Class 1 KNOX Gene Expression Supports the Selaginella Rhizophore Concept

The spikemoss is marked by the unique root-producing pleurogeous rhizophore as well as the lycophytic microphyll. Imaichi and Kato (Bot Mag Tokyo 102:369–380, 1989; Am J Bot 78:1694–1703, 1991) revealed that the exogenous developmental process in the rhizophore is clearly distinguishable from the developmental process in the endogenous root, argued that the axial organ could be coordinate with other fundamental organs including the root and stem, and demonstrated the “rhizophore concept.” In this paper, we report on the expression pattern of the spikemoss Selaginella class 1 KNOX gene, SuKNOX1, in the rhizophore. We show that the SuKNOX1 mRNA is specifically accumulated at the tip of the rhizophore as well as the shoot apical apex, but not in the root tip. This result supports the “rhizophore concept” at the molecular level.