Expression profiles of organogenesis-related genes over the time course of one-step de novo shoot organogenesis from intact seedlings of kohlrabi

Kohlrabi (Brassica oleracea var. gongylodes) is an important vegetable crop that is able to undergo shoot regeneration in culture from intact seedlings in a single-step regeneration process, using cytokinin as the only plant growth regulator. In this work, we present the expression profiles of seven organogenesis-related genes over the time course of shoot regeneration from intact seedlings of kohlrabi cv. Vienna Purple on shoot regeneration media containing trans-zeatin, cis-zeatin, benzyl adenine or thidiazuron. Two auxin transporter genes - PIN3 and PIN4, a cytokinin response regulator - ARR5, two shoot apical meristem-related transcription factors - CUC1 and RGD3, and two cell cycle-related genes - CDKB2;1 and CYCB2;4 - displayed bimodal expression patterns on most cytokinin-containing media when their expression levels were normalized against control plants grown on hormone-free media. The first expression peak corresponded to direct upregulation by cytokinin from the growth media, and the second one reflected transcriptional events related to callus formation and/or acquisition of organogenic competence, corresponding to the shoot regeneration phases that have already been characterized in Arabidopsis thaliana. We demonstrate that the genes involved in the two-step shoot regeneration of Arabidopsis display their expected expression profiles during the single-step shoot regeneration of its close phylogenetic relative kohlrabi confirming the universality of their roles in the distinct phases of the regeneration process in Brassicaceae. The results presented here represent a first step towards genetic characterization of the morphogenetic processes in this important crop species.