Rpp16 and Rpp17, from a common origin, have different protein characteristics but both genes are predominantly expressed in rice phloem tissues

The genes for two types of rice phloem protein (RPP16 and RPP17) were isolated and characterized. Conservation of five exon sizes as well as splicing positions between the two genes suggest that either RPP16 or RPP17 is a resultant of gene duplication. By protein blot analysis, RPP16 and RPP17 proteins were specifically detected in soluble and insoluble fractions of a crude extract of rice plants, respectively, suggesting that these proteins play different roles in individual cells. The expression of Rpp16 and Rpp17 was monitored by the P-glucuronidase (gusA) reporter-gene method. Rpp16-gusA and Rpp17-gusA were expressed preferentially in the phloem tissues from different parts of the plant, but almost no GUS staining was observed in the rest of the tissues. In roots of both constructs, interestingly, stronger GUS-accumulation was detected in younger vascular tissues than in aged vascular tissues. In situ hybridization also showed that Rpp17 was more strongly expressed in vascular tissues of tiller buds. These results suggest that transcript of these genes was more abundant in young tissues. The presence of two copies of the gene in higher plants, from a common origin, which have different protein characteristics, indicates that evolutionary diversification might have occurred in the gene function.