Introduction

Plant growth and development is highly plastic and dynamic. A single genotype possesses the capacity for a startling variety of morphologies and phenotypes, and within its lifetime, a single plant can adopt a variety of morphologies depending on the environmental conditions it is exposed to (de Jong and Leyser 2012). Climatic factors, such as extreme temperatures, drought, and saline soils are major abiotic environmental stressors that limit plant growth, reproductive success and ultimately yield. During the evolutionary shift from water to land, plants have had to acquire the tolerance to a number of osmotic stresses such as periods of soil and atmospheric water deficit (drought) and to high soil salinity (salinity), and they have thus acquired many strategies to survive exposure to osmotic stress (Chaves and Oliveira 2004). One such response to osmotic stress is an inhibition of growth, resulting in reduced stature but also reduced fertilization and ultimately reduced yield (Hsiao et al. 1976). The plant’s growth response to stress conditions is characterized by a rapid and acute inhibition of growth termed “acute response”, followed by recovery and acclimation to the new condition (Skirycz and Inzé 2010).

Virtually every aspect of plant growth and development, from embryogenesis to senescence, requires the plant hormone auxin (Vanderhoef and Dute 1981; Rayle and Cleland 1992; Benjamins and Scheres 2008). Processes governed by auxin in concert with other hormones include, among others, shoot and root meristem maintenance, tropic responses, leaf primordia and lateral root initiation, development of vascular tissues, control of apical dominance, and shoot and root elongation (Went and Thimann 1937; Abel and Theologis 1996). At the cellular level, auxin profoundly affects cell division, elongation and differentiation to drive shaping forces of morphogenesis. Here, we review recent studies integrating osmotic stresses with the auxin pathway in plants. We dissect the auxin pathway into four molecular processes, auxin biosynthesis, transport, perception and cellular auxin response, and illustrate how osmotic stresses modulate these processes at the molecular level to modulate plant growth and development.

Auxin biosynthesis in response to osmotic stress

Auxin biosynthesis

Auxins comprise an important group of hormones that are found in all land plants. Auxin biosynthesis is a tightly regulated process contributing to the control of auxin concentration in the cell and the formation of important auxin gradients in tissues. IAA (Indole-3-acetic acid) is the main auxin in higher plants (Bartel 1997), where a Trp- (tryptophan)-dependent two-step auxin biosynthesis pathway is proposed to be the major contributor to auxin production (Zhao et al. 2001; Stepanova et al. 2008; Tao et al. 2008; Yamada et al. 2009). In this pathway, Trp is first converted into IPA (indole-3-pyruvate) by the TAA1 (l-tryptophan pyruvate aminotransferase) family of aminotransferases, and then converted to IAA by the YUC (YUCCA) family of flavin monooxygenases (Mashiguchi et al. 2011; Won et al. 2011) (Fig. 1). Although additional Trp-dependent and Trp-independent IAA synthesis pathways are active in some species and in specific developmental processes [reviewed in (Tivendale et al. 2014; Wang et al. 2015)], the highly conserved TAA/YUC pathway seems to have a predominant role in plants (Zhao 2012; Gao and Zhao 2014).

Fig. 1
figure 1

A molecular model for osmotic stress integration into the cellular auxin pathway. Auxin Biosynthesis, catabolism and conjugation: Tryptophan is first converted into IPA by the TAA1 aminotransferase and IAA is subsequently produced from IPA by the YUC gene family. Both IAA biosynthetic steps are affected by osmotic stress. IAA could be further conjugated to other small molecules such as sugars by UGT and amino acids by the GH3 families. The latter is reversible by the ILR/ILL amino acid hydrolases. Osmotic stress induces GH3 genes and inhibits miR167a which targets IAR3. Auxin Transport: Auxin levels are modulated by controlling its cellular transporter levels and intracellular localization. Osmotic stress affects PIN protein levels and localization by endocytosis and recycling. Auxin Perception: In the nucleus, auxin mediates the interaction between the auxin co-receptors Aux/IAA and SCFTIR1/AFB, leading to Aux/IAAs degradation and the release of ARFs from transcription inhibition. Osmotic stress effects the transcription level of several Aux/IAAs and triggers miR393, leading to TIR1/AFB2 repression. Notably, the effect of the osmotic stress stimuli is not necessarily direct

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Auxin biosynthesis and osmotic stress

Direct measurements of endogenous IAA content in leaves and roots showed that IAA levels were significantly lower in plants growing in salt (NaCl) and water deficit conditions (Du et al. 2013; Liu et al. 2015). These results are consistent with a lower signal of the reporter DR5 in root tips transferred to salty growth media, which is indicative of lower auxin response in this tissue (Wang et al. 2009; Zolla et al. 2009; Liu et al. 2015). Further molecular analyses revealed that plants exposed to water deficit conditions show a reduced expression of 6 out of 7 YUC auxin biosynthesis genes in rice (Oryza sativa) (Du et al. 2013). Inhibition of auxin biosynthesis is thus suggested to play a role in osmotic stress responses (Fig. 1). However, contradictory results show that several YUC genes in Arabidopsis are upregulated only hours after exposure to dehydration (2 h air drying on paper) (Lee et al. 2012). The Arabidopsis activation-tagged mutant yuc7-1D, in which YUC7 is up-regulated had higher total auxin levels compared to the WT but with similar free auxin levels. The plants exhibited phenotypes typical for an auxin overproducer, such as tall slender stems, enhanced apical dominance, curled narrow leaves, and were drought resistant (high water deficit survival rates) (Lee et al. 2012). In potato (Solanum tuberosum) and poplar (Populus alba, P. glandulosa), plants overexpressing Arabidopsis YUC6 displayed phenotypes such as narrow downward-curled leaves and increased height, suggesting a high auxin content. These phenotypes were accompanied by enhanced drought tolerance (water withdrawal) due to reduced water loss (Im Kim et al. 2013; Ke et al. 2015). Consistent with an increased drought resistance obtained by auxin overproduction, Arabidopsis iaaM-OX transgenic lines, where higher IAA levels can be obtained by constitutively expressing Agrobacterium tumefaciens tryptophan monooxygenase (iaaM) gene, exhibited enhanced drought stress resistance (withdrawal of water for 21 days). At the same time, yuc1/yuc2/yuc6 triple mutants with lower endogenous IAA level showed decreased stress resistance (Shi et al. 2014). Additional experiments suggest that auxin can promote drought stress resistance by modulating root architecture, ABA (abscisic acid)-responsive genes expression, and ROS (reactive oxygen species) metabolism (Shi et al. 2014).

The inconsistent results regarding auxin biosynthesis response to stress can arise from temporal stress response of minutes/hours versus days, thus pointing on a possible acute versus moderate stress response (Skirycz and Inzé 2010). In addition, the inconsistency may reflect the differential spatial response of different tissues and cell types to osmotic stress (Table 1). For example, specific YUC genes showed a differential stress response in different rice cultivars as well as an opposite response between water deficit and cold stresses (Du et al. 2013). An alternative explanation comes from a recent study that proposed that YUC genes confer drought tolerance independently of auxin biosynthesis by a novel thiol-reductase activity (Cha et al. 2015). These results suggest that the YUC proteins are playing a dual role of regulating plant development and conveying stress defence responses. Finally, the inconsistency may be a result of the dramatic effect of increased auxin biosynthesis on growth, which in turn may cause drought resistance/sensitivity. These secondary effects are discussed later.

Table 1 Summary of osmotic stress integration into the auxin pathway
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The availability of water and minerals can be expected to have a major influence on root growth (Davies and Zhang 1991). Indeed, in experiments conducted in various growth media as well as in natural soil, it was found that roots growing along the surface of a wet medium produce more lateral roots on the side directly contacting the wet surface compared to the air (Bao et al. 2014; Robbins and Dinneny 2015). In contrast, the air-exposed side has a higher abundance of root hairs. The term hydropatterning was coined to describe the patterning mechanism that positions lateral roots toward available water in the soil (Bao et al. 2014). Importantly, ABA signaling, known to inhibit the development of lateral roots, does not have a significant effect on hydropatterning. At the same time, water availability around the root promotes auxin biosynthesis and auxin accumulation in the root. Arabidopsis roots grown on media with a lower concentration of agar showed a significant increase in IAA concentration, increased auxin responses as suggested by an increased DR5-VENUS signal (Ulmasov et al. 1997) and reduced levels of the DII-VENUS (Brunoud et al. 2012) auxin response reporters in the outer tissue layers of the root (Bao et al. 2014; Robbins and Dinneny 2015). These apparent increases in auxin could be specifically explained by an increased expression of the TAA1 auxin biosynthesis gene, which showed significant expression differences between the water-contact and the air-exposed sides of the root. In line with the differential expression of TAA1, the taa1 mutant showed a significant reduction in hydropatterning that could be rescued by expressing TAA1 specifically in the epidermis or throughout the root (Bao et al. 2014). Taken together, these results suggest that the local differential water availability promotes auxin accumulation and auxin response.

Auxin catabolism and conjugation in response to osmotic stress

Auxin catabolism and conjugation

Auxin catabolism and conjugation may determine the actual level of active IAA in a given cell and consequently play an important role in many developmental processes. Importantly in higher plants, the bulk of IAA is stored in a conjugated form, providing free active IAA once needed in various developmental processes (Ludwig-Müller 2011). IAA can be converted to ester conjugates with sugars via UGTs (UDP-glucose transferases) or to amide conjugates with amino acids by IAA amino acid conjugate synthetases of the GH3 (Gretchen Hagen3) family (Staswick et al. 2005; Ludwig-Müller 2011) (Fig. 1). Most IAA amino acid conjugates such as IAA–Ala (alanine), IAA–Leu (leucine), IAA–Phe (phenylalanine) can then again be hydrolyzed by the ILR1/ILL (IAA-LEUCINE RESISTANT1/IAA-LEUCINE RESISTANT1 (ILR1)-LIKE) proteins to release free IAA (Bartel and Fink 1995; Woodward and Bartel 2005) whereas amino acid conjugates such as IAA–Asp (aspartic acid) and IAA–Glu (glutamic acid) are thought to be precursors of an IAA degradation pathway that cannot be reconverted. Local auxin levels could be buffered by the irreversible oxydization of IAA, leading to the inactive product oxIAA (Peer et al. 2013).

Auxin catabolism and conjugation under osmotic stress

GH3.5 is rapidly induced by various stress conditions. The dominant gh3.5 allele of the gene WES1 exhibits several auxin-deficient traits, including growth retardation and altered leaf shape, and it shows resistance to biotic (P. syringae infections) and abiotic stresses (withdrawal of water and salinity) (Park et al. 2007). The results suggest that auxin homeostasis plays an important role in mediating growth in response to stress. It is therefore suggested that coordinating auxin-mediated growth control is an important mechanism to cope with the changing stressed environment.

In rice shoots, the IAA-amidosynthetase GH3.13 (rice TLD1, increased number of tillers, enlarged leaf angles, and dwarfism) is induced by auxin and water deficit stress (Hagen and Guilfoyle 1985; Staswick et al. 2005). Furthermore, in sorghum, maize, and rice, altered expression of GH3 family gene members was shown to occur in response to several abiotic stresses (Zhang et al. 2009; Wang et al. 2010; Du et al. 2012; Feng et al. 2015) (Fig. 1). The activation of TLD1 reduced local DR5:GUS signal, resulting in subsequent alterations in plant architecture and enhanced drought tolerance (Zhang et al. 2009; Liu et al. 2014). OsGH3-2 is rapidly induced by water deficit stress in rice. Interestingly, overexpression of OsGH3-2 results in drought hypersensitivity (low water deficit survival rate) and increased resistance to cold stress (Du et al. 2012). Altogether, the molecular data indicate that IAA conjugation plays an important role in plant stress response, albeit some molecular and physiological inconsistency is apparent. Spatiotemporal stress response variation as well as secondary growth affects (discussed later in the conclusions) may explain this inconsistency (Table 1).

The active and regulated process of IAA hydrolysis leads to a rapid release of free IAA from IAA sugar or amino acid conjugates. IAR3 hydrolyzes an inactive form of auxin (IAA-alanine) and releases bioactive IAA (Bartel and Fink 1995; Woodward and Bartel 2005). Recent work shows that high osmotic stress inhibits miR167a levels thereby increasing the mRNA levels of its direct target IAR3 (Fig. 1). iar3 mutants show reduced levels of free IAA and display reduced osmotic stress–induced root architecture to achieve partial drought tolerance (Kinoshita et al. 2012; Liu et al. 2014). Further studies are needed to explore the molecular mechanisms of auxin homeostasis during plant acclimation to water deficit. For example, it will be important to understand whether subcellular compartments, such as vacuoles (Löfke et al. 2015), regulate the spatial distribution of conjugated and free auxin in response to stress.

Auxin transport in response to osmotic stress

Auxin transport

Auxin is polarly transported in plants by cell-to-cell trafficking through specific transporter proteins. Their combined activity results in the formation of IAA gradients and local maxima that account for many important developmental processes such as embryo development, lateral root and leaf primordia initiation (Jacobs and Rubery 1988; Reinhardt et al. 2003; Zažímalová et al. 2010). Several auxin transporters act in concert to control polar auxin transport, local auxin concentrations and auxin homeostasis: PIN (PIN-FORMED) proteins are auxin efflux carriers and their polar distribution in many cells of the plant promotes directional auxin efflux out of the cells (Gälweiler et al. 1998; Wiśniewska et al. 2006; Petrášek and Friml 2009). PGP/ABCB1,19 (P-glycoproteins/ATP-binding cassette class B proteins) are uniformly distributed in the plasma membranes and control non-polar auxin efflux (Geisler and Murphy 2006; Peer et al. 2011). Once exported, auxin can enter cells by diffusion when protonated or through the AUX1/LAX (AUXIN TRANSPORTER CARRIER1/AUXIN TRANSPORTER-LIKE PROTEINS) influx carriers (Swarup et al. 2001; Kramer and Bennett 2006) (Fig. 1). Recent studies have identified two additional auxin transporters, namely PILS (PIN-LIKES) and WAT1 (WALLS ARE THIN1), located on intracellular membrane compartments, the ER, and vacuoles, respectively (Barbez et al. 2012; Ranocha et al. 2013).

Auxin transport and osmotic stress

Salt stress inhibits primary root growth through a decrease in cell division and elongation (West et al. 2004). In addition, salt stress inhibits lateral root formation (Wang et al. 2009; Galvan-Ampudia and Testerink 2011). The inhibition of root growth by salt stress correlates with reduced auxin responses as indicated by a reduced DR5 signal in the root tip and in potential lateral root initiation sites (Wang et al. 2009; Zolla et al. 2009; Liu et al. 2015). Interestingly, mild salinity stimulates both initiation and emergence of lateral roots (Zolla et al. 2009; Zhao et al. 2011). Since auxin transport is an important level of regulation in auxin spatiotemporal homeostasis, it is interesting to understand whether osmotic stress affects this process. This can be approached in two different ways by choosing auxins that are independent from the auxin efflux and influx proteins or by manipulating the abundance of the auxin transporter proteins. Unlike IAA, 2,4-D (2,4-dichlorophenoxyacetic acid), is not efficiently effluxed from cells. Interestingly, 2,4-D treatments have a strong effect on the position of lateral roots toward available water in the soil (hydropatterning). Furthermore, manipulations of PIN proteins, e.g. by PIN1 overexpression or in the pin2/3/7 triple mutant, resulted in a disrupted hydropatterning, indicating that PIN-mediated auxin efflux is required for hydropatterning (Bao et al. 2014). Furthermore, PIN3 translational reporter (ProPIN3:PIN3:GFP) had a specific enrichment in cortex and endodermal cells (ground tissue) overlying early-stage lateral-root primordia on the contact side with an agar medium in comparison to the air side. These results suggest that PIN3 plays an important role in mediating the hydraulic conductivity driving hydropatterning (Bao et al. 2014).

The mechanism that allows directional growth of roots away from regions of high salt concentrations has been coined halotropism (Rosquete and Kleine-Vehn 2013). Salt stress modulates the direction of root growth by reducing the gravity response (Dinneny et al. 2008; Sun et al. 2008). Since auxin has a major role in mediating the root gravitropic response, recent studies have searched for the molecular mechanism linking halotropism and auxin transport. Galvan-Ampudia et al. show that the auxin response reporter DII-VENUS is reduced in response to a directional gradient salt treatment, thus indicating a possible increase in auxin levels at the side of the root that is most distant to the higher salinity (Galvan-Ampudia et al. 2013). Exogenous auxin treatment abolished the root’s halotropic response to salt (Galvan-Ampudia et al. 2013). A recent study shows that hydrotropism, which is the directional growth of roots toward regions of highest water potential does not require auxin redistribution (Shkolnik et al. 2016), suggesting for an exceptional auxin independent tropism.

Several recent reports suggest that osmotic treatments impair maintenance of PIN polarity, but the individual results contradict each other with regard to the PINs involved and the type of stress (Nakayama et al. 2012; Galvan-Ampudia et al. 2013; Zwiewka et al. 2015) (Table 1). With regard to root growth orientation, the role of PIN2 is particularly interesting since PIN2 also regulates root gravitropism (Abas et al. 2006). Salt gradients (anionic osmotic stress) but not mannitol (non-anionic osmotic stress) induce the internalization of PIN2 specifically at the side of the root facing a high salt concentration. Thus, halotropism response correlates with PIN2 protein localization (Sun et al. 2008). No effect of salt treatment was found on the subcellular localization or internalization of the other auxin transporters involved in tropic response, suggesting that PIN2 specifically mediates roots halotropic response to salt by affecting auxin flow (Galvan-Ampudia et al. 2013). Experiments testing PIN1 localization in tomato shoot apical meristems showed that osmotic stress affects PIN1 protein level and intracellular localization (Nakayama et al. 2012). Salt stress represses the expression of PIN genes, resulting in short root meristems (Liu et al. 2015). Consistently, osmotic treatments of NaCl and manitol to root meristem cells immediately increased the internalization of PIN1 and PIN2 and decreased the recycling of internalized PINs to the plasma membrane (Zwiewka et al. 2015), thus supporting the finding that acute hyperosmotic stress enhances PIN endocytosis (Nakayama et al. 2012; Galvan-Ampudia et al. 2013; Zwiewka et al. 2015). Further work should try to address why halotropism is specific to salt stress while the effects on PIN maintenance and polarity are promoted by several different osmotic conditions.

Auxin perception in response to osmotic stress

Auxin perception

Auxin is perceived by a co-receptor complex consisting of an F-box protein from the TIR1/AFB (TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX PROTEIN) family and a member of the Aux/IAA (AUXIN/INDOLE-3-ACETIC ACID) family of transcriptional repressors (Wang and Estelle 2014). Auxin acts as a “molecular glue” to promote the interaction between TIR1/AFBs and Aux/IAAs, leading to ubiquitination and 26S proteasome-mediated degradation of the Aux/IAAs. Aux/IAA degradation de-represses the transcription responses of the Aux/IAA-interacting ARF (AUXIN RESPONSE FACTOR) proteins (Bargmann and Estelle 2014; Salehin et al. 2015). Thus, auxin triggers a vast response network by increasing the affinity between the TIR1/AFB and Aux/IAA coreceptor families, which in turn modulates ARF-dependent molecular outputs (Fig. 1). Importantly, all three protein families involved in auxin response are comprised of numerous family members (29 Aux/IAA, 23 ARF, and 6 TIR1/AFB members in Arabidopsis) and the formation of a large heterogenous number of protein complexes can be envisioned to differentially affect auxin responses (Wang and Estelle 2014). Recent structural studies suggested that formation of Aux/IAA multimers is required for efficient repression of auxin signaling (Korasick et al. 2014; Nanao et al. 2014; Guilfoyle 2015; Korasick et al. 2015), while ARF proteins homodimerize to generate cooperative DNA binding (Boer et al. 2014; Korasick et al. 2015).

Auxin perception and osmotic stress

In rice, water deficit and salt stress were shown to modulate auxin responses (Du et al. 2013). Microarray gene expression under stress conditions revealed the differential regulation of several members of the GH3, Aux/IAA, SAUR (Small Auxin Upregulated RNA), and ARF families, which all represent typically auxin-regulated genes (Song et al. 2009; Du et al. 2013). Specifically, several members of the Aux/IAAs have shown a response to water deficit stress (Song et al. 2009) (Fig. 1). Overexpression of the water deficit -induced Aux/IAA6 improved drought tolerance in rice (Jung et al. 2015). As Aux/IAA6 overexpression affected the expression of auxin biosynthesis genes, it is difficult to interpret whether the drought tolerance reflects the outcome of a direct auxin signaling response or a secondary response that results in altered auxin levels by modulating auxin biosynthesis gene expression (Jung et al. 2015). In agreement with a reduction of auxin responses after salt stress, it was found in Arabidopsis that the stabilization of IAA17 leads to a salt-mediated inhibition of root meristem growth (Liu et al. 2015).

The auxin resistant double mutant tir1 afb2 is more tolerant to salt and oxidative stress compared to wild-type (Iglesias et al. 2010), suggesting that down-regulation of auxin signaling might mediate plant acclimation to stress. Salinity triggers miR393 expression (Chen et al. 2014), which targets the TIR1 and AFB2 auxin receptors to mediate the repression of lateral root development during salinity (Iglesias et al. 2014) (Fig. 1). Furthermore, overexpression of a miR393-resistant TIR1 gene (mTIR1) showed increased tolerance to salt stress as well as reduced hydrogen peroxide (H2O2) and superoxide anion (O2 −) levels (Iglesias et al. 2010; Chen et al. 2014). These findings suggest that osmotic stress might suppress TIR1/AFB-mediated auxin signaling in the root, thereby incorporating stress signals into growth inhibition response under stress conditions.

The studies described above illustrate that osmotic stress rapidly affects the auxin response, and that this response may lead to drought tolerance. Several other studies suggest however different physiological mechanisms correlating auxin signaling and drought tolerance. Three recent papers have shown that auxin is an important regulator of stomatal development. In response to water deficit stress, plants trigger stomatal closure to reduce water loss (Schroeder et al. 2001). Perturbations of polar auxin transport, resulting either from treatment with polar auxin transport inhibitors or from mutations in multiple genes of the PIN family of auxin efflux carriers, cause stomatal clustering (Le et al. 2014). Accordingly, application of exogenous auxin reduces the stomata development, whereas mutants disrupted in the taa1 auxin biosynthesis pathway display increased stomatal clustering (Balcerowicz et al. 2014; Zhang et al. 2014). Auxin response reporter activities drop at specific stages of the meristemoid differentiation, suggesting that auxin depletion might be required for the switch from unequal to equal division (Le et al. 2014). Loss-of-function mutations in the functionally redundant TIR1 and AFB1 auxin receptor genes, or the ARF gene MP (MONOPTEROS) as well as a gain-of-function mutations in AUX/IAA12 (BDL) and AUX/IAA17 (AXR3) result in strong stomatal clustering (Balcerowicz and Hoecker 2014; Balcerowicz et al. 2014; Zhang et al. 2014). Altogether, these results identify auxin as an important repressor of stomata differentiation (Balcerowicz and Hoecker 2014). Future work should test how moderate osmotic stress integrates into the auxin-signaling pathway to drive long-term stomata developmental acclimation.

In addition to the effects of auxin on stomata and drought tolerance, auxin was also shown to coordinate specific aquaporin function. Auxin promotes lateral root development by regulating the distribution of aquaporin-dependent root water transport (Péret et al. 2012). During lateral root initiation, auxin reduces the expression of several AtPIP and AtTIP genes, including AtPIP2;1, which is excluded from the lateral root primordia but maintained at their base. Thus, auxin mediates cell and tissue hydraulic properties during lateral root development (Péret et al. 2012).

While stomata development and aquaporin position are straightforward in terms of drought tolerance mechanisms, a recent study suggests that auxin signaling affects seed germination in salty environment. NTM2 (NAC transcription factor) integrates auxin and salt signals during seed germination (Park et al. 2011). Seed germination is delayed by auxin under high salinity in a NTM2 dependent manner, possibly by a rapid induction of IAA30 in response to salt stress (Park et al. 2011). It will be interesting to explore whether the auxin mediated germination response regulates growth and cell division processes that are modulated by stress, or rather the process involves other known stress and germination hormones such as GA (gibberellin) and ABA.

Conclusions

Environmental stress potentially disrupts cellular structures and impairs key physiological functions. Water deficit, salinity and dehydration stresses impose an osmotic stress that restricts water uptake. As a consequence, membranes may become disorganized, proteins may denature, chloroplasts may lose their photosynthetic activity and reactive oxygen species (ROS) are often produced leading to oxidative damage, all contributing to growth perturbation (Zhu 2002). In addition, recent studies show that growth retardation is a direct response to osmotic stress (Skirycz and Inzé 2010). Why do plants limit their growth in response to stress as a direct and active process? Elegant studies from the 1970’s show that plants with smaller cell size are more tolerant of low water potential and water deficit (Cutler et al. 1977; Steudle et al. 1977; Bradford and Hsiao 1982). Therefore, plants that rapidly respond to the changing stressed environment and limit their growth, in particular their cell size, have better physical parameters to survive water deficit periods. This raises the possibility that plants utilize the auxin response as a direct mechanism to rapidly regulate growth under osmotic stress. Several studies outlined in this review support this hypothesis. However, it is difficult to exclude the possibility that auxin manipulation, which in many cases drives direct growth inhibition and alters many morphological parameters, generates drought tolerance indirectly. A large number of studies showing overexpression phenotypes, either related to auxin or not, which display growth defects, claim for drought resistance/sensitivity gene function, while it is difficult to separate between the direct effects of the respective genes and secondary growth defects resulting from gain-of-function manipulation leading to drought resistance or sensitivity. Both, auxin and osmotic stress signaling pathways are complex and highly redundant, limiting the effectiveness of forward genetics studies (Bouché and Bouchez 2001; Takeda and Matsuoka 2008; Mittler and Blumwald 2010). Further precise and creative work is needed to explore the direct crosstalk between osmotic stress and auxin response.

So far, the studies exploring the stress and auxin pathway lack the spatiotemporal resolution. Temporally, both stress and auxin pathways are rapid and transient pulses that might drive growth processes days and weeks later. Most experiments exploring the transcriptional response to osmotic stress were carried in a minute and hour timescales, and therefore primarily give insight on the acute osmotic response. However, most physiological growth experiments are carried out on a time scale of days and weeks, therefore primarily representing the adaptive growth stage. In order to advance our understating of plant acclimation to the stressed environment, the scientific community will need to generate novel approaches and tools to monitor and integrate the continuous acute and adaptive growth processes. Spatially, both stress and auxin responses are often tissue and cell-type specific (Dinneny et al. 2008; Bargmann et al. 2013). The studies carried so far in the intersection of auxin and stress responses were of a general nature of the integrative plant. Recent elegant work showed that the endodermis is an important signaling center during the regulation of lateral root growth in high saline conditions (Duan et al. 2013; Geng et al. 2013; Duan et al. 2015). Corresponding studies are necessary to fill the spatial gaps in the auxin/stress cell-type specific crosstalk.

While this review has focused on the intersection between osmotic stress conditions and its effect on the auxin response at the molecular, physiological and developmental aspects, it is clear that these processes also require a broad set of additional proteins and regulators. The plant hormones ABA (Zwack and Rashotte 2015), CK (Peleg et al. 2011; Zwack and Rashotte 2015), ethylene (Morgan and Drew 1997; Alonso et al. 1999) and gibberellin (Achard et al. 2006, 2008; Nir et al. 2014) are all known to mediate the drought response in plants (review in (Peleg and Blumwald 2011)). The knowledge about the crosstalk between auxin and these hormones (Stepanova et al. 2007; Weiss and Ori 2007; Moubayidin et al. 2009; Shkolnik-Inbar and Bar-Zvi 2010) further supports the link between osmotic stress and auxin. Future studies should reveal the direct molecular mechanisms behind auxin regulation of stress-induced growth patterning in plants.