Different Roles of a Transcription Factor Gene RDD2 with Close Sequence Similarity to RDD1 Controlling Nutrient Ion Accumulation in Rice

Gene families are groups of genes that share similar nucleotide or amino acid sequences, and these similarities are used to identify genes with functions that are similar to the gene of interest within same family but also have additional specific properties. We investigated expression patterns and functions of RDD2 , which has the highest sequence similarity to the transcription factor gene RDD1 controlling nutrient ion accumulation among rice transcription factor genes. RDD2 expression was induced by light, and its transcript levels in leaf blades were higher than those in leaf sheaths, although RDD2 protein levels in leaf sheaths were higher than in leaf blades. RDD2 proteins were localized to vascular bundles, whereas RDD1 proteins accumulated at high levels in vascular bundles and in vascular bundle sheaths. RDD2 -overexpressing ( RDD2 -OX) plants showed no additional accumulation of the essential nutrient NH 4 + under low-nutrient conditions. Comparisons of nutrient ion contents between leaf blades and leaf sheaths indicated that distributions of NH 4 + and SO 4 2− in leaf sheaths were promoted in RDD2 -OX plants. Subsequent microarray analyses showed that numbers of genes with changed expression levels in RDD2 -OX plants were greater in leaf sheaths than in leaf blades. The effects of overexpression on grain productivity were different between RDD2 -OX and RDD1 -overexpressing plants. These results suggest that the roles of RDD2 differ from those of RDD1 , indicating that the functions and expression patterns of RDD1 and RDD2 differentiated after divergence from a common ancestral gene.