The starch-statolith hypothesis proposes that perception is mediated by the interaction of dense starch-filled organelles (termed statoliths) with other cytoplasmic structures in order to provide directional information to the plant (Kiss, 2000; Saito et al., 2005). A change in the position of the plant leads to a change in the potential energy of the amyloplasts, the statoliths in flowering plants. This energy is then transferred to the plasma membrane as the statoliths settle at a new position in the direction of the new gravity vector. This new position can then be relayed to the rest of the plant via the plant hormone auxin (Simmons et al., 1995; Firn et al., 2000) and then a gravitropic response can be initiated.

Roots most often display positive gravitropism, growing in the direction of the gravity vector and thus into the soil (Figure 1). When measuring the gravitropic response of roots of Arabidopsis starchless mutants, it was observed that the time of response was severely reduced and delayed compared to the response in wild-type genotypes. A similar response was observed in roots and hypocotyls of Nicotiana reduced starch mutants (Kiss and Sack, 1989, 1990). The gravitropic response of roots has also been shown to be linked to the actual rate at which amyloplast sedimentation occurs in Arabidopsis (MacCleery and Kiss, 1999). Furthermore, the gravitropic response has also been correlated to the total mass of statoliths in the root columella cells (Kiss et al., 1996, 1997).

In contrast to roots, stems and stem-like organs exhibit negative gravitropism, growing upward and away from the gravity vector. Mutants lacking amyloplasts in the endodermal cell layer lack gravitropic responses (Fujihira et al., 2000). In addition, stems of starch-deficient Arabidopsis correlated the total mass of starch in endodermal tissue to a change in the response to gravity (Kiss et al., 1997). In addition, gravity perception plays a role in plant development. Stem-like organs often maintain specific angles in relation to gravity, known as a gravitropic setpoint angle (GSA; Digby and Firn, 1995). The GSA operates through the existence of an antigravitropic offset mechanism that works in tension with gravitropism. The magnitude of the antigravitropic offset in relation to gravitropism determines the magnitude of the lateral stem’s angle. GSA values are modulated via the plant hormone auxin in the gravity sensing cells of root and shoot tissue, further implicating the role of auxin in tropism and plant architecture (Roychoudhry et al., 2013).

Further support for the starch-statolith hypotheses is provided by studies which reported that plants lacking starch in the stem endodermal amyloplasts also have a severely reduced gravitropic response (Weise and Kiss, 1999). This suggests that in stem the gravitropic response is similarly regulated by starch-filled amyloplasts. In addition, amyloplasts lacking a full complement of starch show reduced ability to perceive gravity in Arabidopsis (Kiss et al., 1996, 1997, 1998a,b). However, starch-deficient mutants lacking a full complement but grown in hypergravity environments (2–10 × g) restored the gravitropic response (Fitzelle and Kiss, 2001).

An alternative model to statolith-based gravity sensing is the protoplast-pressure hypothesis, where the total mass of the cytoplasm causes tension on the top and bottom of the plasma membrane (relative to the gravity vector; Wayne et al., 1990; Wayne and Staves, 1996). This model is largely based on studies of Characean algae Nitellopsis and Chara, both of which do not contain starch-filled amyloplasts yet still respond to gravity stimulus by exhibiting a gravity-dependent cytoplasmic streaming (Wayne et al., 1992; Staves, 1997). In addition, support for the protoplast-pressure hypothesis arises from the fact that starchless mutants of Arabidopsis, while greatly reduced, can still sense and respond to gravity (Kiss et al., 1989). Furthermore, proponents of the protoplast-pressure hypothesis point out that studies using starch mutants (key evidence in support of the starch-statolith hypothesis) do not discriminate effectively between the two competing hypothesis (Staves et al., 1997a). A study of gravitropism in the roots of rice (Oryza sativa, Poaceae) used variable densities of an external media to exert more or less force on root tissue without changing statolith sedimentation rate, resulting in changes in gravitropic response (Staves et al., 1997b).

While some debate still exists between the validity of the two models, it is likely a combination of multiple gravity sensing mechanisms that control gravitropism. It has been proposed that similar to the way in which a plant senses light, where multiple photosensory mechanisms exist, plants may contain multiple mechanisms which help to perceive the gravity vector (Sack, 1997). In addition, Perbal (1999) proposed that statoliths and the protoplast both can act in gravity sensing, with statoliths being the more sensitive mechanism. A commentary suggests that throughout evolution, higher plants have acquired multiple gravisensing mechanisms as evolution is unlikely to select against a process that aids in fitness (Barlow, 1995). Additionally, Barlow (1995) supposes that gravity sensory redundancy allows gravity to play a larger role in plant development through the evolution of distinct signaling pathways.

Upon sensing of the gravity stimulus, the cell needs to convey the message to the elongation zone before differential growth leading to curvature can occur. In the case of stem tissue, the elongation region is nearby, requiring lateral transmission of the signal to this tissue (Figure 2). However, in the case of roots, the signal must be conveyed over a relatively longer distance to the root elongation zone (Figure 1). While various signal transduction mechanisms have been proposed, there is a great deal of evidence that supports the role of the plant cytoskeleton being involved in gravity transduction. It has been suggested that microtubules may be involved in the perception of gravity in coleoptiles (Blancaflor, 2002). In addition, depolymerization of the F-actin cytoskeleton results in promotion of gravitropic curvature in stems and roots, suggesting that the actin cytoskeleton may play a role in regulation of the gravitropic response (Yamamoto and Kiss, 2002; Hou et al., 2003).

While a role for the cytoskeleton in gravitropic response seems evident, little is known about the mechanism that associates the cytoskeleton with amyloplasts. The actin-tether model proposes that amyloplasts are physically associated with the cytoskeleton, and reorientation in respect to the gravity vector causes the amyloplasts to exert tension or slack on the cytoskeleton (Baluška and Hasenstein, 1997). This change in tension is then relayed to the plasma membrane, where the signal can then be transduced. A second hypothesis, the tensegrity model, proposes that amyloplasts are not attached to the cytoskeleton. Instead, as the amyloplasts reorient to a new gravity vector, they come into contact and disrupt the actin microfilament network. The signal is then transduced to the plasma membrane, where Ca²⁺ stores within the endoplasmic reticulum (ER) are released (Yoder et al., 2001).

As stated previously, the new relative position of the amyloplast (serving a statolith) is perceived by the cell. One observation is the statocyte responds to gravistimulation by increasing the cytosolic Ca²⁺ concentration and reducing the cytosolic proton concentration, which leads to auxin being differential distributed in the direction of the gravity vector (Tasaka et al., 1999; Morita and Tasaka, 2004). The resulting differential distribution of auxin results in unilateral inhibition of root growth on one side, causing the root to grow in a new downward orientation toward the gravity vector. It has been proposed that the ER acts as a Ca²⁺ reservoir; once statoliths settle in the direction of the gravity vector and contact the ER, stored Ca²⁺ is released into the cell (Perbal and Driss-Ecole, 2003). A study using high-resolution electron tomography adds support to this hypothesis, finding statolith sedimentation on the ER was sufficient in locally deforming the ER membrane, and in turn had the potential to activate local mechanosensitive ion channels (Leitz et al., 2009). The authors of this study further suggest that transmission of the gravisensory signal results from a combination of kinetic energy being transferred to the ER from the statolith, rapid release of kinetic energy from the ER upon initial reorientation of the columella cell, as well as statolith-driven motion of the cell cytosol. Furthermore, it is possible that calcium release acts as a signaling molecule during gravistimulation. A recent study indicates that an enzyme in the ethylene biosynthesis pathway (1-aminocyclopropane-1-carboxylate synthase) requires Ca²⁺ to elicit a gravitropic response, suggesting Ca²⁺ may be a signaling element for downstream growth regulation (Huang et al., 2013).

Perception of gravity by the root cap has been shown to result in asymmetrical auxin distribution across the root, leading to differential growth in the root elongation zone (Ottenschläger et al., 2003). A mathematical model inferring auxin distribution suggests a rapid (within 5 min) twofold increase in the auxin content on the lower side of the root, which leads to inhibition of growth and bending of the stem (Band et al., 2012). However, the mechanism and genes involved in the formation for the auxin gradient still remains unclear. Various genes (e.g., PIN, ABCB, and AUX/LAX families) have been implicated in the transport of auxin from the root cap to the distal elongation zone, yet a defined mechanism remains elusive (Luschnig et al., 1998; Petrášek and Friml, 2009). In addition to the identification of genes involved in auxin transport, regulators of the proposed transport genes such as the PIN regulator GOLVEN (GLV), which encode for small secretory peptides, have been shown to regulate the distribution of PIN2 and therefore the formation of the auxin gradient responsible for differential growth in gravitropism (Whitford et al., 2012).

A potentially important facilitator of auxin transport is AtPIN3. PIN3 has been implicated in both hypocotyl and root tropisms. In the hypocotyl, PIN3 is expressed in the shoot endodermis, suggesting it may mediate the lateral distribution of auxin (Friml et al., 2002). In the columella cells of the root, PIN3 localizes along the plasma membrane symmetrically, but quickly (within ∼2 min) relocalizes to the bottom plasma membrane upon gravistimulation by reorientation. This relocalization of PIN3 coincides well with the redistribution of auxin along the root cap and endodermal cells to the elongation zone. This results in auxin inhibiting the growth of cells in the elongation zone nearest the gravity vector, causing the stem to grow downward toward the new gravity vector. Evidence shows that auxin efflux complexes such as PIN proteins cycle along the actin cytoskeleton between the plasma membrane and endosome (Geldner et al., 2001). These results are in accordance with models that suggest that the actin cytoskeleton reorganizes in concert with statolith sedimentation (Baluška and Hasenstein, 1997; Yoder et al., 2001). This reorganization would help to facilitate PIN3 relocalization to the new perceived bottom of the cell.

A recent study has shown that the regulation of PIN2 on opposing sides of gravistimulated root tissue is controlled by an auxin feedback mechanism (Baster et al., 2013). Auxin was shown to control the plasma membrane PIN abundance though vacuolar targeting and degradation through a specialized auxin receptor. This observation indicates that auxin plays a role in recycling the PIN proteins to the plasma membrane or to send them to the vacuole for degradation. Interestingly, the same study indicated that auxin leads to degradation of PIN in the presence of low auxin content as well. These results suggest that during a gravitropic response, auxin leads to the degradation of the PIN proteins coinciding with the different auxin gradients present during a gravitropic response. Additionally, a study by Löfke et al. (2013) has proposed that another plant hormone, gibberellic acid (GA), plays a key role in the trafficking of PIN proteins. Results showed that high GA promoted trafficking of PIN back to the plasma membrane, while low GA concentrations resulted in trafficking to the vacuole for degradation (Löfke et al., 2013). Taken together, these studies suggest that trafficking of PIN proteins is important in regulating auxin flow and initiating the differential growth response.

It is likely that AUXIN RESISTANT1 (AUX1) is responsible for the uptake of auxin in the gravity sensing tissue (endodermis and columella of the root cap), while PIN2 facilitates the transport from the root cap to the elongation zone (Friml, 2003). Genetic expression analysis of AUX1 suggests that AUX1 is required for root gravitropic response and is associated with basipetal auxin transport from the root cap to the elongation zone (Swarup et al., 2001). Recently, it has been proposed that AUX1/LAX genes are responsible for tissue specificity of high auxin levels, while PIN proteins are responsible for directing the flow of auxin from the root cap (Band et al., 2014). This evidence suggests that PIN and AUX/LAX proteins are key components of the signal transduction pathway from the columella cells to the elongation zone (Swarup et al., 2005). However, to date, many aspects of signal transduction between statocytes and the growth response remain unclear.

While sensing of the gravity vector is similar in both roots and shoots, it is possible that the mode of signal transduction is different. In roots, gravity perception takes place in the columella cells, and transferred to the elongation zone which is proximal to the columella cells (Kiss, 2000). Differential auxin concentrations between the two sides of the root cap have been correlated with bending of root tissue (Luschnig et al., 1998). In addition, while there is spatial separation of perception and response in roots, in shoots, perception and response occur in the same area (Molas and Kiss, 2009). It also has been suggested that perception of gravity occurs in the root columella cells and is transferred distantly to the shoot tissue, suggesting a larger role for root graviperception (Hopkins and Kiss, 2012).

Recent studies have also implicated plastid membrane proteins playing a key role in gravitropism. A mutation in Arabidopsis termed ARG1 (altered response to gravity) results in altered root and hypocotyl gravitropism without negative effects on other processes such as phototropism, root growth or starch accumulation (Sedbrook et al., 1999). The ARG1 gene encodes a DnaJ-like domain similar to cytoskeleton-interacting proteins. Additionally, studies have demonstrated that ARG1 is a membrane protein that potentially co-localizes with the PIN proteins (Boonsirichai et al., 2003). Furthermore, this study demonstrated that targeting of ARG1 to the endodermis or columella cells was sufficient in rescuing the phenotype in gravitropism-deficient mutants. ARG1 and its paralog ARL2 (ARG1-LIKE2) have been shown to associated in-complex with actin (Harrison and Masson, 2008a). Additionally, these two proteins are required for the asymmetrical distribution of auxin during gravitropic stimulation (Harrison and Masson, 2008b). ARG1 mutants have also shown to exhibit reduced plastid sedimentation, suggesting a role in both perception and signaling arising from gravitropic stimulation (Kumar et al., 2008). A study by Strohm et al. (2014) has shown that mutants of the Translocon at the Outer membrane of Chloroplast complex (TOC) show decreased gravitropic response in arg1 mutants of Arabidopsis. This complex is responsible for the transport of nuclear-encoded proteins into plastids, however, the identity of which proteins need to be imported for a gravitropic response remains unclear. The study indicated that plastids not only play a role in gravity perception but also play a role in signal transduction. Another Arabidopsis mutant, eal1 (endodermal-amyloplast less 1), completely lacks gravitropic movement in stems, while gravitropic response in roots remains unaffected (Fujihira et al., 2000). Map based cloning techniques have identified eal1 as an allele of the transcription factor SHORT-ROOT (SHR; Morita et al., 2007).

Another group of compounds that have been implicated in gravitropic response is jasmonic acid (JA). JA increases significantly during gravitropic stimulation (Gutjahr et al., 2005). Not surprisingly JA has been shown to have multiple interactions with auxin (Riemann et al., 2003, and reviewed by Hofmann and Pollmann, 2008 and Wasternack and Hause, 2013). In rice coleoptiles, jasmonates have been shown to be present in a gradient opposite the auxin gradient during gravitropic stimulation (Gutjahr et al., 2005). In addition, lipozygenase, a key enzyme in JA-biosynthesis, has been shown to be up-regulated during gravitropic stimulation (León and Sánchez-Serrano, 1999). JA was induced by light as well as involved in photostimulation, where a rice mutant lacking JA biosynthesis (hebiba) exhibits delayed photodestruction of phytochrome A (Riemann et al., 2009). Furthermore, a study using hebiba identified the gene GDSL CONTAINING ENZME RICE 1 (GER1) as playing a role in gravitropic response, as it mirrors the level of JA in gravity stimulated coleoptiles (Riemann et al., 2007). The reduction in phytochrome A photodestruction results in an elevated growth response to red and far-red light. Jasmonates have also been implicated in growth-mediated response to touch (thigmotropism) in root tissue, suggesting a role for these compounds in multiple growth responses in plants (Falkenstein et al., 1991; Weiler et al., 1993; Stelmach et al., 1998; Blechert et al., 1999).

In flowering plants, phototropins are specialized photoreceptor proteins typically located on the plasma membrane that sense blue light and mediate a phototropic response (Sakamoto and Briggs, 2002). In addition to the blue light response, PHOT1 (Huala et al., 1997) and PHOT2 (Jarillo et al., 2001; Sakai et al., 2001) regulate stomatal opening, leaf expansion, and inhibition of stem growth among other processes (Christie, 2007). In the regulation of phototropism, PHOT1 is the primary photoreceptor, whereas PHOT2 only enacts a response when exposed to high intensities of radiation fluence (Sakai et al., 2001). While PHOT2 plays a small role in phototropic mediated signaling, this protein has significant roles in other light responses such as chloroplast movements (Wada et al., 2003) as well as stomatal regulation (Kinoshita et al., 2001). Upon low light conditions, PHOT2 stimulation causes relocation of chloroplasts perpendicular to incident irradiation, while high blue-light stimulation of PHOT2 leads to mobilization of chloroplasts away from the edge of the cell to avoid photobleaching of the organelles (Sakai et al., 2001).

Upon blue-light excitation, PHOT1 and PHOT2 release from the plasma membrane into the cytoplasm (Sakamoto and Briggs, 2002; Wan et al., 2008), while PHOT2 becomes associated with the Golgi apparatus (Sakamoto and Briggs, 2002; Kong et al., 2006). Following excitation with blue light, autophosphorylation of PHOT1 and PHOT2 leads to phototropin-mediated signaling (Inoue et al., 2008, 2011). Upon relocalization, PHOT1 has been shown to associate with clathrin, and is presumed to be internalized through clathrin-mediated endocytosis (Kaiserli et al., 2009). These studies indicate that phosphorylation and subsequent internalization of the PHOT1 is required for phototropic response, however the specific role PHOT1 and PHOT2 play in signal transduction remains unclear. Curiously, activation of phytochrome A, which is stimulated by red/far red light, prevents internalization of PHOT1 and leads to an increased phototropic response (Han et al., 2008).

In addition to the phototropins, there are multiple phototropin-interacting proteins shown to be involved in phototropism. Like the phototropins, NONPHOTOTROPIC HYPOCOTYL3 (NPH3) has been shown to localize to the plasma membrane and to physically interact with PHOT1 (Motchoulski and Liscum, 1999). In addition, NPH3 is required for phototropic response under both low and high intensity stimulation with blue light (Motchoulski and Liscum, 1999; Roberts et al., 2011). However, unlike the phototropins, NPH3 remains membrane associated throughout light stimulation and the phototropic response. It appears that NPH3 plays a role as a substrate in ubiquitination of PHOT1 under both low and high blue-light conditions (Roberts et al., 2011). Under high-blue-light conditions, when the plant is receiving sufficient light for photosynthetic processes, PHOT1 is poly-ubiquitinated and degraded. However, under low blue-light conditions, PHOT1 was shown to be only mono-ubiquitinated, a necessity for initiation of phototropic response (Roberts et al., 2011).

Another protein found to be involved in phototropism is ROOT PHOTOTROPISM2 (RPT2). RPT2 has been implicated in the phototropic response under high blue light conditions, where mutations in the gene result in defective hypocotyl phototropic response (Sakai et al., 2000). This is due in fact to RPT2 being transcriptionally regulated by both blue light and red light in a high-fluency-dependent manner (Sakai et al., 2000). Like the previously mentioned proteins, RTP2 also localizes to the plasma membrane of the cell, where it interacts with both PHOT1 and NPH3 (Inada et al., 2004) It is yet unknown whether or not RTP2 interacts with Cullin-3 (CUL3), however, Hohm et al. (2013) have suggested that both NPH3 and RTP2 could be required for ubiquitination under high blue-light condition.

Upon sensing a light signal, transduction of that signal must take place to induce growth in the elongation zones. The classical Cholodny–Went hypothesis proposes that the differential growth exhibited by a plant when illuminated with unidirectional light is a result of differential concentrations of auxin on the illuminated and shaded sides of the plant (Went and Thimann, 1937). The difference in auxin content results in cessation of cell growth on the illuminated side of the plant, while growth continues in the shaded side, resulting in tropistic movement in the direction of the light. A later study found that introduction of a physical barrier between illuminated and shaded sides of maize coleoptiles prevents the formation of an auxin gradient (Briggs, 1963). Using labeled auxin, Pickard and Thimann (1964) found that auxin moves laterally across the coleoptile when illuminated with continuous light, as well as low light dosages. This results in differential growth on the illuminated and shaded sides of the plant. The differential concentrations in auxin needed to spur differential growth and phototropic response are thought to be the result of local and long-range adaptations in auxin transport (Christie and Murphy, 2013; Hohm et al., 2013).

With the advancement of genetic and molecular techniques, it is possible to investigate the molecular mechanisms responsible for phototropic growth. Not surprisingly, similarities between phototropic stimulated signal transduction and gravitropic stimulated signal transduction have been identified. Like gravitropic response, PIN proteins play a significant role in auxin efflux during phototropic response. Five PIN family proteins (PIN1, PIN2, PIN3, PIN4, and PIN7) reside within the plasma membrane and appear to facilitate auxin efflux during phototropism, with PIN1 and PIN2 functioning as the main efflux carriers (Christie and Murphy, 2013). However, mutant screens of the PIN proteins suggest that all five contribute to phototropic response under differing conditions (Christie et al., 2011; Ding et al., 2011; Haga and Sakai, 2012). These proteins likely play an important role in formation of the lateral auxin gradient, however, it is likely that other auxin transporters exist to help facilitate in creation of the gradient (Christie et al., 2011).

Another family of proteins, ATP BINDING CASSETE B (ABCB) has been implicated in mediating phototropic response in Arabidopsis. Specifically, ABCB19 mutants have been shown to exhibit increased phototropic response (Noh et al., 2003; Kumar et al., 2011). It has been suggested that ABCB19 acts in concert with PIN1 to facilitate lateral transport of auxin through stabilization of PIN1 in the plasma membrane. Loss of function of ABCB19 results in reduced lateral auxin transport arising from the destabilization of PIN1, and the resulting loss of lateral auxin transport reduces the phototropic response.

In another similarity to gravitropic response, the AUX1 and LIKE-AUX1 (LAX) family of genes appears to play a role in influx of auxin and the resulting phototropic response. However, at the present time, the specific role of these proteins is unclear at best. Mutations to the AUX1/LAX genes results in very subtle loss of phototropic response when mutated alone (Okada and Shimura, 1992). However, when loss of function is introduced to AUX1 in combination with LAX2 and LAX3, reduced phototropic response in hypocotyl occurs (Christie et al., 2011).

All adult plants contain meristematic tissues, composed by populations of undifferentiated, totipotent cells with a high capacity of cell proliferation and cell growth. Any specialized tissue can be formed from meristems at any time in the life of the plant. Indeed, plant growth and development, which rely on cellular functions, greatly depend on the balance between cell proliferation and cell differentiation that exists in meristems, which is controlled, in turn, by hormones, among them auxin playing a central role (Perrot-Rechenmann, 2010).

Regulators of plant growth and differentiation (i.e., differentiation and developmental signals) are capable of activating key modulators of cell growth and cell division in a coordinated manner in meristems. Therefore, cell growth and cell proliferation are closely interconnected to one another and this coordination is called “meristematic competence” (Mizukami, 2001).

The concept of cell proliferation is intimately linked to the existence of the cell division cycle, or simply the cell cycle. Proliferating cells grow, duplicate DNA and divide in a cyclic manner. The process is strictly regulated at two specific checkpoints, the first of them at the transition G1/S, allowing DNA replication to proceed, and the second at the transition G2/M, in which the ability of cells to divide is checked. Specific proteins called cyclins, and specific cyclin-dependent kinases (CDKs) play essential roles in these regulatory processes (De Veylder et al., 2003). The concept of cell growth mostly affects the production of cell biomass, essentially proteins, so it is largely determined by the activity of RNA polymerase I, which controls ribosomal RNA synthesis and ribosome biogenesis (Baserga, 2007). It has been largely established, in particular for plants, that the rate of ribosome biogenesis can be estimated through certain features of the molecular cytology of the nucleolus (Medina et al., 2000; Shaw and Doonan, 2005; Sáez-Vásquez and Medina, 2008). Furthermore, regulation of ribosome production is controlled by a subset of nucleolar proteins, among which nucleolin is, a protein conserved in animals, plants and yeast, whose levels are correlated with the rate of functional activity of the nucleolus (Ginisty et al., 1999; González-Camacho and Medina, 2006; Pontvianne et al., 2007).

In this context, it is widely known that environmental conditions modulate meristematic activities, directly or indirectly, at different levels of regulation (Komaki and Sugimoto, 2012). Since many factors involved in light and gravity sensing are also acting in regulating cell cycle and ribosome biogenesis, it is interesting to review how these fundamental environmental factors affect plant development by affecting the regulation of these cellular processes.

The influence of gravity on meristematic cell functions has been approached up to now in a relatively small number of experiments performed in space and in ground-based devices of simulated microgravity (Herranz et al., 2013a). In early pioneering studies on plant space biology it was reported an increase of mitotic index of lentil roots grown was found in microgravity (Darbelley et al., 1986). The interpretation was not unequivocal, since this effect could be due either to a shortening of the interphase or to a lengthening of mitosis. In a further study on board of Spacelab (IML-1 Mission), also using lentil seedlings, a lower mitotic index was shown in root cortical cells of samples grown in space, but no apparent perturbations in the mitosis were observed (Driss-Ecole et al., 1994). In the same work, the authors observed that microgravity promoted the arrest in the G2 phase of the cell cycle. All these experiments were performed with a relatively short exposure of plants to microgravity (28–29 h). In lentil seedlings grown for 30 h in microgravity, the progression of cell cycle was modified, even though the cell elongation did not appear affected (Legué et al., 1996; Yu et al., 1999). The densitometric analysis of nuclear DNA content of meristematic cells from roots grown in microgravity showed a decrease in the proportion of cells in S phase correlated with an increased proportion of cells in G1 phase, suggesting that the G1/S transition of the cell cycle is modulated by gravity.

The first European experiment on plant biology on board the ISS revealed that one of the most relevant effects of altered gravity is the disruption of the meristematic competence in cells of the root apical meristem (Matía et al., 2005, 2010). Under microgravity conditions, cell proliferation and cell growth appear uncoupled, losing their coordinated progress which is characteristic of these cells under normal ground gravity conditions.

Further experiments performed on ground-based facilities for microgravity simulation, including sequential sampling at different growth times and the analysis of gene expression, have confirmed the uncoupling of cell proliferation and ribosome biogenesis caused by altered gravity, showing that the weightlessness environment is a stress condition for plant proliferating cells. The effects of the gravitational stress are detected from the very beginning of germination, in 2-day-old seedlings (Matía et al., 2009; Herranz et al., 2013b; Manzano et al., 2013). The enhanced cell proliferation rate is not accompanied by an increase in the levels of cyclin B1, a regulator of the G2/M transition, as would be the normal in ground gravity, but, on the contrary, these levels appear depleted. At the same time, a lower cellular growth was observed, since ribosomes, the cellular factories of proteins, were produced at a lower rate. This depletion of ribosome biogenesis was already observed in meristematic root cells grown in simulated microgravity (Shen-Miller and Hinchman, 1995; Sobol et al., 2005, 2006), but it was not put in relation to other cellular processes. Since cyclin B1 is synthesized in the G2 phase of the cell cycle, and also this period is the most active in ribosome production (Sáez-Vásquez and Medina, 2008), a shortening of G2 phase is compatible with the mentioned observed uncoupling. The causes of this shortening could be found in a failure or malfunction of the cell size checkpoint which immediately precedes mitosis (De Schutter et al., 2007; González et al., 2007).

Whereas the effects of the lack of detection of a gravity vector on meristematic cells have been clearly identified, we still need to elucidate the factor triggering the cascade of functional events that eventually result in the alteration of meristematic cell proliferation and growth and in the disruption of meristematic competence. According to previously published data, the change in the hormonal signaling pathway mediated by the auxin polar transport could be a possible candidate to play this triggering role (Medina and Herranz, 2010).

It has been demonstrated that auxin plays a fundamental role in the connection between stimuli perceived by the plant and the cellular responses to them (Muday and Murphy, 2002). As stated above, mechanical sensing of a gravity change by columella cells is converted into a relocation of PIN proteins, finally resulting in changes in the auxin gradient in the root (Friml et al., 2002; Kleine-Vehn et al., 2010). There are experimental results showing that this change in the gradient is associated with the inhibition of the auxin polar transport, at least partially (Hoshino et al., 2007; Boucheron-Dubuisson et al., submitted). Auxin influences multiple aspects of plant growth and development, including the regulation of cell cycle progression and the coordination between cell growth and cell division (David et al., 2007; Jurado et al., 2010; Perrot-Rechenmann, 2010). Therefore, all available data point to auxin as the key mediator between altered gravity sensing and the observed effects on root meristematic cells (Medina and Herranz, 2010; Figure 3).

Since gravity can be considered as an abiotic component of the environment, it would be interesting to consider to what extent the response to gravity alteration follows a similar pattern as the response to other abiotic stresses, or, in the case of gravity, whether plants have developed specific mechanisms of defense and/or adaptation. It is widely known that abiotic stresses, such as thermal shock (heat or cold), drought or saline stress, usually cause arrest of the cell cycle on actively proliferating cells (Sacks et al., 1997; Burssens et al., 2000; West et al., 2004; De Veylder et al., 2007; Doerner, 2008; Komaki and Sugimoto, 2012). In the case of gravitational stress, the cycle is not arrested, but the regulation is severely affected through the failure of the G2/M checkpoint, which allows cells to enter mitosis before a critical size is reached. Apparently this scenario results in an increase of cell proliferation, but the daughter cells resulting from these divisions are abnormal, due to a reduced size.

Regarding the long-term consequences of these alterations, it could be foreseen that they could be dramatic for the plant survival, due to a serious impairment of plant development caused by the loss of meristematic competence. However, the reality is that plants finally are capable of surviving in space, even though some features of adult plants are not normal (Miyamoto et al., 1999; Link et al., 2003; Wolverton and Kiss, 2009). This observation suggests that plants would be capable of implementing some countermeasures against the gravitational stress, leading to some sort of adaptation or acclimation to the weightlessness environment, but the molecular processes and mechanisms involved in this presumed adaptation process are unknown at present. Indeed the genomic and transcriptomic data available on the specific cell cycle regulators whose expression is affected by gravity alteration are very scarce and limited to those scarce studies in which the analyzed material was enriched in proliferating cells, such as the root tip (Kimbrough et al., 2004). Therefore, the identification and discrimination of genes and proteins involved in cell proliferation and cell cycle playing a role in the response to gravitational stress is one of the most attractive challenges of space plant biology for the near future (Figure 3). Dedicated transcriptomic/proteomic studies using isolated meristems and/or proliferating in vitro cultured cells will be necessary to find accurate solutions to this problem.

Whereas the role of auxin appears to be clear in connecting the alterations found in meristematic cells with the gravitropic response, it does not provide an explanation to the alterations found in cellular systems devoid of cells specialized in gravity sensing, such as in vitro cultured cells. A response to mechanical signals by these cells, in which the perception mechanism is unknown, has been reported in both plant and animal cells (Cogoli and Cogoli-Greuter, 1997; Kordyum, 1997; Dai et al., 2007). In plants, an altered gravity environment (including spaceflight) has been shown to induce transcriptomic and proteomic effects of on Arabidopsis callus cell cultures (Martzivanou et al., 2006; Barjaktarovic et al., 2009; Manzano et al., 2012; Paul et al., 2012; Hausmann et al., 2013). These cellular systems do not show any type of gravitropism, and cultured cells are not integrated in any organism possessing specialized mechanisms for gravity sensing. In this sense, cultured cells can be considered as “non-professional” cells (Van Loon, 2007). Regarding the effects of altered gravity on cell growth and proliferation parameters in this kind of cellular system, no specific reports have been published yet, but preliminary results indicate that the alterations are quite similar to those found on meristems (Herranz and Medina, 2014 and unpublished data).

Interestingly, whereas functional alterations on undifferentiated cells grown in microgravity were similar either in meristematic cells from seedlings or in cultured cells, transcriptomic changes were substantially different from seedlings or callus cultures (Paul et al., 2012). This suggests a specialized response by different cell types, such that both meristems and cell cultures would share the condition of being homogeneous populations of undifferentiated proliferating cells, whereas each seedling is composed by a heterogeneous collection of differentiated cells, differing in function, structure, and gene expression.

An intermediate case between gravitropic seedling roots and in vitro cell cultures is provided by seedling roots exposed to magnetic levitation (Manzano et al., 2013). In this case, root cells are subjected to the levitation force caused by the diamagnetic levitation of water, which counteracts the force due to gravity but it is not capable of inducing any displacement of statoliths. The response at the meristematic cell level is the disruption of meristematic competence associated to an altered polar auxin transport. This means that there should be an intermediate factor, independent of statoliths, capable of linking the signal sensed in the cell and the alteration of the polar auxin transport.

Therefore, depending on the type of cell, gravity sensing may or may not involve statolith movement and, consequently it may or may not produce gravitropic effects; furthermore, the transduction of the signal may or may not affect the polar auxin transport. In all cases, both in planta and in vitro, the response of undifferentiated proliferating cells to the perception of the lack of a gravity vector is an alteration in growth and proliferation capable of disrupting meristematic competence. It is conceivable that different mechanisms of gravity sensing and signal transduction (within a cell or throughout cells) involving different molecular and cellular players and mediators may exist in different biological systems and even co-exist in a biological model (plants or cell cultures; real or simulated microgravity; mechanical; or magnetic simulation). The nature and the localization of the mechanosensor in “non-professional” cells was hypothesized to be related to the membrane-cytoskeleton associations (Van Loon, 2007). There is a process, called gravity resistance, which is the capacity of plants to resist the gravity force by developing rigid structures, whose cellular basis is the existence of mechanosensitive ion channels at the plasma membrane and the reorientation of cortical microtubules (Hoson et al., 2005, 2010). This mechanism of gravity sensing is compatible with the available data on graviresponse in “non-professional” cells and may represent an initial explanation, which does not exclude the need of dedicated investigation (Figure 3). Furthermore, it has been proposed that cells outside the cap root area are capable of producing a partial gravitropic response in maize (Wolverton et al., 2002; Mancuso et al., 2006).

An alternative (or complementary) mechanism of gravity sensing, not directly related to columella cells, is the protoplast-pressure hypothesis, already mentioned and discussed in a preceding section of this paper.