The inhibition of ammonium uptake in excised birch (Betula pendula) roots by batatasin-III

In northern Sweden, plants growing in association with the clonal dwarf shr ub Empetrum hermaphroditum usually exhibit limited growth and are N-deplete d. Previous studies suggest that this negative effect by E. hermaphroditum may be explained, at least in part, by the release of phenolic compounds, p articularly the dihydrostilbene, batatasin-III from foliage to soil. In the present work, we investigated whether batatasin-III has the potential to i nterfere with NH4+ uptake in birch (Betula pendula) roots. Excised birch ro ots were exposed to batatasin-III during brief periods in (NH4+)-N-15 solut ions, and then analyzed for labeled N. Batatasin-III inhibited N-NH4+ uptak e by 28, 89 and 95% compared with the control, when roots were treated with 0.1, 1.0 and 2.8 mM of batatasin-III, respectively. The effect of 1.0-mM b atatasin-III was greater at pH 4.2 than at pH 6.8. In addition, the inhibit ion of N-NH4+ uptake by batatasin-III was not reversed after rinsing the ro ots in water and transferring them to a batatasin-III free solution. Furthe rmore, birch seedlings immersed in a 1.0-mM batatasin-III solution for 2 h, and then replanted in pots with soil, had decreased growth, such that 10 w eeks after treatment, the dry mass of both shoots and roots was reduced by 74 and 73%, respectively, compared with control seedlings. This suggests th at a brief exposure to batatasin-III may have a long-term inhibitory effect on whole plant growth. Using plasma membrane vesicles isolated from easily extractable spinach (Spinacia oleracea) leaves, it was found that batatasi n-III strongly inhibited proton pumping in isolated plasma membrane vesicle s, while it only slightly inhibited ATP hydrolytic activity. The uncoupling of proton pumping from ATP hydrolytic activity suggests that batatusin-III disturbs membrane integrity. This hypothesis was further supported by a gr eater efflux of ions from birch roots immersed in a batatasin-III solution than from roots in a control solution.