Cucumber PDR8/ABCG36 and PDR12/ABCG40 plasma membrane proteins and their up-regulation under abiotic stresses

The cucumber genes CsPDR8/CsABCG36 and CsPDR12/CsABCG40 encode two similar pleiotropic drug resistance proteins (ABCGPDRₛ) belonging to the large ABC family of multispecific ATP-dependent transporters. We have already shown that the amount of root CsPDR8/CsABCG36 and CsPDR12/CsABCG40 transcripts is markedly elevated by phytohormones related to the plant response to environmental constraints, suggesting the involvement of both genes in hormone-mediated reactions to stresses. To further characterize the function and regulation of CsPDR8/CsABCG36 and CsPDR12/CsABCG40, we determined the subcellular localization of the predicted CsPDR8/CsABCG36 and CsPDR12/CsABCG40 proteins in cucumber and performed a transcriptional analysis of genes encoding these proteins under different abiotic stresses (heavy metals, salinity, osmotic stress, and oxidative stress) and redox perturbations. In addition, the activities of antioxidative enzymes as well as the content of hydrogen peroxide and superoxide were measured in cucumber roots to monitor the redox perturbations under all experimental conditions. Western blot analysis of membrane fractions prepared from cucumber roots with specific antibodies raised against the peptides corresponding to sequences unique to CsPDR8/CsABCG36 and CsPDR12/CsABCG40 revealed that both proteins localize to the plasma membrane. The transcript abundance and the plasma membrane protein content closely correlated with the stress severity and the hydrogen peroxide content but not with the superoxide anion content. Based on the results obtained so far, we may conclude that CsPDR8/CsABCG36 and CsPDR12/CsABCG40 are up-regulated under multiple stress conditions and redox perturbations and that the H₂O₂ and stress-related phytohormones can act as signaling molecules affecting the expression of both cucumber genes.