Evolution and diversification of Ca2+/H+ exchangers: insights into karst-plant adaption to high-calcium environment.

Background: Calcium (Ca²⁺) is a critical mineral essential for plant growth, development, and signal transduction. The CAX (Ca²⁺/H⁺ exchanger) family represents a group of key transmembrane transporters that mediate Ca²⁺ transport and homeostasis, thereby playing a central role in regulating various physiological processes. Although previous studies have addressed the functional characterization of certain CAX proteins, a comprehensive understanding of their evolutionary origins and trajectory remains limited.

Results: We identified 216 CAX proteins from 49 green plant species and performed in-depth phylogenetic and expression analyses. Our phylogenetic results suggest that CAX proteins originated early in plant evolution, with a presence detectable even in red algae. Notably, two major CAX clades diverged prior to the emergence of seed plants. Following the whole-genome duplication (WGD) events, CAX further expanded in seed plants, with the Clade 1 splitting into two distinct subgroups. Expression analysis revealed tissue-specific expression patterns within the CAX family, with Clade 1-1 potentially involved in the development of reproductive organs. Additionally, the genus Primulina exhibited lineage-specific variations in key structural domains, which may contribute to its adaptation to high-calcium environments.

Conclusions: Our study provides new insights into the phylogenetic relationships of CAX family members across green plant lineages and highlights the significant role of CAXs in plant adaptation and diversification.