NEW GENETIC INSIGHTS INTO THE CYTOKININ PATHWAY AS REVEALED BY THE CHARACTERIZATION OF CICHORIUM ENDIVIA LEAVES TRANSCRIPTOME AND METABOLITES ANALYSIS

Cytokinins (CK) are key hormones that control plant growth, leaf development, morphogenesis and senescence, nutrient allocation and stress responses, all crucial traits for crop productivity. We reconstructed the genetic pathway of cytokinins in the leafy crop Cichorium endivia and in the reference species for Asteraceae Lactuca sativa. Genes involved in cytokinin metabolism, perception and signaling, and candidates to nucleotide-nucleoside interconversion and transport were identified, classified and manually annotated. RNAseq expression profiles of edible leaves of four C. endivia commercial cultivars pointed to genes involved in cytokinin activation, O-glycosilation and response as the major source of variability among cultivars. Cytokinin metabolites analysis indicated cis-zeatin (cZ) derivatives as the most abundant forms of cytokinins in C. endivia leaves. Metabolite-transcript correlation identified candidate genes for cis- and trans-zeatin reversible O-glycosylation. Candidate genes involved in the two-step interconversion of cytokinin bases, nucleosides and nucleotides, which have remained elusive for a long time, were also found. Robust gene co-expression networks from RNAseq profiles of two production years validated our cytokinin pathway and revealed the intrinsic intra-modular structure of the CK pathway in leaves. Members of the TALE superclass of homeobox transcription factors were also identified as regulatory hubs that antagonize the AHK4/CRE1-driven CK co-expression network in C. endivia leaves.