In this study, we investigated the expression and functions of the transcription factor gene RDD4 (rice Dof daily fluctuations 4), which has sequence similarity to RDD1 that controls nutrient ion accumulation in rice. RDD4 protein was highly accumulated in leaf sheaths and localized to vascular bundles. RDD4-overexpressing plants (RDD4-OX) improved the accumulation of various nutrient ions, irrespective of nutrient concentration in a hydroponic solution. K+ and Cl- deficiencies induced the accumulation of other cations and anions, respectively. Interestingly, in RDD4-OX plants K+ and Cl- deficiencies increased PO4 3- and Mg2+ contents, respectively, despite opposite electric charges. Furthermore, PO4 3- deficiency induced NO3 - and Mg2+ accumulation in RDD4-OX plants. These data show that RDD4 is associated with the control of nutrient ion contents within plants. Also, photosynthetic CO2 assimilation in RDD4-OX plants was higher than in wild-type (WT) plants, although the sizes of shoots and panicles decreased in RDD4-OX plants. Subsequent microarray analysis indicated that OsFWL7, similar to maize CNR1 that negatively regulates plant size, showed the most significant difference in its expression levels between WT and RDD4-OX plants. Based on these results, it is hypothesized that a prominent increase in the OsFWL7 expression reduces plant size in RDD4-OX plants.
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