Romera et al. (1999) showed that Fe-deficient roots of several Strategy I species produced more ethylene than the Fe-sufficient ones, even before the plants showed any other symptom of deficiency. This excludes that the higher ethylene production could be due to stimulation of wound ethylene in necrotic tissues (Lynch and Brown, 1997). After this report, the higher ethylene production by Fe-deficient roots of different Strategy I species has been confirmed by other authors (Waters and Blevins, 2000; Li and Li, 2004; Molassiotis et al., 2005; Zuchi et al., 2009; Wang et al., 2012; Li et al., 2014a).

The higher ethylene production described for Fe deficiency has been further supported by results showing upregulation of genes implicated in ethylene synthesis, such as SAMS, ACS, and ACO (Figure 3; García et al., 2010; Stein and Waters, 2012 and references therein; Li et al., 2014a; Moran Lauter et al., 2014; Romera et al., in press and references therein). At the proteomic level, several studies have also shown a significant increase in the SAMS protein (Figure 3) under Fe deficiency (reviewed by López-Millán et al., 2013). Additionally, MTK, an enzyme involved in methionine recycling for a sustained ethylene production (Figure 3), is also induced by Fe deficiency (García et al., 2010; Zamboni et al., 2012; Romera et al., in press and references therein).

Besides ethylene synthesis, Fe deficiency can also affect ethylene responsiveness by altering the expression of genes implicated in ethylene signaling. Several of these genes are upregulated in different Strategy I species under Fe deficiency, like ETRs and ERSs (coding for ethylene receptors), EIN2, EIN3, EILs, and ERFs (see above; O'Rourke et al., 2007; García et al., 2010, 2014; Wang et al., 2014a). Whether the expression of these genes enhances or decreases the sensitivity to ethylene is not known yet. It is possible that ethylene sensitivity increases at the earlier stages of Fe deficiency and then decreases as a dampening mechanism, slowing down the ethylene response once it has been initiated. In any case, this deserves further investigation.

There are few publications relating ethylene and Fe deficiency in Strategy II species. Morgan and Hall (1962) showed that Fe-deficient sorghum plants treated with 2,4-D (2,4-dichlorophenoxyacetic acid, a synthetic auxin) produced more ethylene than the Fe-sufficient ones. However, they determined ethylene when plants were very chlorotic and, consequently, the higher ethylene production could be an indirect effect of the advanced stress. After this report, Romera et al. (1999) found that roots from several Fe-deficient Strategy II species (maize, wheat, barley) did not produce more ethylene than the Fe-sufficient ones. Recently, Wu et al. (2011) showed similar results with barley plants. However, Fe-deficient roots from rice, that presents a combined strategy (Figure 2; see Section Introduction), produced more ethylene than the Fe-sufficient ones (Wu et al., 2011). Moreover, several ethylene synthesis genes, like OsACS, OsACO, OsSAMS, and OsMTK, were up-regulated in rice under Fe deficiency (Figure 3; Kobayashi et al., 2005; Zheng et al., 2009; Kobayashi and Nishizawa, 2012; Itai et al., 2013). Zheng et al. (2009) also found upregulation of a gene coding for a transcription factor relating to ethylene signaling in rice (Os03g64260) under Fe deficiency.

Additionally to rice, SAMS and MTK genes are also up-regulated in Strategy II species, such as barley and maize, under Fe deficiency (Figure 3; Suzuki et al., 2006; Li et al., 2014b). This is in agreement with the participation of both genes in NA and PS synthesis, besides their participation in ethylene synthesis (Figure 3).

As previously stated, Strategy I species need to reduce Fe³⁺, the most abundant form in soils, to Fe²⁺, prior to uptake. This reduction is mediated by a plasma membrane ferric reductase (encoded by AtFRO2 in Arabidopsis; Robinson et al., 1999) and then Fe²⁺ is taken up by a Fe²⁺ transporter (encoded by AtIRT1 in Arabidopsis; Eide et al., 1996). Ferric reductases and iron transporter genes have also been cloned from other plant species, like tomato (Eckhardt et al., 2001; Li et al., 2004), pea (Waters et al., 2002; Cohen et al., 2004), and cucumber (Waters et al., 2007). When grown under Fe deficiency, Strategy I species enhance both ferric reductase activity (due to increased expression of AtFRO2-like genes) and Fe²⁺ uptake capacity (due to increased expression of AtIRT1-like genes) (Walker and Connolly, 2008; Kobayashi and Nishizawa, 2012).

In addition to enhanced ferric reductase activity and Fe²⁺ uptake capacity, Strategy I species can develop other Fe deficiency responses aimed to facilitate Fe mobilization and uptake from the soil. This includes the capacity to acidify the rhizosphere medium (due to increased expression of plasma membrane proton-ATPase genes, such as AtAHA2, AtAHA7, CsHA1, and MxHA7), which contributes to Fe solubilisation (Santi et al., 2005; Waters et al., 2007; Zha et al., 2014); and the increased synthesis and release of Fe³⁺-related compounds such as flavins and phenolics (Jin et al., 2007; Rodríguez-Celma et al., 2013; Fourcroy et al., 2014; Schmid et al., 2014; Schmidt et al., 2014). The exact function of flavins and phenolics is not totally clear, but it has been proposed that they could act as “Iron Binding Compounds” contributing to Fe mobilization in the rhizosphere (Rodríguez-Celma and Schmidt, 2013). Alternatively, their main function could be related to Fe³⁺reduction, acting as long distance electron shuttle (Rodríguez-Celma and Schmidt, 2013).

The regulation of the Fe-related genes described above is not totally understood but in the last years several transcription factors that participate in their activation have been found. The master regulator of most of the responses to Fe deficiency is the tomato SlFER, identified as a bHLH transcription factor (Ling et al., 2002), or its homologs AtFIT in Arabidopsis (bHLH29; Colangelo and Guerinot, 2004; Jakoby et al., 2004; Bauer et al., 2007) and MxFIT in Malus xiaojinensis (Yin et al., 2014). The tomato fer mutant (Figure 5) and the Arabidopsis fit mutant are very chlorotic and lack the ability to activate most Fe responses in roots (Brown et al., 1971; Ling et al., 2002; Colangelo and Guerinot, 2004; Jakoby et al., 2004). In Arabidopsis, the AtFIT regulatory network comprises other bHLH transcription factors, such as AtbHLH38, AtbHLH39, AtbHLH100, and AtbHLH101. All of them have redundant functions and can interact with AtFIT to form heterodimers that activate the expression of the Fe acquisition genes AtFRO2 and AtIRT1 (Figure 1; Yuan et al., 2008; Wang et al., 2013a; Maurer et al., 2014; Brumbarova et al., 2015). FIT(FER) is induced in roots in response to Fe deficiency while the other bHLHs are induced in both roots and leaves in response to Fe deficiency (Colangelo and Guerinot, 2004; Jakoby et al., 2004; Brumbarova and Bauer, 2005; Wang et al., 2007; Yuan et al., 2008; Brumbarova et al., 2015). In addition to the Arabidopsis AtFIT regulatory network, the AtPYE (AtPOPEYE) regulatory network has been described and implied in the regulation of a different subset of stele expressed Fe-related genes (Long et al., 2010; Ivanov et al., 2012; Brumbarova et al., 2015; Zhang et al., 2015). Other transcription factors related to Fe deficiency responses in Arabidopsis are AtMYB72 and AtMYB10 (Colangelo and Guerinot, 2004; Palmer et al., 2013; Zamioudis et al., 2014). They have redundant functions and have been implicated in the Fe deficiency induced up-regulation of AtNAS4 (encoding NicotianAmine Synthase), playing an opposite role to AtPYE (Palmer et al., 2013).

The addition of ethylene inhibitors to Fe-deficient plants inhibits the induction of most of their physiological Fe responses, such as ferric reductase activity (Figure 4), Fe²⁺ uptake capacity, rhizosphere acidification, and flavin excretion (Romera and Alcántara, 1994, 2004; Lucena et al., 2006; Romera et al., 2007; Waters et al., 2007). The ferric reductase activity and other Fe responses are also drastically inhibited by ethylene inhibitors in mutants that show constitutive Fe responses and, as a consequence, accumulate toxic concentrations of Fe in their leaves, like the pea brz (bronze) and dgl mutants. Upon treatment with ethylene inhibitors, these mutants lower Fe uptake and, consequently, avoid Fe toxicity symptoms (Figure 6; Romera et al., 1996, 2014; see Supplementary Material). In contrast to ethylene inhibitors, the addition of ethylene, ACC or ethephon to Fe-sufficient plants induces some physiological Fe responses, such as enhanced ferric reductase activity (Figure 4), located in the subapical regions of the roots where formation of root hairs is also induced (Figure 5; Romera and Alcántara, 2004; Lucena et al., 2006; Waters et al., 2007; Romera et al., 2007; García et al., 2010, 2011). In supporting the role of ethylene, it should be mentioned that Fe-efficient cultivars of pea (Kabir et al., 2012) and Medicago truncatula (Li et al., 2014a) produce more ethylene than the Fe-inefficient ones. This suggests that the higher ethylene production would allow these cultivars to better activate their responses to Fe deficiency.

The participation of ethylene in the activation of physiological responses is further supported when analyzing its effects on the genes controlling these responses (Lucena et al., 2006; Waters et al., 2007; García et al., 2010; Lingam et al., 2011; Yang et al., 2014; Romera et al., in press). Ethylene up-regulates the expression of the key transcription factors AtFIT (or its tomato homolog SlFER), AtbHLH38, AtbHLH39, and AtMYB72 (Figure 1; Lucena et al., 2006; García et al., 2010, 2013, 2014; Lingam et al., 2011). The link between ethylene and AtFIT has been recently reinforced. It has been shown that AtEIN3 and AtEIL1, two transcription factors in the ethylene signaling pathway (see Section Ethylene Synthesis and Signaling under Fe Deficiency), interact with AtMED16 (Mediator) and AtMED25, to form a complex implicated in the transcription of AtFIT (Figure 7; Yang et al., 2014; Zhang et al., 2014; Brumbarova et al., 2015). Moreover, AtEIN3 and AtEIL1 can also influence the posttranscriptional stability of AtFIT (Lingam et al., 2011). AtbHLH38 and AtbHLH39 interact with AtFIT to form heterodimers that activate the expression of the Fe acquisition genes AtFRO2 and AtIRT1 (Figures 1, 7; Yuan et al., 2008; Wang et al., 2013a). Consequently, the ferric reductase activity (depending on AtFRO2-like genes) and the Fe²⁺ uptake capacity (depending on AtIRT1-like genes) can be regulated by ethylene through AtFIT, AtbHLH38, and AtbHLH39. Similarly, the acidification capacity (depending on AtAHA-like genes, also activated by AtFIT-like transcription factors; Colangelo and Guerinot, 2004) can be regulated by ethylene too (Figure 1; Romera and Alcántara, 1994, 2004; Waters et al., 2007). Flavin excretion is another response inhibited by ethylene inhibitors (Romera and Alcántara, 2004). Since AtbHLH38 and AtbHLH39 have been implicated in the activation of flavin production (Vorwieger et al., 2007), and both transcription factors can be induced by ethylene (García et al., 2010), these results reinforce the hypothesis that flavin production is also controlled by ethylene (Figure 1). In relation to the excretion of phenolics, another response to Fe deficiency (Römheld and Marschner, 1986), some indirect studies suggest that the biosynthesis of some phenolics, like caffeic, coumaric and ferulic acids, could be stimulated by ethylene through an increase in the activity of the phenylalanine ammonia lyase enzyme (Rhodes and Wooltorton, 1973; Heredia and Cisneros-Zevallos, 2009; Liang et al., 2013). Very recently, Zamioudis et al. (2014) have found that the β-glucosidase AtBGLU42 is very important for the secretion of phenolic compounds under Fe deficiency. Furthermore, they have shown that the expression of AtBGLU42, as well as the expression of the phenolic efflux transporter AtABCG37 (formerly named AtPDR9) and other genes involved in phenolic synthesis, are dependent on the transcription factor AtMYB72 (Figure 1; Zamioudis et al., 2014). Since AtMYB72 expression is activated by ethylene (García et al., 2010), probably through AtFIT (AtMYB72 is one of its targets; Sivitz et al., 2012), this suggests that ethylene can also regulate phenolic secretion (Figure 1). In supporting this view, it should be noted that AtBGLU42 expression is also activated by ethylene (García et al., 2010).

Besides genes related to Fe acquisition from the growth medium, there are other genes that are induced under Fe deficiency in roots and/or leaves, like AtFRD3 (Ferric Reductase Defective3; Rogers and Guerinot, 2002) and AtNASs (NicotianAmine Synthase; Klatte et al., 2009), that are very important for internal Fe mobilization and homeostasis. The FRD3 protein belongs to the multidrug and toxin efflux (MATE) family and has been implicated in the loading of citrate into the xylem, which is necessary for Fe translocation from roots to shoots (Rogers and Guerinot, 2002; Durrett et al., 2007; Roschzttardtz et al., 2011). The higher expression of FRD3 under Fe deficiency is generally associated with increased synthesis of organic acids, like citrate and malate (Landsberg, 1981; Kabir et al., 2012; Li et al., 2014a). In toto, both responses cooperate for the internal translocation of Fe from roots to shoots. NAS (NA Synthase) proteins participate in the synthesis of NA (nicotianamine), which is a chelating agent implied in the long-distance transport of Fe (and other metals) and that facilitates the transport of Fe through the phloem to sink organs (Klatte et al., 2009; Schuler et al., 2012). NA is also the precursor for the biosynthesis of PS, that play a key role in Strategy II species (Figure 3; Kobayashi and Nishizawa, 2012). The mutants related to these genes (Arabidopsis frd3 and nas4x, and the tomato NAS mutant chloronerva) show constitutive activation of Fe responses, even when grown under Fe-sufficient conditions (Rogers and Guerinot, 2002; Klatte et al., 2009; García et al., 2013). This suggests that the precise distribution of Fe throughout the plant has a decisive role in the control of Fe deficiency responses. Ethylene has been implicated in the activation of both AtNAS (AtNAS1 and AtNAS2) and AtFRD3 genes (García et al., 2010).

In addition to physiological responses, Strategy I species can develop some morphological responses in their roots under Fe deficiency, like subapical root hairs (Figure 1), root epidermal transfer cells and cluster roots (also named proteoid roots; Kramer et al., 1980; Römheld and Marschner, 1986; Romera and Alcántara, 1994, 2004; Schmidt et al., 2000a; Waters and Blevins, 2000; Schmidt and Schikora, 2001; Schikora and Schmidt, 2002; Zaid et al., 2003; Romera et al., 2007; García et al., 2015). Transfer cells have increased surface area, due to invaginations of the plasma membrane (Kramer et al., 1980; Schmidt et al., 2000a; Schikora and Schmidt, 2002). Proteoid roots are clusters of closely spaced short lateral rootlets formed in some plant species adapted to poor soils (Waters and Blevins, 2000; Zaid et al., 2003; Wang et al., 2014b). Most of these root modifications are also formed under P deficiency since both Fe and P may be poorly available in soils (Schmidt and Schikora, 2001; Schikora and Schmidt, 2002; Wang et al., 2014b). Root hairs, root epidermal transfer cells and cluster roots enhance nutrient uptake by increasing the surface of contact of roots with soil and by chemically modifying the soil environment (Wang et al., 2014b). Besides these specialized morphological responses, Fe deficiency, depending on the extent of the deficiency (mild, moderate, or severe), can also change root system architecture by altering the number, length, and diameter of roots. Generally, Fe deficient plants exhibit a shallower architecture that results from inhibition of primary root elongation (Kramer et al., 1980). Additionally, Fe deficiency can cause an increase in lateral root density (Kramer et al., 1980; Jin et al., 2008).

Ethylene and auxin, along with other hormones and signaling substances, have been implicated in all the morphological changes described in the above paragraph (Romera and Alcántara, 1994, 2004; Schmidt et al., 2000a; Waters and Blevins, 2000; Schmidt and Schikora, 2001; Schikora and Schmidt, 2002; Zaid et al., 2003; Romera et al., 2007; Muday et al., 2012; Wang et al., 2014b; García et al., 2015). Both hormones synergistically inhibit root elongation while play an antagonistic role on lateral root formation (Muday et al., 2012). Moreover, ethylene influences auxin distribution at the root tip, thus affecting the development of subapical root hairs (Muday et al., 2012; Lee and Cho, 2013).

The implication of ethylene on the regulation of morphological responses to Fe deficiency has been based on the use of ethylene inhibitors and precursors, and the use of ethylene mutants. The addition of ethylene inhibitors to Fe-deficient plants inhibited subapical root hairs, transfer cells and cluster roots, while the addition of ethylene or ethylene precursors (ACC or ethephon) to Fe-sufficient plants promoted them (Romera and Alcántara, 1994, 2004; Schmidt et al., 2000a; Schmidt and Schikora, 2001; Schikora and Schmidt, 2002; Zaid et al., 2003; Romera et al., 2007). In the same way, some ethylene insensitive mutants, like the Arabidopsis etr1 and ein2, the soybean etr1 and the Medicago truncatula sickle, did not develop subapical root hairs either under Fe-deficiency or upon ACC treatment, while the wild-types did (reviewed in Romera and Alcántara, 2004). By contrast, the Arabidopsis ethylene constitutive mutant ctr1 developed subapical root hairs even under Fe-sufficient conditions (Romera and Alcántara, 2004).

The implication of ethylene on the development of transfer cells has also been reinforced by some studies with cotyledons. Ethylene functions as a key inductive signal for wall ingrowth and, consequently, transfer cell formation in epidermal cells of cotyledons (Zhou et al., 2010) while glucose functions as a negative regulator (Andriunas et al., 2011). Glucose modulates the amplitude of the ethylene-stimulated wall ingrowth induction by down-regulating the expression of ethylene synthesis and signaling genes, such as EIN3/EILs (Andriunas et al., 2011). These antagonistic effects of ethylene and glucose suggest that EIN3/EILs (see Section Ethylene Synthesis and Signaling under Fe Deficiency) act as integrators of glucose and ethylene regulation of transfer cell formation.

Since ethylene has been implicated in the regulation of most of the physiological and morphological responses to Fe deficiency in Strategy I species (Figures 1, 7), a question emerged: Are all of them regulated by ethylene in the same way? The answer to this question, based on results with ethylene mutants, is that different responses can be regulated by ethylene through different signaling pathways. On the other hand, many results suggest that ethylene acts in conjunction with other hormones and signaling substances to regulate the responses. Some of these signals (auxin, nitric oxide) act positively on the regulation of Fe deficiency responses in Strategy I species while other ones (ABA, cytokinins, jasmonic acid, brassinosteroids) act negatively (Giehl et al., 2009; Hindt and Guerinot, 2012; Brumbarova et al., 2015; García et al., 2015; Romera et al., in press).

Results from ethylene insensitive mutants suggest that physiological and morphological responses can be regulated through different signaling pathways. For instance, the development of subapical root hairs is impaired in the Arabidopsis ethylene insensitive mutants ein2 and etr1, either under Fe deficiency or upon ACC treatment (Schmidt and Schikora, 2001; Romera and Alcántara, 2004), while the enhanced ferric reductase activity and the expression of Fe acquisition genes is not impaired (Lucena et al., 2006; García et al., 2010). On the other hand, both the Arabidopsis ethylene constitutive mutant ctr1 and the Arabidopsis ethylene overproducer mutant eto have constitutive subapical root hairs (a Fe deficiency response) in complete nutrient solution; however, neither of these mutants have full constitutive activation of Fe physiological responses (Figure 8; Schmidt et al., 2000b; Romera and Alcántara, 2004; García et al., 2007, 2014, 2015; see Supplementary Material). In the same way, root hairs, transfer cells and cluster roots are almost fully induced by ACC or ethephon in plants grown with high levels of Fe while physiological responses are activated to a lesser degree than when applied to plants grown with low levels, or in absence, of Fe (Figure 4; Romera and Alcántara, 1994; Schmidt et al., 2000a; Zaid et al., 2003; Lucena et al., 2006; García et al., 2013).

From the results above, several conclusions can be drawn. First, morphological and physiological responses can be differently regulated by ethylene. Second, for some morphological responses, like root hairs, ethylene acts through a signaling pathway including ETR1, CTR1, and EIN2 (Figure 7). Third, for some physiological responses, like ferric reductase activity, ethylene could act through a pathway where EIN2, and possibly CTR1, are not strictly required (Figure 7). Fourth, for the regulation of physiological responses, ethylene could act in conjunction with Fe-related repressive signals since the ctr1 and eto mutants do not have full constitutive activation of these responses (see above).

The existence of an alternate route for ethylene signaling, besides the conventional one including CTR1 and EIN2 (Figure 7; Shakeel et al., 2013), is further supported by results showing that the Arabidopsis ctr1 and ein2 mutants respond to both ACC (García et al., 2010, 2014) and ethylene inhibitors (García et al., 2007) for physiological responses. Does this mean that CTR1 and EIN2 do not participate at all in the regulation of these responses? Some experimental results suggest that, although not strictly required, CTR1 and EIN2 can participate in this regulation. As examples, hypoxia inhibited physiological responses in Fe-deficient wild type plants but not in ctr1 mutant plants (García et al., 2014), which suggests that CTR1 can play a role in their regulation. A possible explanation for the participation of CTR1 and EIN2, without being strictly required, could be the possible interaction between the conventional route for ethylene signaling and the alternate one under certain circumstances. In such cases, both routes could converge downstream through EIN3/EILs (Figure 7). In supporting this view, it should be noted that EIN3/EILs transcription factors have been involved in the up-regulation of FIT (activator of most physiological responses; Figure 7; Yang et al., 2014; Brumbarova et al., 2015) but also in the development of morphological responses, such as root hairs (Figure 7; Zhu et al., 2011) and transfer cells (Andriunas et al., 2011).

In addition to physiological responses (Figures 1, 7), FER (FIT homolog) has also been implicated in the activation of morphological responses. The tomato fer mutant does not develop either subapical root hairs or transfer cells under Fe deficiency (Schmidt et al., 2000a). However, it does develop both subapical root hairs (Figure 5; Romera and Alcántara, 2004; see Supplementary Material) and root transfer cells (Schmidt et al., 2000a) upon ACC treatment. This suggests that FER could indirectly activate morphological responses by influencing ethylene synthesis. This suggestion is further supported by two experimental results. First, the AtMTK gene, implicated in ethylene synthesis (Figure 3), is activated by ethylene through AtFIT (Figure 1; Colangelo and Guerinot, 2004; García et al., 2010). Second, one AtSAMS gene implicated in ethylene synthesis (Figure 3) is up-regulated by the AtMYB72 transcription factor (Zamioudis et al., 2014), that is a direct target of AtFIT (Figure 1; Sivitz et al., 2012).

In relation to the existence of Fe-related repressive signals, we can speculate that since physiological responses are not fully activated by ACC in “high Fe” plants (Figure 4), and are not fully activated in Fe-sufficient ctr1 (Figure 8) and eto mutants, some Fe-related signals are acting negatively to block physiological responses. It has been proposed that these Fe-related repressive signals are not associated with the total Fe in roots but with some Fe compound(s) moving through the phloem, which could negatively interact with ethylene signaling to regulate Fe physiological responses (García et al., 2013; Mendoza-Cózatl et al., 2014; Zhai et al., 2014; Romera et al., in press). This does not preclude that these signals can also affect ethylene synthesis.

A possible step affected by the Fe-related repressive signals would be the Shk1 binding protein 1 (SKB1/AtPRMT5) in Arabidopsis (Fan et al., 2014). SKB1 associates with the chromatin of the Ib subgroup bHLH genes (AtbHLH38, AtbHLH39, AtbHLH100, and AtbHLH101) and symmetrically dimethylates histone H4R3, thereby increasing H4R3sme2 and inhibiting transcription (Figure 7). The quantity of SKB1 that associates with the chromatin of these bHLH genes depends on the Fe status of the plants, in such a way that Fe sufficiency increases its association, thus inactivating the transcription of the Ib subgroup bHLH genes (Figure 7; Fan et al., 2014; Brumbarova et al., 2015). The possible effect of the Fe-related repressive signals through SKB1 would explain several experimental results. First, it would explain why the overexpression of AtFIT does not activate physiological responses in plants grown with high levels of Fe (Colangelo and Guerinot, 2004; Jakoby et al., 2004) while the overexpression of AtFIT with either AtbHLH38 or AtbHLH39 does (Yuan et al., 2008). Second, it would explain why the foliar application of Fe inhibits more drastically the expression of AtbHLH38 and AtbHLH39 than that of AtFIT (García et al., 2013). Third, it would explain why Fe physiological responses are not fully activated in Fe-sufficient ctr1 (Figure 8) and eto mutants. Finally, it would explain why the application of ACC to Fe-sufficient plants does not fully activate physiological responses (Figure 4; Romera and Alcántara, 1994; Lucena et al., 2006; García et al., 2013). Taken together, the results described above indicate that ethylene can not activate Fe physiological responses until some Fe-related repressive signal is removed. This combinatorial control would provide Fe-specificity to the system, suggesting that both Fe deficiency and ethylene action are necessary for full transcriptional activation (Lucena et al., 2006).