REORIENTATION OF CORTICAL MICROTUBULES IN THE SUB-APICAL REGION DURING TUBERIZATION IN SINGLE-NODE STEM SEGMENTS OF POTATO IN CULTURE

The initial events in tuberization were examined in single-node stem s egments of potato, in which the tuberization was easily regulated in c ulture. The addition of 8% sucrose to the culture medium caused the ce ssation of elongation of lateral shoots and the swelling of the sub-ap ical region of each shoot. Swelling was first induced by lateral cell expansion, which was followed by periclinal cell division. The divided cells then expanded laterally. The alteration in the direction of gro wth was accompanied by the reorientation of arrays of cortical microtu bules (MTs), which was monitored by immunofluorescence microscopy. Cel ls in the sub-apical region of elongating shoots had prominent transve rse arrays of MTs. The MTs in swelling cells were oriented longitudina lly with respect to the axis of the shoot. Finally, the arrays of MTs became completely disorganized. By contrast, the elongation of lateral shoots continued in GA(3)-treated segments and the cells in the sub-a pical region of such shoots retained conspicuous transverse arrays of MTs during culture, even in the presence of a high concentration (8%) of sucrose.