Type: Response Essays
Sample donated: Doug Floyd
Last updated: December 22, 2019
Acquisition of trace elements, suchas iron (Fe), is indispensable for metabolic pathways, crop yield and high-qualitynutritious food. Biofortification offers the chance to reduce Fe deficiencyanemia in humans (Haas et al., 2016) but relies on elaborate knowledge of plant nutrient sensing, uptake andsignaling. Under Fe deficient conditions, Arabidopsisthaliana, like other non-graminaceous angiosperms, induces Fe uptake andhomeostasis genes, and follows a reduction-based Fe acquisition strategy (StrategyI) (Marschner et al.,1986).
Due to high amounts of insoluble Fe in the soil (Guerinot and Yi, 1994), plants first acidify the rhizosphere via protonextrusion, chelate ferric Fe (Fe3+) and subsequently reduce it to ferrousFe (Fe2+), which is mediated by FERRIC REDUCTION OXIDASE 2 (FRO2) (Brumbarova et al.,2015). IRON REGULATED TRANSPORTER 1 (IRT1), which displays a robust Fedeficiency marker gene (Ivanov et al., 2012), then takes up Fe2+ into root epidermalcells (Eide et al., 1996;Vert et al., 2002).
The expression of components involved in Strategy I Fe uptake is controlledby a complex regulatory network comprising the central regulator FER-LIKE IRONDEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT), a subgroup IIIa basic helix-loop-helix(bHLH) protein (Heim et al., 2003;Colangelo and Guerinot, 2004; Jakoby et al., 2004). FIT robustly controls the transcription of 34 Fe responsive genes (Mai et al., 2016). The fit-3 loss-of-functionmutant allele develops a strong Fe deficiency leaf chlorosis and has a lethalphenotype (Jakoby et al., 2004).
This displays thepivotal role of FIT in the Fe deficiency response. An interplay between external andinternal signaling pathways affects the activity of FIT on a molecular level. FITprotein activity is controlled by Fe availability (Jakoby et al., 2004) and is modulated by different signaling molecules aswell as hormones (Brumbarova et al.,2015; Le et al.
, 2016). FIT activity is further controlled by different protein-proteininteractions (Brumbarova et al.,2015; Le et al., 2016; Gratz et al.
, 2018 (submitted)). BHLH proteins are known to form homo- and hetero-dimers (Heim et al., 2003). The hetero-dimerization of FIT and members of subgroup Ib bHLHproteins, bHLH038/039/100/101 is needed for downstream activation of FIT targetgene expression (Heim et al., 2003;Wang et al.
, 2007; Yuan et al., 2008; Wang et al., 2013). Interestingly, FITexpression requires the presence of FIT protein, which displays the existenceof a FIT auto-regulatory loop (Jakoby et al., 2004;Wang et al., 2007). Hence, the dynamicaddition of FIT interactors enables the cell to quickly react to changes in Feavailability by fine-tuning FIT protein activity.
In a FIT overexpression situation, the FIT transcript is present under both Fe sufficient and deficientconditions. However, induction of FIT target genes, FRO2 and IRT1, only occursupon Fe deficiency. Thus, it was suggested that FIT is divided into active and inactivepools (Lingam et al., 2011;Meiser et al.
, 2011; Sivitz et al., 2011). Further, it was shown that FIT undergoes proteasomal degradation (Lingam et al., 2011;Meiser et al., 2011; Sivitz et al.
, 2011). Probably, mostly active FIT forms are degraded, but in order tomaintain Fe deficiency responses, however, inactive FIT is constantlyactivated. This might be beneficial for the cell to remain responsive to quick changesin nutrient abundance (Lingam et al., 2011;Meiser et al., 2011; Sivitz et al., 2011).
This raises the question what the molecular difference between activeand inactive FIT forms is. One potential mechanism is the involvement of post-translationalmodifications. This would be in agreement with the finding that theexperimentally observed molecular weight of FIT is higher than predicted (Sivitz et al., 2011). Recently, we identified CBL-INTERACTINGPROTEIN KINASE CIPK11 as a novelFIT interaction partner. We demonstrated that phosphorylation of the C-terminalpart of FIT (FIT-C) at position Ser272 by CIPK11 has a positive impact on itslocalization, mobility and dimerization capacity. Overall, Ser272phosphorylation renders the FIT protein active, which displays a valuablemolecular tool for the fine-tuning of the Fe uptake from the soil (Gratz et al., 2018(submitted)).
The activity of manytranscription factors, such as thebHLH protein INDUCER OF CBF EXPRESSION 1(ICE1), involved in cold-stress responses (Chinnusamy et al.,2003), is regulated by phosphorylation. Cold-induced phosphorylation ofSer278, mediated by SnRK2.6 / OPEN STOMATA 1 (OST1), increases ICE1 stabilityand hence promotes downstream target gene expression (Ding et al., 2015).
Likewise, MITOGEN-ACTIVATED PROTEIN KINASEs (MAPKs)MAPK3/MAPK6 facilitates additional ICE1 phosphorylation. However, this hasdiametrical effects on its stability and leads to proteasomal degradation ofICE1 (Li et al., 2017; Zhaoet al.
, 2017). Hence, activating phosphorylation events can contribute to theactivity of a protein, but an antagonistic mechanism is likewise needed toensure responsiveness to quickly changing conditions by a balanced proteinactivity. Plant tyrosine phosphorylation ismainly associated with proteins having kinase or transferase activity and isoverrepresented on nuclear proteins (Sugiyama et al., 2008). Only a few transcription factors were identified tobe tyrosine-phosphorylated until now, such as Coptis japonica WRKY-type transcription factor CjWRKY1, involved inalkaloid biosynthesis (Kato et al., 2007;Yamada and Sato, 2016).
A CjWRKY1 tyrosine phospho-mimicking mutant displays enhancedcytosolic localization and reduced transactivation activity. Additionally,protein turn-over was shown to be connected to the tyrosine-phosphorylationstatus (Yamada and Sato, 2016).This prompted us to questionwhether FIT activity is similarly regulated. Hence, it is of interest toinvestigate, whether FIT possesses additional phosphorylation target sites and whetherone site reflects a negative regulatory unit. We further asked if a tyrosine phosphorylation-sitecan be identified that might contribute to FIT stability. It is also conceivablethat different phosphorylation sites exhibit different significances and hencedisplay a certain hierarchy.