/WBC9 and AtABCG31 proteins results in reduced steryl glycoside levels and altered pollen coat morphology. Correct morphology is essential for pollen coat maturation. A double mutant exhibited increased water loss upon exposure to dry air (Choi et al., 2014). The AtABCG26/WBC27 protein participates in exine deposition in the pollen, presumably by transporting sporopollenin precursors (Quilichini et al., 2010; Choi et al., 2011; Dou et al., 2011). An ortholog in O. sativa (OsABCG15) is crucial for anther cuticle and pollen exine formation (Wu et al., 2014).

Once cuticular components have been moved across the cell membrane, they must pass the hydrophilic CW to reach the cuticle. Movement through the extracellular matrix appears to be facilitated by LTPs. Two members of the LTP family (LTPG1 and LTPG2) are defined as GPI-anchored (glycosylphosphatidylinositol –anchored) proteins. These proteins are located at the exterior face of the PM and are possibly involved in wax export (see Figure 3; Debono et al., 2009; Lee et al., 2009a; Kim et al., 2012). Disruption of AtLTPG1 or its homolog AtLTPG2 modifies the structure of the cuticular layer and reduces the amount of C29 alkane; this is also seen in AtABCG12 and AtABCG11 mutants (Debono et al., 2009; Lee et al., 2009a; Kim et al., 2012). The expression of LTPG1 can complement wax scarcities in ltpg1 mutant lines (Debono et al., 2009). Compared to each single mutant, the double mutant exhibited a more severe phenotype (Kim et al., 2012). Taken together, it seems likely that LTPG1 and LTPG2 are functionally overlapping wax exporters. These proteins act either directly by associating with ABC transporters or indirectly by creating suitable environment for wax Efflux (Debono et al., 2009; Kim et al., 2012). The former model assumes that LTPGs are loaded with C29 alkane cargo and transfer it to other extracellular LTPs, which then carry the lipid load to the plant surface (Debono et al., 2009).

To increase the volume of potentially accessible water, the integrated action of ABA and auxins promotes root growth and development (Xu et al., 2013b). Several transporters, which belong to distinct gene families, act as key modulators of auxin translocation and subsequent root morphology changes (Petrasek and Friml, 2009; Zazimalova et al., 2010). There are only few examples of extensive crosstalk between the mechanisms that regulate auxin transport, root development, and drought resistance. The expression of an A. thaliana vacuolar H⁺-pyrophosphatase (AVP1) confers tolerance against drought in many plant species, including Lycopersicon esculentum (Park et al., 2005), Saccharum officinarum (Kumar et al., 2014b), N. tabacum (Arif et al., 2013), and Gossypium hirsutum (Pasapula et al., 2011). AVP1 overexpressing plants exhibited greater PP_i -driven sequestration of ions and sugars into the vacuole, increased water retention and increased cell turgor. Auxin transport is also stimulated, enhancing root system development (Park et al., 2005). This adaptation to stress conditions is also possible due to the different subcellular and tissue distribution of isoforms of transporters with similar substrate profiles. The MFS carrier ZIFL1 protein (Remy et al., 2013) is one example. The two splice isoforms of ZIFL1, ZIFL1.1, and 1.3, both have K⁺ transport activity that likely influences membrane proton gradients, resulting in distinct biological roles in roots and guard cells, respectively. The change of pH that occurs as a consequence of K⁺ translocation results either in stomata closure or the modulation of auxin transport. In this way, plants can control stomata aperture and change root morphology if necessary.

The effects of ABA on root morphogenesis differ between various plant species, especially the effects on the formation of the lateral roots. In A. thaliana, ABA inhibits lateral root formation. In legumes such as Medicago truncatula, ABA promotes the growth of lateral roots and affects the nodulation process. ABA also acts as a negative regulator of the early stages of nodulation (affecting epidermal Nod factor signaling and cytokinin-activated cortex cell division) and suppresses symbiotic nitrogen fixation (Liang et al., 2007; Ding and Oldroyd, 2009). Nodule formation is an energetic cost that must be strictly controlled; nodulation is highly influenced by environmental conditions. It has been proposed that alternations in hormone levels, especially stress hormones such as ABA, serve as shoot derived signals directed at the roots (Ding and Oldroyd, 2009). This fluctuation in ABA levels is the bridge joining nodulation control and the environmental status of the plant. Despite their importance, the mechanisms behind the translocation/control of such signals are still largely unknown.

Root hydraulic conductivity (L), the water flux across the root surface area, is governed by drought and ABA. In water deficit conditions, root hydraulic conductivity decreases to prevent water outflow from the roots due to progressively decreasing soil water potential (Aroca et al., 2012). Morphological changes in the root tissue and the existence of water channels constitute the main long-term and short-term determinants of root hydraulic conductivity, respectively. The formation of apoplastic barriers, such as Casparian bands and suberin lamellae, prevents cellular death by limiting water outflow into the soil (Lobet et al., 2014). The half-size ABCGs, StABCG1, and AtABCG11, affect the root suberin content. Therefore, these proteins might be responsible for the proper formation of suberin lamellae (Panikashvili et al., 2010; Landgraf et al., 2014). Rapid and reversible alterations in water permeability are done by controlling the expression and activity of AQPs. AQPs allow for the passive movement of water through a cell-to-cell pathway, which is the major water path during drought (Aroca and Ruiz-Lozano, 2012). The expression of the two aforementioned MIP subgroups, PIPs and TIPs, is greater in roots than in leaves. Moreover, the localization of PIPs has been associated with the presence of root hydrophobic barriers; this finding suggests that these proteins are involved in the regulation of water movement during drought (Chaumont and Tyerman, 2014). Upon water deficit, the expression of several PIPs is down-regulated to reduce water loss (Moshelion et al., 2014). According to Jang et al. (2004), nine out of the thirteen PIP genes expressed in the root (three from the PIP1 group and six from PIP2 group) showed significant reductions in mRNA accumulation in A. thaliana plants exposed to drought. Similarly, in N. tabacum, the expression of PIPs is significantly down-regulated in a manner corresponding to the level of drought stress (Mahdieh et al., 2008); this phenomenon has also been observed in Fragaria vesca (Surbanovski et al., 2013) and Camellia sinensis roots (Yue et al., 2014). Additionally, the ectopic overexpression of A. thaliana PIP1;4 and PIP2;5 in N. tabacum resulted in increased water loss under dehydration stress (Jang et al., 2007). By contrast, the heterologous expression of the Triticum aestivum aquaporin gene (TaAQP7), which belongs to PIP2 subgroup, conferred drought tolerance in transgenic N. tabacum; plants were able to maintain their water status, reduce the accumulation of ROS and prevent membrane damage via an ABA-dependent pathway (Zhou et al., 2012). Concerning TIP AQPs, plants may respond to drought through cell elongation, osmotic adjustment, and the mediation of water transport across the tonoplast (Wudick et al., 2009). The constitutive expression of TIP1 from Panax ginseng, TIP1;2 from the highly drought-resistant Thellungiella salsuginea and TIP2 from Solanum lycopersicum confer tolerance to water deprivation in A. thaliana; an increase in water absorption requires roots elongation and high water permeability in the tonoplast (Peng et al., 2007; Sade et al., 2009; Wang et al., 2014). Conversely, it has been demonstrated that TIPs from Glycine soja (TIP2;1) and T. aestivum (TIP2;2) have a negative impact on A. thaliana drought tolerance (Wang et al., 2011; Xu et al., 2013a).

Increasing evidence indicates that arbuscular mycorrhiza (AM) play a crucial role in enhancing tolerance to drought (Abbaspour et al., 2012; Li et al., 2013, 2014; Barzana et al., 2014). Glomeromycota fungi are intimately associated with 80% of land plants; these organisms, supply water and nutrients to the roots of host plants in return for carbon compounds (Schmitz and Harrison, 2014). The improvement in water status is possible by harnessing the hyphae growing in the soil, enlarging the absorption surface, and exploiting the ability of the fungus to take up water despite the low soil water potential (Ruiz-Lozano et al., 2012). In addition, AM symbiosis affects the hydraulic properties of roots and maintains root water permeability by the regulating the expression of plant AQPs (PIPs and TIPs) and the activity of fungal AQPs (Li et al., 2013; Barzana et al., 2014). Considering the involvement of AM in plant response to drought stress, transport mechanisms play a crucial role in the establishment of plant–fungus interactions. The successful recognition of root-exuded plant metabolites, in particular flavonoids (F), 2-hydroxy fatty acids (2OH-FA), strigolactones (SL), and 16C cutin monomers, is a prerequisite for host-root colonization via hyphopodium development (Nadal and Paszkowski, 2013). Despite the importance of these transporters, the knowledge of this pre-symbiotic molecular crosstalk is limited. The isolation of the Petunia hybrida PDR1 transporter has shed light on SL transport, as it encodes for a putative root SL exporter (Kretzschmar et al., 2012). A corresponding protein has been found in the PM of hypodermal passage cells (HPC), the non-suberized cortical entry points for AM hyphae (Kretzschmar et al., 2012). The entrance into cortical cells, where hyphae develop a treelike structure, is called an arbuscule. Arbuscules form through the PM and tonoplast invagination. Formed arbuscules are enveloped by a plant-derived periarbuscular membrane (PAM), a key symbiosis interface that contains several plant transporters and mediates bi-directional nutrient flux (Recorbet et al., 2013).

An increased synthesis of sugar and polyol compounds is triggered by various abiotic stresses. However, types of accumulated sugars may differ among plant species and nature of abiotic stressors (Zhuo et al., 2013; Folgado et al., 2014; Moyankova et al., 2014). Within the cell, carbohydrates are stored in chloroplasts and in the vacuole in the forms of starch and soluble sugars, respectively. In unfavorable conditions, the cytosolic concentrations of sugars and their derivatives are regulated by membrane-embedded transporters. There are three major families of mono- and disaccharide transporters, as follows: monosaccharide/polyol transporters (MST), sucrose transporters/carriers (SUT/SUC) and the SWEET sugar family of transporters (Lalonde and Frommer, 2012). The vast majority of sugar transport is an active process, requiring a proton motive force. However, recent findings indicate that sugar-specific facilitated diffusion transporters also exist in plant cells.