Rice aquaporins and their responses to environmental stress

Abstract

Rice (Oryza sativa) can adapt to a wide range of hydrological conditions, and control the balance of water uptake and transpiration under fluctuating environmental conditions. However, the mechanism underlaid remains obscure. Aquaporins (water channels) are membrane proteins which usually transport water. Rice has 33 aquaporin isoforms. Genes of many rice aquaporins were expressed in the roots where hydraulic conductance of the whole plants is regulated. Aquaporins were localized mainly in endodermis which controls water flow from root surface to the cylinder. Among these genes, the expression levels of genes for root-specific aquaporins such as OsPIP2;5 responded sensitively to the changes in light-dark cycle, temperature, humidity, drought, nitrogen availability around roots. In addition to these observations obtained from controlled environmental condition such as chambers, close correlation of the aquaporins to a meteorological factor (potential evaporation) was observed under field weather condition. The common response to several different environmental changes suggests that the transpirational demand is delivered from shoots to the roots and triggers root aquaporin gene expression through unknown mechanism. Thus, aquaporins may contribute to adaptation of the hydraulic conductance of whole plants through changes in the hydraulic conductance of the roots. On the contrary of roots, two aquaporin isoforms were abundantly expressed in rice grains and they were localized in tissue specific manner.