Calcium (Ca) is essential for plant growth, but too much of it can be toxic. In the karst regions of southwestern China, soils are notoriously rich in calcium—posing a serious challenge for native plants. Now, researchers have uncovered how members of the Rhododendron genus have evolved an extraordinary genetic strategy to cope with this high-calcium stress. By analyzing the genomes and transcriptomes of multiple Rhododendron species, the team discovered that a family of calcium-efflux proteins called Ca²⁺-ATPases (CAPs) has undergone massive amplification in species growing in karst environments. These duplicated genes, particularly those in the CAP-a subfamily, appear to function as a "storage repository"—silent under normal conditions but ready to be activated when stress strikes. The findings reveal new insights into how plants balance growth and stress responses in challenging environments.
For decades, plant stress research has largely relied on laboratory models such as Arabidopsis, where conditions are controlled and simplified. But in the real world, plants face a complex web of environmental pressures—high temperatures, drought, and mineral toxicity—all at once. Karst landscapes, which cover approximately 15% of Earth's land area and about 33% of China's territory, present one such multifactorial challenge. High calcium levels are a defining feature of these regions, and plants must constantly pump excess Ca²⁺ out of their cells to avoid toxicity. Yet the evolutionary strategies that wild plants use to manage calcium stress have remained poorly understood, particularly in woody species like Rhododendrons. Due to these challenges, there is a pressing need for in-depth research into the genetic and evolutionary mechanisms underlying high-calcium adaptation in natural plant populations.
Now, a team of researchers from Guizhou University, the Guizhou University of Engineering Science, and the Yazhouwan National Laboratory has published (DOI: 10.1093/hr/uhag174) a comprehensive study on the evolution and function of Ca²⁺-ATPases (CAPs) in Rhododendrons. The work appears in Horticulture Research on August 13, 2026. The researchers combined pan-genome and pan-transcriptome analyses across 12 high-quality Rhododendron genomes, along with experimental validation in yeast, to reveal how these calcium pumps have evolved, how they are expressed under different conditions, and how they function in alleviating high-calcium stress.
The study revealed that CAPs emerged early in plant evolution—traces appear even in algae and archaea—and diversified into six major clades (CAP-a through CAP-f). In Rhododendrons, the CAP-a subfamily has undergone the most extensive duplication, with many copies clustered in two large gene blocks on chromosome 2. Interestingly, these CAP-a genes are barely expressed under normal conditions, but specific environmental triggers—such as karst habitats, high altitude, or certain hormones—can induce their activity. The researchers describe this as a "storage expression mode": a reservoir of genes kept in reserve until needed, allowing the plant to conserve energy during growth while retaining the capacity to mount a rapid stress response. In contrast, CAP-b and CAP-d subfamilies showed consistently high expression in stems, old leaves, and floral organs, suggesting they play more constitutive roles in stress mitigation and development. Through co-expression network analysis, the team identified key regulatory partners, including Calcineurin B-like Proteins (CBLs) and CBL-interacting protein kinases (CIPKs), that work alongside CAPs. Experimental validation in yeast demonstrated that two key members—CAPb1 and CAPd1—significantly alleviate high-calcium stress, and when paired with specific CIPK-CBL modules (CAPb1-CIPK1-CBL1 and CAPd1-CIPK2-CBL1), their protective effects were markedly enhanced.
The authors said, "What excites us most is the discovery of this 'storage' strategy—it's like the plant keeps a genetic toolbox locked away until the environment demands it. This could be a broader principle in how wild plants adapt to extreme conditions, not just for calcium but for other stresses as well. The fact that we could then validate the function of these genes in yeast and show that specific partner proteins boost their activity gives us a clear path forward for engineering more stress-tolerant plants." They added that the karst-adapted species R. bailiense, R. delavayi, and R. irroratum—all collected from Guizhou's Baili Rhododendron Nature Reserve—carry the highest CAP copy numbers, underscoring the strong link between gene duplication and environmental pressure.
The findings have direct implications for forestry and ecological restoration. Karst desertification is a major environmental challenge in southwestern China, and understanding how native species tolerate high-calcium soils could inform breeding programs for resilient tree species. The CAP genes and their regulatory modules identified in this study could serve as targets for genetic improvement, not only in Rhododendrons but also in other woody plants grown in calcium-rich regions. Moreover, the "storage expression" model offers a new perspective on how plants balance the trade-off between growth and stress defense—a question that has long puzzled plant biologists. As the researchers continue to develop genetic transformation systems for Rhododendrons, these findings may eventually contribute to the restoration of fragile karst ecosystems and the sustainable management of these unique landscapes.
