ARABIDOPSIS CAP1 FUNCTIONS IN THE TIP GROWTH OF ROOT HAIRS
Cells must sense and regulate their internal NH₄⁺ levels to modulate nitrogen levels and avoid NH₄⁺ toxicity. Bai et al. (pages 1497–1511) identify an Arabidopsis [Ca²⁺]_cyt-associated protein kinase (CAP1), a receptor-like kinase that mediates NH₄⁺ homeostasis. CAP1 also regulates the polar growth of root hairs by maintaining tip-focused cytoplasmic Ca²⁺ gradients. The cap1-1 mutation specifically affects root hair tip elongation and the morphology of root hairs on Murashige and Skoog medium and produces elevated levels of cytoplasmic NH₄⁺. Ammonium depletion from the medium reestablished the Ca²⁺ gradient necessary for normal root hair tip growth in the mutant. The image shows abnormal root hairs in a cap1-1 mutant grown on Murashige and Skoog medium.