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

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. 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. 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.