γ-Aminobutyric acid plays a key role in alleviating Glomerella leaf spot in apples.

The fungal disease Glomerella leaf spot (GLS) seriously impacts apple production. As a nonprotein amino acid, γ-aminobutyric acid (GABA) is widely involved in biotic and abiotic stresses. However, it is not clear whether GABA is involved in a plant's response to GLS, nor is its molecular mechanism understood. Here, we found that exogenous GABA could significantly alleviate GLS, reduce lesion lengths, and increase antioxidant capacity. MdGAD1 was identified as a possible key gene for GABA synthesis in apple. Further analysis indicated that MdGAD1 promoted antioxidant capacity to improve apple GLS resistance in transgenic apple calli and leaves. Yeast one-hybrid analysis identified the transcription factor MdWRKY33 upstream of MdGAD1. Electrophoretic mobility shift assay, ß-glucuronidase activity, and luciferase activity further supported that MdWRKY33 bound directly to the promoter of MdGAD1. The content of GABA and the transcription level of MdGAD1 in the MdWRKY33 transgenic calli were higher than that of the wild type. When MdWRKY33 transgenic calli and leaves were inoculated with GLS, MdWKRY33 positively regulated resistance to GLS. These results explained the positive regulatory effects of GABA on apple GLS and provided insight into the metabolic regulatory network of GABA.

GABA-mediated inhibition of cadmium uptake and accumulation in apples.

GABA, a four-carbon non-protein amino acid, plays an important role in animals and plants. We previously found GABA could alleviate alkali stress in apple seedlings. However, its physiological mechanism under heavy metal cadmium (Cd) stress need to be further studied. Thus, we explored its biological role in response to Cd stress. It was verified that 0.5 mM GABA could effectively alleviate Cd toxicity. Using NMT technique, we found that exogenous GABA could significantly reduce the net Cd2+ fluxes in apple roots, and Cd content was significantly lower than that in roots under Cd stress. Further analysis indicated exogenous GABA could significantly reduce the expression of genes related to the uptake and transport of Cd in apples under Cd stress. In addition, exogenous GABA could significantly increase the content of amino acids in apple roots under Cd stress. GAD is a key enzyme in GABA synthesis, we obtained transgenic apple roots of overexpression MdGAD1. Compared with the control, transgenic roots accumulated less Cd, maintained lower Cd uptake by roots, and lower expression of related transport genes. These results showed that GABA could effectively alleviate Cd toxicity in apple seedlings and provide a new perspective of GABA to alleviate Cd stress.