ScMT10, a metallothionein-like gene from sugarcane, enhances freezing tolerance in Nicotiana tabacum transgenic plants

Sugarcane (Saccharum spp. hybrid) is of tropical origin, yet generally cultivated in subtropical areas. Cold stress is a major environmental factor that impairs the production of sugarcane in that area. Metallothioneins, a family of small, cysteine-rich proteins, are proved to be involved in metal homeostasis and detoxification and thus alleviate stress-induced ROS accumulation. Here, a cDNA clone deducing a single Cys-rich domain type 2 metallothionein-like protein, designated ScMT10, was isolated from sugarcane. The level of ScMT10 expression was up-regulated by salicylic acid (SA), cold, and oxidative stress (H2O2) in sugarcane seedlings, but it was down-regulated in response to abscisic acid (ABA), methyl jasmonate (MeJA), heavy metal ions (Cu2+, Cd2+), and salinity. Transgenic tobacco (Nicotiana tabacum) plants (ScMT10-OE) showed a remarkable enhancement in freezing tolerance compared with wild-type plants (WT). Furthermore, ScMT10-OE tobacco showed a lower electrolyte leakage, reduced malondialdehyde contents, and slower ROS (including H2O2) accumulation under freezing stress and retained higher proline, soluble sugar, and relative chlorophyll content compared to WT plants. The expression profiles of ScMT10 in the transgenic tobacco were similar to those in sugarcane seedlings under different stresses. Besides, the level of H2O2 in the ScMT10-OE plants was comparable to that observed in WT plants under heavy metal, salt, and drought stress. Therefore, the transgenic plants did not perform better than WT plants under those stresses. Taken together, we present a hypothesis that ScMT10 was exclusively and positively involved in the cold stress-induced accumulation of reactive oxygen species (ROS). The overexpression of ScMT10 in tobacco decreased the contents of endogenous ROS, MDA, and relative electric conductivity. It increased the accumulation of proline, soluble sugar, and relative chlorophyll under freezing stress, which alleviated the oxidative damage to the cell membrane during freezing exposure. ScMT10 can be considered as a candidate gene for cold-tolerance trait improvement.

Automated summary

ScMT10, a metallothionein-like protein in sugarcane, is up-regulated by cold, oxidative and salicylic acid stress. The overexpression of ScMT10 in transgenic tobacco enhances freezing tolerance, reduces damage and increases the accumulation of antioxidants. However, the performance of transgenic plants under heavy metal, salt and drought stress is comparable to that in the wild type.