Abscisic Acid Mediates Salicylic Acid Induced Chilling Tolerance of Grafted Cucumber by Activating HO Biosynthesis and Accumulation.

Grafting is widely applied to enhance the tolerance of some vegetables to biotic and abiotic stress. Salicylic acid (SA) is known to be involved in grafting-induced chilling tolerance in cucumber. Here, we revealed that grafting with pumpkin (, ) as a rootstock improved chilling tolerance and increased the accumulation of SA, abscisic acid (ABA) and hydrogen peroxide (HO) in grafted cucumber () leaves. Exogenous SA improved the chilling tolerance and increased the accumulation of ABA and HO and the mRNA abundances of , , , and . However, 2-aminoindan-2-phosphonic acid (AIP) and L-a-aminooxy-b-phenylpropionic acid (AOPP) (biosynthesis inhibitors of SA) reduced grafting-induced chilling tolerance, as well as the synthesis of ABA and HO, in cucumber leaves. ABA significantly increased endogenous HO production and the resistance to chilling stress, as proven by the lower electrolyte leakage (EL) and chilling injury index (CI). However, application of the ABA biosynthesis inhibitors sodium tungstate (NaWO) and fluridone (Flu) abolished grafting or SA-induced HO accumulation and chilling tolerance. SA-induced plant response to chilling stress was also eliminated by N,N'-dimethylthiourea (DMTU, an HO scavenger). In addition, ABA-induced chilling tolerance was attenuated by DMTU and diphenyleneiodonium (DPI, an HO inhibitor) chloride, but AIP and AOPP had little effect on the ABA-induced mitigation of chilling stress. NaWO and Flu diminished grafting- or SA-induced HO biosynthesis, but DMTU and DPI did not affect ABA production induced by SA under chilling stress. These results suggest that SA participated in grafting-induced chilling tolerance by stimulating the biosynthesis of ABA and HO. HO, as a downstream signaler of ABA, mediates SA-induced chilling tolerance in grafted cucumber plants.