The CATION CALCIUM EXCHANGER 4 (CCX4) regulates LRX1-related root hair development through Ca ²⁺ homeostasis

Calcium, as a cellular second messenger, is essential for plant growth. A tip-focused Ca ²⁺ gradient in polarized cells is considered to drive cell expansion. The cell wall polysaccharide pectin is a major Ca ²⁺ binding structure and Ca ²⁺ homeostasis is influenced by the cell wall architecture. LRR-extensin (LRX) proteins are extracellular regulators of cell wall development that are anchored in the cell wall by their extensin domain. The extensin-less LRX1ΔE14 variant of the root hair-expressed LRX1 of Arabidopsis induces a dominant-negative effect resulting in aberrant root hairs. In an effort to identify the underlying mechanism of the root hair defect caused by LRX1ΔE14 , we isolated a su ppressor of dominant- ne gative effect mutant, sune42 . It codes for the CATION CALCIUM EXCHANGER 4 (CCX4) that localizes to the Golgi apparatus and was shown to have Ca ²⁺ transport activity. A detailed investigation of the Ca ²⁺ dynamics revealed that LRX1ΔE14 coincides with a defect in tip-focused cytoplasmic Ca ²⁺ oscillation, and this effect is alleviated by the sune42 mutation. Additionally, reducing Ca ²⁺ availability influences the LRX1ΔE14 -induced root hair defect. We conclude that sune42 suppresses the root hair defect in LRX1ΔE14 through modulating cytoplasmic Ca ²⁺ dynamics, pointing at the importance of the Golgi apparatus for cellular Ca ²⁺ homeostasis.