Abstract: Rhododendron is a significant plant genus, with over 600 identified species in China. The subgenus Hymenanthes holds the largest number of Rhododendron germplasms and showcases strong environmental adaptability. However, there remains a lack of understanding regarding Rhododendron’s evolution and environmental adaptations. R. bailiense, an exceedingly rare species, thrives in the alkaline karst landforms of Guizhou, southwest China, different to the typical growing environment of other Rhododendrons. In this study, we present a chromosome-level genome assembly of R. bailiense, revealing a genome size of 923.3 Mb, a contig N50 of 24.5 Mb, and a total of 47,567 predicted genes. An evolutionary analysis indicated that R. bailiense diverged from its ancestors prior to other subgenus Hymenanthes Rhododendrons, with the expanded and contracted genes being notably enriched in 'stress response' and 'growth', respectively. R. bailiense is predominantly found on limestone soil in the mountains of Guizhou, with only two wild populations known. The genome of R. bailiense contained a high copy number of ankyrin repeats (ANKs) and Ca²⁺-ATPases (CAPs) genes, primarily involved in Ca²⁺ transport, shedding light on how R. bailiense copes with karst high-calcium stress. In contrast, the structures of the ANKs displayed unique characteristics, while the CAPs exhibited conservatism. The R. bailiense genome provides new insights into the adaptation and evolutionary history of Rhododendron plants in karst environments, proving valuable for adaptive breeding and ecological enhancement in such challenging settings.

Key words:

Rhododendron bailiense,  karst, adaptive evolution, ankyrin repeats protein, Ca²⁺-ATPase.