γ-aminobutyric acid contributes to a novel long-distance signaling in figleaf gourd rootstock-induced cold tolerance of grafted cucumber seedlings.

Long-distance signals play a vital role in plant stress response. γ-aminobutyric acid (GABA) has been proposed to be a signal and protects crops against diverse stresses. However, whether GABA acts as a long-distance signal to plant response to stresses remains unknown. Here, we found that the GABA content in cucurbita rootstocks, especially figleaf gourd, was significantly higher than that in cucumber. Figleaf gourd rootstock obviously enhanced cold tolerance and GABA accumulation in roots, xylem sap and leaves of grafting cucumber seedlings. Conversely, GABA synthesis inhibitor 3-mercaptopropionic acid (3-MPA) irrigation was more effective than its foliar application in inhibiting grafting-induced cold tolerance. Moreover, fluorescence microscopy confirmed that GABA can be transported from root to shoot through the xylem when the roots of grafted seedlings were fed with fluorescein isothiocyatate-labeled GABA under normal and cold stress conditions. Importantly, 3-MPA irrigation attenuated grafting-induced cold tolerance, as revealed by a decline in the GABA accumulation, the transcripts of ICE1, CBF1 and COR47, the activities of the antioxidant enzymes, and an increase in stomatal aperture. Collectively, our findings strongly support that GABA functions as a novel long-distance signal in figleaf gourd rootstock-induced cold tolerance of grafted cucumber seedlings by modulating CBF-signalling pathways, antioxidant system and stomatal aperture, providing new evidence for long-distance signaling-mediated cold response of plants.