The homeodomain of the Raphanus sativus WOX4 binds to the promoter of the LOG3 cytokinin biosynthesis gene

Abstract

Background
 The WOX4 transcription factor plays a crucial role in maintaining the organisation of cambium meristem during secondary growth, but its direct targets are unknown. The objectives of our work were to study the effect of WOX4 overexpression on the root development and gene expression in radish (Raphanus sativus L.), a root crop related to Arabidopsis thaliana, and to search for direct targets of the WOX4 in radish.
 Materials and methods 
 Radish line 19 of the St. Petersburg State University radish genetic collection was used. Plants were grown on Murashige-Skoog medium and then in soil at 23оС and 16 h of daylight. Total DNA was extracted from radish seedlings using the CTAB method. The PCR-amplified full-length RsWOX4-2 gene, gene fragments or homeobox sequence were cloned into the vectors for overexpression (pB7WG2D), RNA interference (pH7GWIWG2) and yeast one-hybrid assay (pDEST22), respectively, using the Gateway system. The vectors for overexpression and RNA interference of RsWOX4-2 were transformed into Escherichia coli DH10B and then into Agrobacteium rhizogenes Arqua chemically competent cells. Radish seedlings were transformed transformation with A. rhizogenes containing vectors for overexpression and RNA interference of RsWOX4-2, and GUS-overexpressing A. rhizogenes was used as a control. Total RNA from transgenic radish roots was extracted with Trizol reagent. RNA reverse transcription was performed using dT-18 primers and RevertAid reverse transcriptase. qPCR was performed using the Eva Green reagent kit on a CFX96 thermocycler with fluorescence detection system. Results were processed using the 2-ΔΔCT method.
 Yeast transformation with competent Saccharomyces cerevisiae of Y2H Gold strain cells and yeast one-hybrid assay were performed as described in the article. The obtained yeast colonies transformed with plasmids containing TF homeodomain sequence and promoter regions of genes were grown on DDO and TDO selective media with different concentrations of 3-amino-1,2,4-triazole.
 Statistical processing based on Student's t-test and graphing were performed using the ggplot2 package for the R programming language (v.4.0.2).
 Results
 Overexpression of the RsWOX4-2 gene affects the structure of the radish root stele and alters the number of vessels and cambium cells. Overexpression and RNA interference of the RsWOX4-2 causes changes in the expression levels of putative target genes with the WOX family transcription factor conserved binding sites in their promoters. Using the yeast one-hybrid assay, we have shown that the DNA-binding homeodomain of RsWOX4-2 interacts with the TAATCC site in the promoter of the RsLOG3 gene, which encodes the enzyme for cytokinin biosynthesis.
 Conclusion
 We have demonstrated the effect of RsWOX4-2 overexpression on radish root stele and gene expression and identified the RsLOG3 as the putative direct target of the WOX4 transcription factor in radish.