A solid-phase microextraction and gas chromatography-mass spectrometry utilizing $^{13}C$-formate as an internal standard for the determination of formate was proved to be applicable as a reliable quantitative method in several plants. We were the first to discover that trees contain larger pool sizes of formate than herbs. Our data also showed that the formate level of the leaves increased after the methanol-spraying, suggesting that methanol oxidation could convert formaldehyde into formate. In addition, drought and chilling led to the increase of endogenous formate in Arabidospsis thaliana. These results confirmed that formate is a universal stress signal in plants.