Aquaporins play a significant role in plant water relations.To further understand the aquaporin function in plants underwater stress,the expression of a subgroup of aquaporins,plasma membrane intrinsic proteins(PIPs),was studied at boththe protein and mRNA level in upland rice(Oryza sativa L.cv.Zhonghan 3)and lowland rice(Oryza sativa L.cv.Xiushui63)when they were water stressed by treatment with 20% polyethylene glycol(PEG).Plants responded differently to20% PEG treatment.Leaf water content of upland rice leaves was reduced rapidly.PIP protein level increased markedlyin roots of both types,but only in leaves of upland rice after 10h of PEG treatment.At the mRNA level,OsPIP1;2,Os-PIP1;3,OsPIP2;1 and OsPIP2;5 in roots as well as OsPIP1;2 and OsPIP1;3 in leaves were significantly up-regulatedin upland rice,whereas the corresponding genes remained unchanged or down-regulated in lowland rice.Meanwhile,weobserved a significant increase in the endogenous abscisic acid(ABA)level in upland rice but not in lowland rice underwater deficit.Treatment with 60μM ABA enhanced the expression of OsPIP1;2,OsPIP2;5 and OsPIP2;6 in roots andOsPIP1;2,OsPIP2;4 and OsPIP2;6 in leaves of upland rice.The responsiveness of PIP genes to water stress and ABAwere different,implying that the regulation of PIP genes involves both ABA-dependent and ABA-independent signalingpathways during water deficit. Aquaporins play a significant role in plant water relations. The expression of a subgroup of aquaporins, plasma membrane intrinsic proteins (PIPs), was studied at both the protein and mRNA level in upland rice (Oryza sativa L.cv.Zhonghan 3) and lowland rice (Oryza sativa L.cv.Xiushui63) when they were water stressed by treatment with 20% polyethylene glycol (PEG) .Plants modulated differently to 20% PEG treatment. Leaf water content of upland rice leaves was reduced rapidly. PIP protein level increased markedly roots of both types, but only in leaves of upland rice after 10h of PEG treatment. At the mRNA level, OsPIP1; 2, Os-PIP1; 3, OsPIP2; 1 and OsPIP2; 5 in roots as well as OsPIP1; 2 and OsPIP1; 3 in leaves were significantly up-regulated in upland rice, whereas the corresponding genes were unchanged unchanged or down-regulated in lowland rice.Meanwhile, weobserved a significant increase in the endogenous abscisic acid (ABA) level in upland ric e but not in lowland rice underwater deficit. Treatment with 60 μM ABA enhanced the expression of OsPIP1; 2, OsPIP2; 5 and OsPIP2; 6 in roots and OsPIP1; 2, OsPIP2; 4 and OsPIP2; 6 in leaves of upland rice. PIP genes to water stress and ABA were different, implying that the regulation of PIP genes involves both ABA-dependent and ABA-independent signalingpathways during water deficit.