The WSD-type wax ester synthase is widely conserved in streptophytes and crucial for floral organ formation under high humidity in land plants.

Cuticular waxes are a complex mixture of long-chain aliphatic compounds, including alkanes, aldehydes, alcohols, ketones, and esters, that cover the outer surface of most terrestrial plants. While cuticular waxes play a pivotal role in plant adaptation to various environmental stresses, the specific roles of individual molecular species, particularly wax esters (WEs), remain poorly understood. In this study, we investigate the evolution and function of wax ester biosynthesis in land plants, focusing on the WSD (wax synthase/DGAT) enzyme family. We demonstrate that the ancestral origin of WSD enzymes traces back to streptophyte algae, specifically Klebsormidium nitens (KnWSD1). Our findings provide evidence that KnWSD1 functions as a monofunctional WSD catalyzing WE formation without producing triacylglycerols (TAGs). By generating Arabidopsis mutants with multiple WSD gene knockouts, we uncover a crucial role for WEs in supporting floral organ development under high humidity. Our results reveal that WEs are essential for floral organ development and provide new insights into their evolutionary significance in plant adaptation to terrestrial environments.