Integrated Systems-Level Proteomics and Metabolomics Reveals the Diel Molecular Landscape of Diverse Kale Cultivars

The diverse cultivars of Brassica oleracea and Brassica napus constitute an important global food source. Of those, the Brassica oleracea convar. acephala cultivar group containing var. sabellica and var. palmifolia and Brassica napus var. pabularia, collectively known as kale, are nutritious leafy greens consumed for their abundance of vitamins and micronutrients. Typified by their curly, serrated or wavy leaves, kale varieties have been primarily defined based on their leaf morphology and geographic origin, despite having complex genetic backgrounds. Kale is a very promising crop for vertical farming due to its high nutritional content. However, systematic analysis of kale growth and molecular composition under different light regimes suitable for vertical farming has been lacking. Hence, we selected nine diverse, publicly available and commercially grown kale cultivars for growth under light emitting diode (LED)-replicated sunlight-like conditions. We then analyzed their growth and nutrition using a combination of phenomics, proteomics and metabolomics. As plant growth and development are driven by the diel molecular activities of plants, we harvested kale leaf tissue at both end-of-day (ED) and end-of-night (EN) time-points for all molecular analyses. Our results reveal that diel metabolome and proteome signatures sub-divide the selected kale cultivars into two distinct groups, defined by their amino acid and sugar content, along with significant proteome differences involving carbon and nitrogen metabolism, mRNA splicing, protein translation and light harvesting. Together, our analysis has generated robust quantitative insights into the diel growth and development landscape of kale, significantly advancing our fundamental understanding of this nutritious leafy green for next-generation breeding and biotechnology.