Response of Microtubule Arrays and Ca2+ Signal Transduction in Sugarcane Cultivars of Different Tolerance to Cold Stress.

Microtubules are an important component of cells and their rearrangement is closely related to Ca²⁺ concentration of cell signal transduction under cold stress.  An investigation was conducted in two sugarcane cultivars, GT28 (cold-tolerance) and YL6 (cold-susceptible) to better understand the relationship between the cytoskeleton and signaling of cold stress. Two different temperature regimes were used (0°C, cold stress and 25°C, control) and the plants at early ripening stage were exposed to cold-stress for 6 days. The results showed: (1) before cold stress, the cortical microtubule arrays were mainly oriented transversely to the elongation axis and orderly structured in the cells for both sugarcane cultivars, though the microtubule arrays were more densely structured in GT28 than in YL6. After cold stress, the microtubules in YL6 were disarrayed due to depolymerization and condensation, and the cell boundary became indistinct, while the cortical microtubule arrays in GT28 were re-ordered, re-oriented in all directions after depolymerization, and thus became more densely  arranged; (2) both sugarcane cultivars accumulated Ca²⁺ pellets in cytoplasm and nucleolus at the beginning of cold stress, but Ca²⁺ pellets in GT28 decreased and maintained at a low level while they were maintained at a high level in YL6 with the extension of cold stress. (3) Cold stress had a minimal influence on Ca²⁺-ATPase activity and its distribution in GT28, which was maintained at a high level. On the contrary, Ca²⁺-ATPase activity in YL6 was reduced with extended cold stress. It suggests that a high level of Ca²⁺ in cytoplasm is one of the important reasons for cell injury from cold for the cold sensitive cultivars, while high Ca²⁺-ATPase activity mitigates such effect by reducing Ca²⁺ accumulation in cytoplasm, thus rendering the microtubule arrays more acclimatized to cold stress for the cold tolerant cultivars of sugarcane.