Identification of genes up-regulated in response to Cd exposure in Brassica juncea L.

In this paper, the fluorescent mRNA differential display (DD) technique was applied to analyze transcriptional regulation in response to Cd treatment in a heavy-metal accumulator, Brassica juncea. 154 DD bands were identified, of which fragments corresponding to 15 and 13 cDNAs were successfully cloned from leaves and roots, respectively. Many of the genes were confirmed to have a 2-5 fold increase in expression in both roots and leaves after 48 h Cd exposure (approximately 22.4 ppm). However, several isolated genes, e.g., DD2, DD21, DD22, showed a reversed mRNA expression pattern. Sequencing revealed those Cd-induced up-regulated genes displayed mRNAs corresponding to 19 different genes, 18 of which had a clear identity to Arabidopsis thaliana sequences and a putative function was assigned to 15 of them, including the auxin-responsive GH3, ARF-like small GTPases/ARFs, ARD/ARD', APS reductase, Nop, catalase, zinc finger (C3HC4-type RING finger), diacylglycerol kinase, and haloacid dehalogenase-like hydrolase families. Three cDNAs corresponded to predicted membrane proteins (KOG3491) or a ribosome-associated membrane protein RAMP4. One other clone, DD26, did not show significant identities to any translated sequence in the GenBank database, suggesting it may either encode unidentified proteins, or correspond to un-translated, non-conserved regions of mRNA molecules. These Cd-responsive up-regulated genes are mostly also regulated by abiotic or biotic stresses, e.g., dehydration, chilling, high salt, auxin, heat and infection, in other plants. The present study leads to an increased understanding of genes and/or the biochemical pathways involved in heavy-metal resistance and accumulation in plants.