CHANGES IN THE MICROTUBULAR CYTOSKELETON PRECEDE IN-VITRO TUBER FORMATION IN POTATO

An in vitro system for tuber formation was used to study early morphol ogical and cytological changes occurring during tuber formation in pot atoes, with special emphasis on the orientation of the microtubular cy toskeleton, visualized immunocytochemically. Axillary buds from potato plants were cultured in the presence or absence of gibberellin (GA), resulting in either tuber formation (without GA) or shoot formation (G A added). Tuber formation in the absence of GA was highly synchronous in individual buds, enabling the dissection of various aspects of tube rization. Under both conditions, starch started to accumulate. In the absence of GA, starch levels rapidly increased, concomitantly with tub er formation, whereas it slightly decreased in the presence of GA. Up to 4 days, the cortical MTs in the cells were oriented perpendicular t o the longitudinal axis of the developing buds. Under tuber-inducing c onditions this orientation changed into a longitudinal one at day 5. T his change preceded a change in the direction of cell expansion. In th e presence of GA no such reorientation was observed, cells continued t o grow longitudinally, and a stoloniferous shoot was formed. The cytos keletal changes preceded the visible swelling of the buds, observed af ter day 5, demonstrating that the reorientation of the microtubular cy toskeleton is one of the earliest steps observed so far in tuber forma tion in potatoes.