Cytokinin biosynthesis ISOPENTENYLTRANSFERASE genes are differentially expressed during phyllomorph development in the acaulescent Streptocarpus rexii (Gesneriaceae)

The enzyme ISOPENTENYLTRANSFERASE (IPT) is responsible for the rate limiting step of cytokinin biosynthesis, an important plant hormone with key roles in meristem maintenance and organ development. In this study, we isolated IPT genes from the acaulescent Streptocarpus rexii, a plant that shows an unorthodox development starting with post-germination anisocotyly, in which cytokinins play an integral role. Three adenosine phosphate-IPTs and two tRNA-IPTs were isolated from S. rexii. Their expression levels and patterns in different tissues were compared by means of realtime-PCR and mRNA in-situ hybridization. We found that each SrIPT had a distinctive expression pattern. Interestingly, in vegetative tissues as well as in meristems only the adenosine phosphate-IPT SrIPT5 and the tRNA-IPT SrIPT9 were found. In addition, they were differentially affected by external hormone application, suggesting their different regulation and expression during meristem formation and maintenance and lamina growth. Our results indicate that SrIPTs are involved in shaping the architecture of S. rexii, working differentially and redundantly, and show that differentially expressed IPT genes regulate plant form.