Structure and function of CrACA1, the major PM‐type Ca²⁺‐ATPase, expressed at the peak of the gravity‐directed trans‐cell calcium current in spores of the fern Ceratopteris richardii

Abstract

Spores of the fern Ceratopteris richardii have proven to be a valuable single‐cell system for studying gravity responses. The earliest cellular change directed by gravity in these cells is a trans‐cell calcium current, which peaks near 10 h after the spores are induced to germinate. This current is needed for gravity‐directed axis alignment, and its peak is coincident with the time period when gravity polarises the direction of subsequent nuclear migration and rhizoid growth. Transcriptomic analysis of genes expressed at the 10‐h time point revealed several that encode proteins likely to be key components that either drive the current or regulate it. Notable among these is a plasma membrane (PM)‐type Ca2+ATPase, CrACA1, whose activity pumping Ca2+ out of cells is regulated by gravity. This report provides an initial characterisation of the structure and expression of this protein, and demonstrates its heterologous function complementing the K616 mutant of yeast, which is deficient in PM‐type Ca2+ pump activity. Gravity‐induced changes in the trans‐cell Ca2+ current occur within seconds, a result consistent with the hypothesis that the force of gravity can rapidly alter the post‐translational state of the channels and pumps that drive this current across spore cells. This report identifies a transporter likely to be a key driver of the current, CrACA1, and characterises the role of this protein in early germination and gravity‐driven polarity fixation through analysis of expression levels, functional complementation and pharmacological treatments. These data, along with newly available transcriptomic data obtained at the 10‐h time point, indicate that CrACA1 is present, functional and likely a major contributing component of the trans‐cell Ca2+ efflux. CrACA1 is not necessary for polar axis alignment, but pharmacological perturbations of it disrupt rhizoid development. These data support and help refine the post‐translational modification model for gravity responses.

Plant biology in space: recent accomplishments and recommendations for future research

“…), for germinating spores of the fern Ceratopteris (Bushart et al . ), for graviperception in Chara rhizoids (Limbach et al . ) and Euglena (Daiker et al .…”

Molecular mechanisms of gravity perception and signal transduction in plants

“…Exogenous calcium application enhances root and hypocotyl gravity responses, while inhibitors of mechanosensitive ion channels, calcium channels, and Ca 2+ -ATPases interfere with gravitropism (Bushart et al 2013(Bushart et al , 2014Perera et al 2006;Salmi et al 2011;Urbina et al 2006;Zhang et al 2011a). Specific inhibitors were also used to demonstrate that gravity induces stretch-activated channels to release ATP along the lower side of the spore cell.…”

Intracellular Calcium Decreases Upon Hyper Gravity-Treatment of Arabidopsis Thaliana Cell Cultures

“…Bushart et al (2014) showed that such an ATPase is regulated by gravity. The use of ATPase inhibitors could thus help to elucidate this hypergravity effect in more detail.…”