Genome-Wide Identification and Expression Analysis of Metal Tolerance Protein Gene Family in Medicago truncatula Under a Broad Range of Heavy Metal Stress.

Ahmed H El-Sappah,Jia Li,Zhao Xianming,Ahmed S Elrys,Manzar Abbas,Yu Wang,Khaled A El-Tarabily,Rania G Elbaiomy,Qiulan Huang,Yumin Zhu,Kuan Yan

doi:10.3389/fgene.2021.713224

Abstract

Metal tolerance proteins (MTPs) encompass plant membrane divalent cation transporters to specifically participate in heavy metal stress resistance and mineral acquisition. However, the molecular behaviors and biological functions of this family in Medicago truncatula are scarcely known. A total of 12 potential MTP candidate genes in the M. truncatula genome were successfully identified and analyzed for a phylogenetic relationship, chromosomal distributions, gene structures, docking analysis, gene ontology, and previous gene expression. M. truncatula MTPs (MtMTPs) were further classified into three major cation diffusion facilitator (CDFs) groups: Mn-CDFs, Zn-CDFs, and Fe/Zn-CDFs. The structural analysis of MtMTPs displayed high gene similarity within the same group where all of them have cation_efflux domain or ZT_dimer. Cis-acting element analysis suggested that various abiotic stresses and phytohormones could induce the most MtMTP gene transcripts. Among all MTPs, PF16916 is the specific domain, whereas GLY, ILE, LEU, MET, ALA, SER, THR, VAL, ASN, and PHE amino acids were predicted to be the binding residues in the ligand-binding site of all these proteins. RNA-seq and gene ontology analysis revealed the significant role of MTP genes in the growth and development of M. truncatula. MtMTP genes displayed differential responses in plant leaves, stems, and roots under five divalent heavy metals (Cd2+, Co2+, Mn2+, Zn2+, and Fe2+). Ten, seven, and nine MtMTPs responded to at least one metal ion treatment in the leaves, stems, and roots, respectively. Additionally, MtMTP1.1, MtMTP1.2, and MtMTP4 exhibited the highest expression responses in most heavy metal treatments. Our results presented a standpoint on the evolution of MTPs in M. truncatula. Overall, our study provides a novel insight into the evolution of the MTP gene family in M. truncatula and paves the way for additional functional characterization of this gene family.

Highlights

Metals act as a cofactor, which has essential implications in inactivating enzymes in plant cells to perform specific biological reactions (Thomine and Vert, 2013)
Every gene was assigned with a specific name, i.e., MtMTP1.1, MtMTP1.2, MtMTP2, MtMTP4, MtMTP5, MtMTP7, MtMTP8.1, MtMTP8.2, MtMTP9, MtMTP10.1, MtMTP10.2, and MtMTP11
Twelve identified Metal tolerance proteins (MTPs) genes were phylogenetically divided into three major substrate-specific clusters (Zn/Fe-cation diffusion facilitator (CDFs), Mn-CDFs, and Zn-CDFs), and seven groups seemed to have undergone expansion and gene loss after polyploidization through segmental duplication

Summary

Introduction

Metals act as a cofactor, which has essential implications in inactivating enzymes in plant cells to perform specific biological reactions (Thomine and Vert, 2013). A deficient concentration of non-essential metals, including mercury (Hg), silver (S), cadmium (Cd), and lead (Pb), can cause plant cell toxicity (Clemens, 2001). Plants overcome heavy metal stress by various physiological and molecular mechanisms, including genomic-level and complex biochemical processes (Liu et al, 2019). Some of these mechanisms are part of the homeostatic process and are constitutive (Rai et al, 2019). CDF transporters have been widely identified in many organisms since their first identification in the bacterial cell (Nies and Silver, 1995), which were further classified into three major groups: Mn-CDF, Zn/Fe-CDF, and Zn-CDF, based on either confirmed or hypothesized transported substrate specificities (Montanini et al, 2007)

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Discussion

Conclusion