Enhanced Expression of Vacuolar H+-ATPase Subunit E in the Roots Is Associated with the Adaptation of Broussonetia papyrifera to Salt Stress

Vacuolar H+-ATPase (V-H+-ATPase) may play a pivotal role in maintenance of ion homeostasis inside plant cells. In the present study, the expression of V-H+-ATPase genes was analyzed in the roots and leaves of a woody plant, Broussonetia papyrifera, which was stressed with 50, 100 and 150 mM NaCl. Moreover, the expression and distribution of the subunit E protein were investigated by Western blot and immunocytochemistry. These showed that treatment of B. papyrifera with NaCl distinctly changed the hydrolytic activity of V-H+-ATPase in the roots and leaves. Salinity induced a dramatic increase in V-H+-ATPase hydrolytic activity in the roots. However, only slight changes in V-H+-ATPase hydrolytic activity were observed in the leaves. In contrast, increased H+ pumping activity of V-H+-ATPase was observed in both the roots and leaves. In addition, NaCl treatment led to an increase in H+-pyrophosphatase (V-H+-PPase) activity in the roots. Moreover, NaCl treatment triggered the enhancement of mRNA levels for subunits A, E and c of V-H+-ATPase in the roots, whereas only subunit c mRNA was observed to increase in the leaves. By Western blot and immunocytological analysis, subunit E was shown to be augmented in response to salinity stress in the roots. These findings provide evidence that under salt stress, increased V-H+-ATPase activity in the roots was positively correlated with higher transcript and protein levels of V-H+-ATPase subunit E. Altogether, our results suggest an essential role for V-H+-ATPase subunit E in the response of plants to salinity stress.