Aquaporins Responded To Water Stress In Rice

Drought is a serious problem for agriculture around the world, and animportant factor holding back agricultural and economic development. Droughtresistance in plant has become to immediate problem for agriculture study. Toexplore the mechanism of drought resistance, this research is mainly focused onthe different physiological response and expression pattern analysis on plasmamembrane intrinsic protein between upland rice and lowland rice. Furthermore,we transformed plasma membrane intrinsic protein in rice, and consequentlysome physiological parameters were analyzed. The main results are as follows:(1)The physiological mechanisms of drought resistance were studied bycomparing physiological response between upland rice (Zhonghan 3) andlowland rice (Xiushui 63) during the water deficit mediated by 20% PEG. Weobserved young leaf rolling and the remarkable decline of water content andrelative cumulative transpiration in Zhonghan 3 under PEG treatment. Mercurialcompounds are widely used to study the function of aquaporins because they areinhibitors for aquaporins. Mercury treatment greatly decreased the relativecumulative transpiration of non-stressed plants in both cultivars. However,mercury showed more serious inhibition effects on Zhonghan 3. Zhonghan 3 andXiushui 63 had similar drop of leaf osmotic potential, while Xiushui 63 hadlower root osmotic potential under water deficit. Xiushui 63 probably showedhigher osmotic adjustment because Xiushui 63 had greater drop of root osmoticpotential. We also observed that PEG treatment obviously induced prolinesynthesis in leaves of Zhonghan 3,while they were unchanged in Xiushui 63. Wepostulated proline were involved in the leaf osmotic adjustment of Zhonghan 3under water deficit, root osmotic adjustment of Zhonghan 3 and Xiushui 63possibly accompany through other pathways. Abscisic acid (ABA) accumulationin stressed plants was found to be protective against drought damage, causingstomata closure that reduces water loss via transpiration and increasing hydraulicconductance, promoting water movement from roots to leaves. ABA contentmarkedly increased in leaves and roots of Zhonghan 3, but not in Xiushui63.These results suggested that upland rice and lowland rice mobilized distinctmechanism of drought resistance, i.e. upland rice was drought avoidance(increasing water uptake and decreasing water evaporation) while lowland ricewas drought tolerance (osmotic adjustment). Aquaporins and ABA might playroles in drought avoidance of upland rice.(2)To explore the role of aquaporin and ABA between upland rice and lowlandrice under water deficit, we investigated the PIP expression of upland rice(Zhonghan 3) and lowland rice (Xiushui 63) treated by PEG and ABA,respectively. In protein level, the leaf and root PIPs content of upland rice washigher than that of lowland rice. PIP protein level increased markedly in roots ofboth cultivars but only in leaves of upland rice after 10 h of PEG treatment. Atthe mRNA level, OsPIP1;2, and OsPIP2;5 in roots as well as OsPIP1;2 andOsPIP1;3 in leaves were significantly up-regulated in upland rice whereas thecorresponding genes remained unchanged or down-regulation in lowland rice.Exogenous ABA also induced the up-regulation of OsPIP1;2 and OsPIP2;5 inroot of upland rice. We postulated up-regulation of OsPIP1;2 and OsPIP2;5 inupland rice under water deficit might be ABA-dependent, while up-regulationof OsPIP1;3 and OsPIP2;1 might be ABA-independent.(3)To further understand the role of aquaporins in drought resistance of rice,aquaporins OsPIP1;1 and OsPIP1;3 were transformed respectively into ricecultivars (Zhonghua 11), and consequently some physiological parameters weremeasured. As rice is monocot plant, UBI-promoter was used, which was isolatedin maize, was constitutive strong promoter. UBI::OsPIP1;1 and UBI::OsPIP1;3were transferred into rice mediated by Agrobacterium tumefaciens. By theassays of PCR, the presence of UBI::OsPIP1;1 and UBI::OsPIP1;3 wereconfirmed in the transformed rice. By real time RT-PCR analysis, it was foundthat UBI::OsPIP1;1 and UBI::OsPIP1;3 transformed rice had higher OsPIP1;1and OsPIP1;3 expression levels than control plants, respectively.Over-expression of OsPIP1;1 and OsPIP1;3 accelerated seed germination , plantgrowth rate and improved net photosynthetic rate, stomatal conductanceand .transpiration rate under normal condition. However, over-expression ofOsPIP1;1 and OsPIP1;3 caused faster leaf rolling and lower drought tolerantthan control plants under the PEG treatment.