Genome-Wide Identification and Expression Analysis of Heavy Metal Stress–Responsive Metallothionein Family Genes in Nicotiana tabacum

Tobacco (Nicotiana tabacum L.) is the most important non-food cash crop worldwide and has recently been considered a Cd hyperaccumulator. Metallothionein, a small–molecular weight protein with cysteine-rich domains, plays a key role in plant growth, development, and maintenance of metal homeostasis. The understanding of the tobacco metallothionein (MT) gene family remains unclear. Herein, we systematically characterized twelve NtMT genes in the tobacco genome and classified them into three phylogenetic subfamilies. A number of cis-elements related to plant responses to hormones and abiotic stresses were detected in the promoters of NtMT genes. Tissue expression pattern analysis indicated that NtMT4A/4B were expressed only in seeds, and NtMT2C/2F/2G were mainly expressed in roots. Moreover, most of the NtMT genes were highly induced by heavy metal stress and ion deficiency, suggesting their critical role in relieving metal toxicity and maintaining ion homeostasis in tobacco. This was the first study to describe the genome-wide analysis of the NtMT gene family in tobacco, and the results lay a foundation for understanding the functions of NtMT genes for further enhancing plant tolerance to heavy metal toxicity.