The CATION CALCIUM EXCHANGER 4 (CCX4) regulates LRX1-related root hair development through Ca²⁺homeostasis

Calcium, as a cellular second messenger, is essential for plant growth. A tip-focused Ca²⁺gradient in polarized cells is considered to drive cell expansion. The cell wall polysaccharide pectin is a major Ca²⁺binding structure and Ca²⁺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-lessLRX1ΔE14variant of the root hair-expressedLRX1of 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 byLRX1ΔE14, we isolated a<underline>su</underline>ppressor of dominant-<underline>ne</underline>gative effectmutant,sune42. It codes for the CATION CALCIUM EXCHANGER 4 (CCX4) that localizes to the Golgi apparatus and was shown to have Ca²⁺transport activity. A detailed investigation of the Ca²⁺dynamics revealed that LRX1ΔE14 coincides with a defect in tip-focused cytoplasmic Ca²⁺oscillation, and this effect is alleviated by thesune42mutation. Additionally, reducing Ca²⁺availability influences theLRX1ΔE14-induced root hair defect. We conclude thatsune42suppresses the root hair defect inLRX1ΔE14through modulating cytoplasmic Ca²⁺dynamics, pointing at the importance of the Golgi apparatus for cellular Ca²⁺homeostasis.