Screening and Phenomic Characterization of Heavy Metal Pollution Resistance in Lettuce (Lactuca sativa)

Abstract

Heavy metals are naturally occurring elements originating from the Earth’s crust, but through natural processes and anthropogenic activities they can be released into the soil where they can affect plant life. Some heavy metals are toxic and thus pose a danger to plants when soils are contaminated with them, whereas other heavy metals are essential, meaning they are required in minute amounts for plants, but still pose a threat for soil contamination when present in excessive amounts. As soil is an essential component of most plants’ living environment, plants have developed effective defence mechanisms to cope with heavy metal exposure. Two heavy metal specific mechanisms include the use of phytochelatins (PCs) to bind and sequester heavy metals to the plant vacuole and the use of metal exclusion to prevent heavy metals from entering the plant and reaching sensitive metabolic sites. Another known danger of excessive heavy metal exposure is the formation of reactive oxygen species (ROS), which are highly reactive chemical species capable of reacting with and damaging important biological macromolecules such as DNA, RNA, and lipids. To detoxify ROS, plants have developed an intricate system of antioxidants that are able to scavenge ROS and prevent dangerous reactions from taking place. The aim of this current study was to investigate heavy metal pollution tolerance in plants using lettuce (Lactuca sativa) as a model organism. Through exposure of various lettuce cultivars to two heavy metals, zinc (Zn) and copper (Cu), the goal was to identify metal-tolerant and metal-sensitive cultivars and phenotypically characterize their defence against heavy metal toxicity. Zn-tolerant, mid-tolerant, and sensitive cultivars were selected through high-throughput seedling screening experiments, suggesting that the Legacy and Celtuce Celery cultivars were more Zn-tolerant and the Iceberg cultivar was more Zn-sensitive. Additionally, analysis of oxidative damage and antioxidant activity of these various lettuce cultivars revealed that the metal-tolerant plants generally exhibited lower levels of oxidative stress and higher antioxidant activities than the metal-sensitive plants when exposed to both Zn and Cu. Investigation of PC levels in Zn-exposed lettuce seedlings suggested that none of the selected cultivars were using PCs as a defence mechanism against Zn exposure at the seedling stage. Furthermore, in an analysis of Zn uptake levels in lettuce plant shoots grown in glasshouse conditions, findings suggested that none of the selected cultivars were utilizing metal exclusion as a defence mechanism as well.