Iron (Fe) Acquisition By Plant Roots and Fe Signaling

Introduction - Iron

Iron (Fe) is required by nearly all organisms for continued existence. Fe is a crucial component of redox systems because of its ability of donating and accepting electrons. In plants, it works as a cofactor in many metalloproteins and is found in active sites of Fe-S clusters, which are essential for electron transport chains in photosynthesis and respiration. Fe is also required for DNA and hormone biosynthesis, nitrogen fixation, sulfate assimilation, and chlorophyll biosynthesis. Limited Fe solubility is one of the major determinants of its phytoavailability to plants. Fe solubility in an oxygenated medium at pH 7 is hundred times less than the essential iron concentration for survival of a plant because of the ability of Fe to produce ferric oxides and ferric hydroxides in aerobic environments (Guerinot, 1994; Lindsay, 1995; Palmer and Guerinot, 2009). Fe deficiency in plants generally occurs on well-oxygenated and alkaline soils such as calcareous soils (Frossard et al., 2000). These soils cover one third of agricultural land in the world (White and Broadley, 2009). Fe-deficiency induces intercostal leaf chlorosis due to the limited chlorophyll biosynthesis. This in turn, causes significant yield losses of crops in the field. Fe deficiency also compromises human health since plants are the most common source of dietary iron. Iron deficiency is the most common cause of anemia worldwide. In the United States, 7% of toddlers ages 1 to 2 years old and 9-16% of menstruating women are iron deficient (Clark, 2008). 30-70% of the people have iron deficiency anemia in the underdeveloped countries in the world (Yip and Ramakrishnan, 2002). Moreover, it is estimated that iron-deficiency anemia affects some two billion people, causing almost one million deaths each year (WHO, 2002). Understanding the molecular mechanisms of Fe uptake and regulation of these processes is important for the correction of nutritional disorders in both plants and humans.

Iron Acquisition by Roots

Plants have developed two distinct mechanisms to mobilize insoluble iron in the rhizosphere and uptake it through plasma membrane (Romheld, 1987). In Strategy II plants, grasses and cereals, phytosiderophores are released into the rhizosphere, where they complex with ferric iron (Fe3+) and then the complex is taken up into root cells via Yellow-stripe1 (YS1) transporters (Curie et al., 2001; Kim and Guerinot, 2007). In Strategy I plants, which includes all dicots and non-graminaceous monocots such as Arabidopsis thaliana, Fe is taken up by a set of three activities on plasma membrane of root epidermal cells. First, rhizosphere is acidified by a plasma membrane (PM) H+-ATPase-mediated proton extrusion mechanism (Schmidt et al., 2003). In Arabidopsis, rhizosphere acidification under Fe-deficiency is mediated by AHA2, a member of PM H+-ATPase family. On the other hand, another member of the same family, AHA7 functions in root hair development (Santi and Schmidt, 2009). Rhizosphere acidification is followed by reduction of insoluble Fe3+ to soluble Fe2+ by ferric reductase oxidase (FRO) that possesses the transmembrane electron transport ability. To date, FRO encoding genes have been identified in different Strategy I species such as pea (Waters et al., 2002), tomato (Li et al., 2004), cucumber (Waters et al., 2007) and Arabidopsis (Robinson et al., 1999). In Arabidopsis, FRO family consists of eight members (Jeong and Connolly, 2009). The first identified member of the family, FRO2 is localized to PM of root epidermal cells. Members of FRO family show various specificities of tissue expression and subcellular localization, and some of them are Fe-regulated (Robinson et al., 1999; Feng et al., 2006; Mukherjee et al., 2006; Jeong et al., 2008). All of this information indicates multiple roles of FROs in metal acquisition, distribution and homeostasis in plants. Reduced iron is taken up by a high-affinity transporter, iron regulated transporter 1 (IRT1), which is highly expressed in epithelial root cells and localized to PM (Eide et al., 1996; Varotto et al., 2002; Vert et al., 2002). IRT1 belongs to ZIP metal transporter family. Both FRO2 and IRT1 are under transcriptional and post-transcriptional regulation in Fe-limited conditions because their mRNA and protein levels increase in response to Fe-deficiency (Connolly et al., 2002; Kerkeb et al., 2008). There are other transporters that function in subcellular mobilization of Fe such as AtNRAMP3/4, which are two transporters localized on tonoplast. Its expression increases under Fe deficiency and it functions in iron remobilization from vacuole. Fe toxicity is as important as Fe deficiency as excessive amounts of iron can cause over-production of reactive oxygen species in Fenton reaction (Moller et al., 2007). Therefore, Fe homeostasis is strictly regulated (Martinoia et al., 2007). Excessive Fe is mobilized into vacuole by AtVIT1, which is Arabidopsis ortholog of yeast transporter CCC1 (Kim et al., 2006). Fe mobilization into chloroplast is achieved by AtPIC1, which is an ortholog of cyanobacterial Fe transporter (Duy et al., 2007), and AtFRO7 (Jeong et al., 2008). Excessive Fe is sequestered by ferritins in plastids. Arabidopsis ferritins are encoded by four genes (Ravet et al., 2009). AtFER1 is the most highly expressed in response to Fe excess while AtFER2 is the only gene expressed in seeds (Petit et al., 2001; Ravet et al., 2009).

Iron Signaling in Plants


According to recent studies of Fe deficiency induced signaling in Arabidopsis, a basic helix-loop-helix (bHLH) transcription factor called Fe-deficiency-induced transcription factor 1 (FIT1 or bHLH29/FRU) was identified as the regulator of 71 Fe-deficiency induced genes including FRO2, IRT2, AHA7 and nicotinamine synthase 1 (NAS1) (Colangelo and Guerinot, 2004). FIT1 is Arabidopsis ortholog of tomato LeFER gene and belongs to bHLH family of transcription factors (Ling et al., 2002; Yuan et al., 2005). The transcript levels of FIT1 and four other bHLH genes (bHLH38, bHLH39, bHLH100 and bHLH101) change under Fe deficiency and FIT1 forms heterodimers with bHLH38 and bHLH39 (Wang et al., 2007; Yuan et al., 2008). This indicates the presence of upstream components in Fe signaling pathway that mediates sensing and signaling of Fe deficiency. Moreover, Arabidopsis FER1 promoter contains a 15-bp cis-acting element called iron-dependent-regulatory-sequence (IDRS), which is involved in the repression of AtFER1 expression under Fe deficiency (Petit et al., 2001; Tarantino et al., 2003). Root growth inhibitory conditions such as cytokinin, ABA, mannitol or salt treatments suppress the iron deficiency response genes regardless of the plant iron status (Seguela et al., 2008).

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