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-less LRX1{Delta}E14 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 LRX1{Delta}E14, we isolated a suppressor of dominant-negative effect mutant, 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 LRX1{Delta}E14 coincides with a defect in tip-focused cytoplasmic Ca2+ oscillation, and this effect is alleviated by the sune42 mutation. Additionally, reducing Ca2+ availability influences the LRX1{Delta}E14-induced root hair defect. We conclude that sune42 suppresses the root hair defect in LRX1{Delta}E14 through modulating cytoplasmic Ca2+ dynamics, pointing at the importance of the Golgi apparatus for cellular Ca2+ homeostasis.