Suppressing a mitochondrial calcium uniporter activates the calcium signaling pathway and promotes cell elongation in cotton

Mitochondrial calcium uniporter (MCU) is a conserved calcium ion (Ca2+) transporter in the mitochondrial inner membrane of eukaryotic cells. How MCU proteins regulate Ca2+ flow and modulate plant cell development remain largely unclear. Here, we identified the gene GhMCU4 encoding a MCU protein that negatively regulates plant development and fiber elongation in cotton (Gossypium hirsutum). GhMCU4 expressed constitutively in various tissues with the higher transcripts in elongating fiber cells. Knockdown of GhMCU4 in cotton significantly elevated the plant height and root length. The calcium signaling pathway was significantly activated and calcium sensor genes, including Ca2+ dependent modulator of interactor of constitutively active ROP (GhCMI1), calmodulin like protein (GhCML46), calcium-dependent protein kinases (GhCPKs), calcineurin B-like protein (GhCBLs), and CBL-interacting protein kinases (GhCIPKs), were dramatically upregulated in GhMCU4-silenced plants. Metabolic processes were preferentially enriched, and genes related to regulation of transcription were upregulated in GhMCU4-silenced plants. The contents of Ca2+ and H2O2 were significantly increased in roots and leaves of GhMCU4-silenced plants. Fiber length and Ca2+ and H2O2 contents in fibers were significantly increased in GhMCU4-silenced plants. This study indicated that GhMCU4 plays a negative role in regulating cell elongation in cotton, thus expanding understanding in the role of MCU proteins in plant growth and development.