Cloning of a putative extracellular Cu/Zn superoxide dismutase and functional differences of superoxide dismutases in invasive and indigenous whiteflies.

Superoxide dismutases (SODs) are a group of important antioxidant defense enzymes. In this study， a putative extracellular Cu/Zn superoxide dismutase (ecCuZnSOD) complementary DNA was cloned and characterized from the whitefly， Bemisia tabaci. Quantitative polymerase chain reaction analysis showed that the expression level of BtecCuZnSOD was more than 10-fold higher in the invasive Middle East Asia Minor 1 (MEAM1) than in the native Asia II 3 species of the B. tabaci species complex. After exposure to low temperature (4 °C)， the expression of Bt-ecCuZnSOD gene was significantly up-regulated in MEAM1 but not in Asia II 3. Furthermore， the expression level of B. tabaci intracellular CuZnSOD (Bt-icCuZnSOD)， Bt-ecCuZnSOD and mitochondrial MnSOD (Bt-mMnSOD) was compared after transferring MEAM1 and Asia II 3 whiteflies from favorable (cotton) to unfavorable host plants (tobacco). On cotton， both CuZnSOD genes were expressed at a higher level in MEAM1 compared with Asia II 3. Interestingly， after transferring onto tobacco， the expression of Bt-ecCuZnSOD was significantly induced in Asia II 3 but not in MEAM1. On the other hand， while Bt-mMnSOD was expressed equally in both species on cotton， Bt-mMnSOD messenger RNA was up-regulated in MEAM1 on tobacco. Consistently， enzymatic activity assays of CuZnSOD and MnSOD demonstrated that CuZnSOD might play an important protective role against oxidative stress in Asia II 3， whereas MnSOD activation was critical for MEAM1 whiteflies during host adaptation. Taken together， our results suggest that the successful invasion of MEAM1 is correlated with its constitutive high activity of CuZnSOD and inducible expression of MnSOD under stress conditions.