Composite Cucurbitaceae Plants as a Tool to Study Physiological and Molecular Mechanisms of Root Development

Composite plants are a convenient model for obtaining a whole plant with undisturbed regulatory links between the shoot and root system, while the roots carry the properties specified by the researcher. This approach allows expressing transgenes only in roots, which in some cases may be preferable over the expression of transgenes in all organs of a non-chimeric transgenic plant. Composite plants have been widely used to study the nitrogen fixation process, mycorrhizal symbioses, and the interaction of plant roots with various pathogens. The current protocol for Agrobacterium rhizogenes-mediated transformation of Cucurbitaceae species, cucumber (Cucumis sativus) and squash (Cucurbita pepo), allows obtaining multiple transgenic roots within 4 weeks. These transgenic roots can contain different modifications such as promoter-reporter fusions, altered gene expression levels, and CRISPR/Cas9-edited genomes. Additionally, different selectable and visually screenable markers of transgenicity, as well as the potential cross-activation between promoters in vectors containing multiple expression cassettes, are described.