Sugar starvation induces the central vacuolation with coordinated increase in expression of tonoplast intrinsic protein genes in suspension-cultured rice cells

An efficient monitoring of cytoplasmic vacuolation by fluorescein diacetate (FDA) staining of protoplasts revealed that the major populations of suspension-cultured rice cells undergo a rapid vacuolation upon glucosedepletion. As aquaporin-family proteins, tonoplast intrinsic proteins (TIPs) are known to play in regulating the water balance across the vacuolar membrane. Glucose starvation increased expression of every member of OsTIP family, leading to an enhancement of the total expression by up to 110-fold, which is well matched with an expansion of the vacuolar structure induced by starvation. OsTIPs 1;1, 2;2 and 3;1 are the three most prominently expressed OsTIPs in starved conditions due to their highest responsiveness to sugar deprivation. Feeding experiments with various sugars and glucose analogs indicated that sugar regulated expression of the three major OsTIPs is likely mediated by a hexokinasedependent pathway. Alleviation of sugar-induced suppression of OsTIP expression by co-treatment with the uncoupler of ATP synthesis suggests that sugar signaling for OsTIP regulation is also cross-talked by the energy-deficit conditions. Intriguingly, starvation-induced central vacuolation was effectively prevented by mannose and 2-deoxyglucose, but not by 3-O-methylglucose. These results imply that the hexokinase is able to trigger the signaling to suppress the central vacuolation, independently of fueling the energy metabolism.