Fluorescence Resonance Energy Transfer-Sensitized Emission of Yellow Cameleon 3.60 Reveals Root Zone-Specific Calcium Signatures in Arabidopsis in Response to Aluminum and Other Trivalent Cations

Fluorescence resonance energy transfer-sensitized emission of the yellow cameleon 3.60 was used to study the dynamics of cytoplasmic calcium ([Ca²⁺] _cyt ) in different zones of living Arabidopsis (Arabidopsis thaliana) roots. Transient elevations of [Ca²⁺] _cyt were observed in response to glutamic acid (Glu), ATP, and aluminum (Al³⁺). Each chemical induced a [Ca²⁺] _cyt signature that differed among the three treatments in regard to the onset, duration, and shape of the response. Glu and ATP triggered patterns of [Ca²⁺] _cyt increases that were similar among the different root zones, whereas Al³⁺ evoked [Ca²⁺] _cyt transients that had monophasic and biphasic shapes, most notably in the root transition zone. The Al³⁺-induced [Ca²⁺] _cyt increase generally started in the maturation zone and propagated toward the cap, while the earliest [Ca²⁺] _cyt response after Glu or ATP treatment occurred in an area that encompassed the meristem and elongation zone. The biphasic [Ca²⁺] _cyt signature resulting from Al³⁺ treatment originated mostly from cortical cells located at 300 to 500 μm from the root tip, which could be triggered in part through ligand-gated Glu receptors. Lanthanum and gadolinium, cations commonly used as Ca²⁺ channel blockers, elicited [Ca²⁺] _cyt responses similar to those induced by Al³⁺. The trivalent ion-induced [Ca²⁺] _cyt signatures in roots of an Al³⁺-resistant and an Al³⁺-sensitive mutant were similar to those of wild-type plants, indicating that the early [Ca²⁺] _cyt changes we report here may not be tightly linked to Al³⁺ toxicity but rather to a general response to trivalent cations.