Zinc (Zn) is an essential micronutrient for plants and animals because of its structural and catalytic roles in many proteins. Zn deficiency affects ca. two billion people, mainly those living on plant-based diets that rely on crops from Zn deficient soils. Plants maintain adequate Zn levels through tightly regulated Zn homeostasis mechanisms, involving Zn uptake, distribution and storage, but it was not known how they sense Zn status. We use in vitro and in planta approaches to show that the Arabidopsis thaliana F-group bZIP transcription factors bZIP19 and bZIP23, which are the central regulators of the Zn deficiency response, act as Zn sensors by binding Zn 2+ ions to a Zn sensor motif (ZSM).Deletions or modifications of this ZSM disrupts Zn binding, leading to a constitutive transcriptional Zn deficiency response, which causes a significant increase in plant and seed Zn accumulation. Since the ZSM is highly conserved in F-bZIPs across land plants, the identification of the first plant Zn-sensor will promote new strategies to improve the Zn nutritional quality of plant-derived food and feed, and contribute to tackle the global Zn deficiency health problem.

Root hairs (RHs) function in nutrient and water acquisition, root metabolite exudation, soil anchorage and plant-microbe interactions. Longer or more abundant RHs are potential breeding traits for developing crops that are more resource-use efficient and can improve soil health. RH elongation is controlled by both environmental and endogenous factors. While many genes are known to promote RH elongation, relatively little is known about genes and mechanisms that constrain RH growth. Here we demonstrate that a DOMAIN OF UNKNOWN FUNCTION 506 (DUF506) protein, AT3G25240, negatively regulates Arabidopsis thaliana RH growth. The AT3G25240 gene is strongly and specifically induced during P-limitation. Mutants of this gene, which we call REPRESSOR OF EXCESSIVE ROOT HAIR ELONGATION 1 (RXR1), have much longer RHs, while over-expression of the gene results in much shorter RHs. RXR1 physically interacts with a Rab-GTPase (RXR2), and an rxr2 mutant phenocopies the rxr1 mutant. Overexpression of a Brachypodium distachyon RXR1 homolog resulted in repression of RH elongation in Brachypodium. Taken together, our results reveal a DUF506-GTPase module with a prominent role in repression of RH elongation that is conserved in monocots and dicots.

The zinc deficiency response in Arabidopsis thaliana is regulated by F-group basic region leucine-zipper (F-bZIP) transcription factors, and there is evidence of evolutionary conservation of this regulatory network in land plants. Fundamental knowledge on the zinc homeostasis regulation in crop species will contribute to improving their zinc nutritional value. Legumes are protein-rich crops, used worldwide as part of traditional diets and as animal forage, being therefore a good target for micronutrient biofortification. Here, we identified F-bZIP transcription factors in representative legume species and functionally characterized the two F-bZIPs from Medicago truncatula. Results indicate that MtFbZIP1 is the functional homolog of A. thaliana bZIP19 and bZIP23, while MtFbZIP2 does not play a role in the zinc deficiency response. Additionally, analysis of M. truncatula genes from the Zrt/Irt-like protein (ZIP) family of zinc transporters or encoding nicotianamine synthase enzymes that produce the zinc ligand nicotianamine, support the conservation of the F-bZIP-regulated zinc deficiency response in M. truncatula. Phylogenetic analysis of F-bZIP homologs enriched in legume species reinforces the branching into two groups, with MtFbZIP1 and MtFbZIP2 mapping in Groups 1 and 2, respectively. This phylogeny combined with the functional characterization of MtFbZIPs supports the suggested conservation of the zinc deficiency response associated with Group 1 F-bZIPs, and the more variable evolutionary paths associated with Group 2. Overall, we provide novel insight on the mechanisms of response to zinc deficiency in M. truncatula, which contributes to developing strategies for improving zinc content in legume crops.

AbstractZinc (Zn) is an essential micronutrient for plants and animals because of its structural and catalytic roles in many proteins. Zn deficiency affects ca. two billion people, mainly those living on plant-based diets that rely on crops from Zn deficient soils. Plants maintain adequate Zn levels through tightly regulated Zn homeostasis mechanisms, involving Zn uptake, distribution and storage, but it was not known how they sense Zn status. We use in vitro and in planta approaches to show that the Arabidopsis thaliana F-group bZIP transcription factors bZIP19 and bZIP23, which are the central regulators of the Zn deficiency response, act as Zn sensors by binding Zn2+ ions to a Zn sensor motif (ZSM). Deletions or modifications of this ZSM disrupts Zn binding, leading to a constitutive transcriptional Zn deficiency response, which causes a significant increase in plant and seed Zn accumulation. Since the ZSM is highly conserved in F-bZIPs across land plants, the identification of the first plant Zn-sensor will promote new strategies to improve the Zn nutritional quality of plant-derived food and feed, and contribute to tackle the global Zn deficiency health problem.