Ion uptake from the xylem into the symplasm of the maize leaf

Abstract

Whereas the uptake of ions into the xylem roots is well understood, less is known about the uptake from the xylem to the symplast of the leaves. LAMMA experiments using 85Rb+ as a tracer showed that ions move over the cell membrane between xylem and bundlesheath cells of small veins of Zea maize. A new isolation technique was developed that enabled the separation of mesophyll cells and bundle-sheath cells of maize leaves. Whole-cell patch-clamp experiments revealed three types of channel, MB-1 and MB-2 in bundle-sheath cells and MM-2 in mesophyll cells. Stimulation by acidic lumenal medium suggested that MB-1 and MB-2 take care of charge balancing during uptake of anions, whereas stimulation at high pH would provide MM-2 with a fail-safe function by clamping membrane potential to EK, if the H+ pump is inactive. The distinction between MB-1 and MB-2 was based on current density, temporal behaviour and different conductivity with Rb+ at 100 mM KCl at the lumenal side. MB-2 requires ATP for activation. Freshly prepared bundle-sheath cells are either of MB-1- or of MB-2-type. The isolation procedure developed for the patch clamp experiments was employed to separate c-DNA from vascular strands from that of mesophyll cells and of epidermis cells. Physiological behaviour, especially pH-dependence, indicated that KZM1 found in the vascular strands originated from the phloem. Thus the genetic identification of MB-1 and MB-2 is still an open question.