Expression and functional analysis of rice plastidic maltose transporter, OsMEX1

Abstract

In Arabidopsis, maltose is a major product of the transitory starch degradation pathway at night, and its mobilization from the chloroplasts to the cytosol in leaf tissues via a plastidic maltose transporter, AtMEX1, is essential for normal plant growth. However, such a starch utilization pathway has not yet been characterized in rice (Oryza sativa), a monocot model plant. Examination of expression profiles of a rice plastidic maltose transporter, OsMEX1, by real-time polymerase chain reaction showed that it is abundant in the pollen grain-containing stamens of mature flowers. Consistently, high performance liquid chromatography analysis revealed a relatively high maltose content in mature flowers, suggesting that OsMEX1 mainly functions in the tissues. OsMEX1-green fluorescent protein fusion experiment confirmed that OsMEX1 localizes at the chloroplast envelope in both rice and Arabidopsis. Arabidopsis maltose excess1 (mex1) mutant was transformed with OsMEX1 fused to the cauliflower mosaic virus 35S (CaMV35S) promoter. In the resulting transgenic plants, the typical mutant phenotypes of Arabidopsis mex1, such as chlorosis, stunted growth, and maltose and starch deposition at the end of the night, are clearly rescued. This result demonstrates that OsMEX1 functions as a plastidic maltose transporter in Arabidopsis. Our present findings thus suggest that whereas the Arabidopsis MEX1 gene essentially functions in source leaf tissues, its rice counterpart likely has a role in the pollens of mature flowers.