Simulated weightlessness and hyper-g results in opposite effects on the regeneration of the cortical microtubule array in protoplasts from Brassica napus hypocotyls

Enzymatic digestion of the cell wall of Brassica napus hypocotyls gave a heterogeneous suspension of protoplasts with the cortical microtubules (CMTs) randomly organised or CMTs organised in parallel. The effect of variable g- influences has been tested on CMT organisation. In contrast to the 1 g- protoplasts, which reorganised the CMTs into parallel arrays during the 96 h test period, the frequency of randomly-oriented CMTs in the protoplasts exposed to simulated weightlessness (0 g ) on a 2-D clinostat increased significantly during the same period. The opposite effect was obtained when the protoplasts were exposed to hyper -g (7 or 10 g ), where the reorganisation of the CMTs into parallel arrays was accelerated compared to the 1 and 0 g- protoplasts. These results indicate that a unidirectional gravity force is a necessity for the reorganisation of CMTs in protoplasts to parallel arrays and that CMTs act as responding elements that are able to sense different levels of gravity. Besides the inability of the protoplasts to reorganise the CMTs into parallel arrays, the quantity of CMTs in the individual protoplast decreased during 4 days of simulated weightlessness, both compared to the CMTs quantity in the protoplasts immediately after isolation and compared to the 1 g- and hyper -g- protoplasts after 24 and 48 h of g- exposure. The size of the protoplasts was also affected by the g- exposure. Protoplasts exposed to simulated 0 g increased significantly after 24 and 48 h, whereas the 1 g- and 10 g- protoplasts maintained the same size during the 48 h test period.