Overexpression of Arabidopsis thaliana tryptophan synthase beta 1 ( AtTSB 1) in Arabidopsis and tomato confers tolerance to cadmium stress

Tryptophan (Trp) is an essential amino acid in humans, and in plants, it plays a major role in the regulation of plant development and defence responses. However, little is known about Trp‐mediated cadmium (Cd) tolerance. Gene expression analysis showed that Arabidopsis thaliana tryptophan synthase beta 1 ( AtTSB 1) is up‐regulated in plants treated with Cd; hence, we investigated whether this gene is involved in Cd tolerance. Exogenous application of Trp to wild‐type Arabidopsis enhances Cd tolerance. Cd tolerance in the Trp‐overproducing mutant trp5‐1 was associated with high chlorophyll levels and low lipid peroxidation, as indicated by malondialdehyde 4‐hydroxyalkenal level, whereas the wild‐type developed symptoms of severe chlorosis. Moreover, the Trp‐auxotroph mutant trp2‐1 was sensitive to Cd. CaMV 35S promoter‐driven AtTSB 1 enhanced Trp accumulation and improved Cd tolerance in transgenic Arabidopsis and tomato plants without increasing the level of Cd. Moreover, reverse transcription‐polymerase chain reaction confirmed that enhanced level of Trp in AtTSB 1 transgenic Arabidopsis plants affected the expression of AtZIP 4 and AtZIP 9 metal transporters, which interfered with Cd ion trafficking, a mechanism of transcriptional regulation that does not exist in wild‐type plants. Overexpression of AtTSB 1 in transgenic tomato also produced higher Trp synthase‐ β enzyme activity than that in wild‐type plants. These results implicate that Trp could be involved in Cd defence.