Bushart T J, Cannon A, Clark G, Roux S J
Section of Molecular Cell & Developmental Biology, The University of Texas at Austin, Austin, TX, USA.
Plant Biol (Stuttg). 2014 Jan;16 Suppl 1:151-7. doi: 10.1111/plb.12107. Epub 2013 Oct 29.
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 Ca(2+) ATPase, CrACA1, whose activity pumping Ca(2+) 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 Ca(2+) pump activity. Gravity-induced changes in the trans-cell Ca(2+) 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 Ca(2+) 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.
已证明，蕨类植物里氏鹿角蕨的孢子是研究重力反应的宝贵单细胞系统。在这些细胞中，重力引导的最早细胞变化是跨细胞钙电流，该电流在孢子诱导萌发后约10小时达到峰值。这种电流是重力引导的轴对齐所必需的，其峰值与重力使后续核迁移和假根生长方向极化的时间段一致。对在10小时时间点表达的基因进行转录组分析，发现了几个编码可能是驱动或调节该电流的关键成分的蛋白质的基因。其中值得注意的是一种质膜（PM）型Ca(2+) ATP酶CrACA1，其将Ca(2+)泵出细胞的活性受重力调节。本报告对该蛋白质的结构和表达进行了初步表征，并证明了其对酵母K616突变体的异源功能互补作用，该突变体缺乏PM型Ca(2+)泵活性。重力诱导的跨细胞钙电流变化在几秒钟内发生，这一结果与重力可迅速改变驱动该电流穿过孢子细胞的通道和泵的翻译后状态这一假设一致。本报告鉴定出一种可能是该电流关键驱动因素的转运蛋白CrACA1，并通过分析表达水平、功能互补和药理学处理，表征了该蛋白质在早期萌发和重力驱动的极性固定中的作用。这些数据，连同在10小时时间点获得的新转录组数据，表明CrACA1存在且具有功能，可能是跨细胞Ca(2+)外流的主要促成成分。CrACA1对于极轴对齐不是必需的，但对其进行药理学干扰会破坏假根发育。这些数据支持并有助于完善重力反应的翻译后修饰模型。
