Changes in the microtubular cytoskeleton precede in vitro tuber formation in potato

An in vitro system for tuber formation was used to study early morphological and cytological changes occurring during tuber formation in potatoes, with special emphasis on the orientation of the microtubular cytoskeleton, 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 (GA added). Tuber formation in the absence of GA was highly synchronous in individual buds, enabling the dissection of various aspects of tuberization. Under both conditions, starch started to accumulate. In the absence of GA, starch levels rapidly increased, concomitantly with tuber formation, whereas it slightly decreased in the presence of GA. Up to 4 days, the cortical MTs in the cells were oriented perpendicular to the longitudinal axis of the developing buds. Under tuber-inducing conditions this orientation changed into a longitudinal one at day 5. This change preceded a change in the direction of cell expansion. In the presence of GA no such reorientation was observed, cells continued to grow longitudinally, and a stoloniferous shoot was formed. The cytoskeletal changes preceded the visible swelling of the buds, observed after day 5, demonstrating that the reorientation of the microtubular cytoskeleton is one of the earliest steps observed so far in tuber formation in potatoes.