Host-guest synergistic hydrogen bond triggered signal amplification for visualizing the plant hormone salicylic acid

Salicylic acid (SA), the crucial phytohormone, is biosynthesized by plants in response to fight against multifarious phytopathogens and is a central mediator for regulating plant physiology and pathology processes including biotic and abiotic stress. However, current research still lacks of effective and simple tactics to track SA for understanding the underlying mechanisms in plants. Herein, we constructed a supramolecular host–guest nanoprobe (RAD&CB[7]) based on cucurbit[7]uril (CB[7]) and adamantane (AD)-modified rhodamine derivative, the nanoprobe selectivity and sensitivity for SA were very satisfactory. The nanoprobe RAD&CB[7] has been employed to (1) improve the fluorescent signal for detecting SA about 330-fold at lower concentration of SA (20 μM) in vitro, which is very difficult to realize by previous SA fluorescent probes. The density functional theory (DFT) calculations demonstrated the signal amplification is significantly higher than other fluorescent probes reported in literature due to the synergetic effects of host–guest and hydrogen-bond interaction. (2) realize the detection of SA in living Nicotiana glutinosa L. callus, Arabidopsis thaliana, and tomato seedlings even at ultra-low concentrations of RAD&CB[7] (minimum 1 μM) and SA (minimum 5 μM), compared with previous probes (minimum 10 μM) for detecting SA (minimum 25 μM) in vivo, thus realized quantitatively detected SA in callus and Arabidopsis thaliana roots. (3) verify SA-induced stomatal closure in the leaves of Arabidopsis thaliana by host–guest fluorescent probe technology for the first time. The remarkable in vitro and in vivo performance make this fluorescence signal amplification strategy a powerful tool for studying SA-related physiology and pathology in biological processes and discovering novel immune activators for crop protection.