Cytokinin-Dependent Secondary Growth Determines Root Biomass in Radish (Raphanus sativus L.)

A root serves as an essential organ in plant growth by up-taking nutrients and water from soil and supporting the rest of a plant body. Root apical growth and system architecture have been extensively studied because they strongly affect overall plant growth and yields. Some plant species also utilize roots as storage organs. Many of them, including sweet potatoes (Ipomoea batatas), cassava (Manihot esculenta), and radish (Raphanus sativus) are important crops, however their root development has remained elusive. In this study, we characterized radial root growth in the radish and found that it is very similar to the secondary growth in stems. We identified well-established cambium zones in the actively growing radish roots. Cell proliferation activities in the cambium zones positively correlated with root growth rates and final yields. Through a comparative analysis with genome-wide Arabidopsis root expression data, we selected several transcription factors whose expression is highly enriched in the cambia and then validated their expression in various stages of radish roots. We also compared their expression in two inbred lines with distinctive radial root growth and found that the differences in the root growth of two inbred lines are from changes in cytokinin responses. Taken together, our study indicates that radish is an excellent system for studying root crops and that cytokinin-dependent secondary growth directly affects yields of radish roots.