Phosphate Starvation Alters Abiotic-Stress-Induced Cytosolic Free Calcium Increases in Roots

Phosphate (Pi) deficiency strongly limits plant growth, and plant roots foraging the soil for nutrients need to adapt to optimize Pi uptake. Ca²⁺ is known to signal in root development and adaptation but has to be tightly controlled, as it is highly toxic to Pi metabolism. Under Pi starvation and the resulting decreased cellular Pi pool, the use of cytosolic free Ca²⁺ ([Ca²⁺]_cyt) as a signal transducer may therefore have to be altered. Employing aequorin-expressing Arabidopsis (Arabidopsis thaliana), we show that Pi starvation, but not nitrogen starvation, strongly dampens the [Ca²⁺]_cyt increases evoked by mechanical, salt, osmotic, and oxidative stress as well as by extracellular nucleotides. The altered root [Ca²⁺]_cyt response to extracellular ATP manifests during seedling development under chronic Pi deprivation but can be reversed by Pi resupply. Employing ratiometric imaging, we delineate that Pi-starved roots have a normal response to extracellular ATP at the apex but show a strongly dampened [Ca²⁺]_cyt response in distal parts of the root tip, correlating with high reactive oxygen species levels induced by Pi starvation. Excluding iron, as well as Pi, rescues this altered [Ca²⁺]_cyt response and restores reactive oxygen species levels to those seen under nutrient-replete conditions. These results indicate that, while Pi availability does not seem to be signaled through [Ca²⁺]_cyt, Pi starvation strongly affects stress-induced [Ca²⁺]_cyt signatures. These data reveal how plants can integrate nutritional and environmental cues, adding another layer of complexity to the use of Ca²⁺ as a signal transducer.