Abstract
Zinc (Zn) and iron (Fe) deficiency are widespread among citrus plants, but the molecular mechanisms regarding uptake and transport of these two essential metal ions in citrus are still unclear. In the present study, 12 members of the Zn/Fe-regulated transporter (ZRT/IRT)-related protein (ZIP) gene family were identified and isolated from a widely used citrus rootstock, trifoliate orange (Poncirus trifoliata L. Raf.), and the genes were correspondingly named as PtZIPs according to the sequence and functional similarity to Arabidopsis thaliana ZIPs. The 12 PtZIP genes were predicted to encode proteins of 334–419 amino acids, harboring 6–9 putative transmembrane (TM) domains. All of the PtZIP proteins contained the highly conserved ZIP signature sequences in TM-IV, and nine of them showed a variable region rich in histidine residues between TM-III and TM-IV. Phylogenetic analysis subdivided the PtZIPs into four groups, similar as found for the ZIP family of A. thaliana, with clustered PtZIPs sharing a similar gene structure. Expression analysis showed that the PtZIP genes were very differently induced in roots and leaves under conditions of Zn, Fe and Mn deficiency. Yeast complementation tests indicated that PtIRT1, PtZIP1, PtZIP2, PtZIP3, and PtZIP12 were able to complement the zrt1zrt2 mutant, which was deficient in Zn uptake; PtIRT1 and PtZIP7 were able to complement the fet3fet4 mutant, which was deficient in Fe uptake, and PtIRT1 was able to complement the smf1 mutant, which was deficient in Mn uptake, suggesting their respective functions in Zn, Fe, and Mn transport. The present study broadens our understanding of metal ion uptake and transport and functional divergence of the various PtZIP genes in citrus plants.
Highlights
Zinc (Zn) and iron (Fe) are essential micronutrients for plant growth and development
The ZIP genes were PCR-amplified, cloned, and sequenced from trifoliate orange using sweet orange sequences as references, and were correspondingly named as PtZIPs according to the sequence and functional similarity to A. thaliana ZIPs (Table 1)
12 PtZIP genes were isolated from trifoliate orange plants, and sequence analysis and prediction suggests that they all possessed the basic characteristics of members of the ZIP family
Summary
Zinc (Zn) and iron (Fe) are essential micronutrients for plant growth and development. Shortage or excess of Zn and Fe would cause severe nutritional disorders To cope with this issue, plants have developed a tightly regulated cellular homeostasis system to balance the uptake, distribution and utilization of these metal ions (Clemens, 2001; Grotz and Guerinot, 2006). In this system, various members of the zinc/iron-regulated transporter (ZRT/IRT)-related protein (ZIP) family, natural resistance associated macrophage protein (NRAMP) family, cation diffusion facilitator (CDF) family, yellow stripe-like (YSL) family, major facilitator super family (MFS), P1B-type heavy metal ATPase (HMA) family, vacuolar iron transporter (VIT) family, and the cation exchange (CAX) family have been shown to play key roles (Vigani et al, 2013; Boutigny et al, 2014; Bashir et al, 2016)
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