Plant roots navigate in the dark soil environments following the gravity vector. Cell divisions in the meristem and rapid cell growth in the elongation zone propel the root tips through the soil. Actively elongating cells acidify their apoplast to enable cell wall extension by the activity of plasma membrane AHA H+-ATPases. The phytohormone auxin, central regulator of gravitropic response and root development, inhibits root cell growth, likely by rising the pH of the apoplast. However, the role of auxin in the regulation of the apoplastic pH gradient along the root tip is unclear. Here we show, by using an improved method for visualization and quantification of root surface pH, that the Arabidopsis thaliana root surface pH shows distinct acidic and alkaline zones, which are not primarily determined by the activity of AHA H+-ATPases. Instead, the distinct domain of alkaline pH in the root transition zone is controlled by a rapid auxin response module, consisting of the AUX1 auxin influx carrier, the AFB1 auxin co-receptor and the CNCG14 calcium channel. We demonstrate that the rapid auxin response pathway is required for an efficient navigation of the root tip.
Autophagy is the major catabolic process in eukaryotes and a key regulator of plant fitness. It enables rapid response to stress stimuli, essential for plastic adaptation of plants to changes in the environment. Fluorescent reporters and confocal microscopy are among the most frequently used methods for assessing plant autophagic activity. However, detection of dynamic changes in the pathway activity has been hampered by stresses imposed on living plant tissues during sample mounting and imaging. Here we implemented RoPod, a toolkit optimized for minimally-invasive time-lapse imaging of Arabidopsis roots, to reveal a time-resolved response of plant autophagy to drug treatments typically used for pathway modulation and discovered previously overlooked cell type-specific changes in the pathway response. These results not only give an insight into the complex dynamics of plant autophagy, but also provide necessary information for choosing sampling time for the end-point assays currently employed in plant autophagy research. RoPods are inexpensive and easy-to-use devices that are based on commercial or custom designed chambers, compatible with inverted microscopes. We describe a detailed protocol for the fabrication and use of RoPods and provide a complete pipeline including semi-automated image analysis for root hair growth assays, demonstrating the broader applicability of the RoPod toolkit.
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