Watercore Pear Fruit Respiration Changed and Accumulated γ-Aminobutyric Acid (GABA) in Response to Inner Hypoxia Stress

Abstract

Watercore is a physiological disorder which often occurs on the pear fruit and the excessive accumulation of sorbitol in fruit intercellular space is considered to be an important cause of watercore. Our previous studies found that the metabolic disorder of sugars may lead to hypoxia stress and disturb respiration, resulting in aggravated fruit rot and the formation of bitter substances. However, the further changes of respiration and the fruit response mechanism are not well understood. A comprehensive transcriptome analysis of ‘Akibae’ pear watercore fruit was performed in this study. The transcriptome results revealed the hypoxia stress significantly induced the expression of several key enzymes in the TCA cycle and may lead to the accumulation of succinic acid in watercore fruit. The glycolytic pathway was also significantly enhanced in watercore fruit. Moreover, the γ-aminobutyric acid (GABA) synthesis related genes, glutamate decarboxylase (GAD) genes and polyamine oxidase (PAO) genes, which associated with the GABA shunt and the polyamine degradation pathway were significantly upregulated. In addition, the PpGAD1 transcript level increased significantly along with the increase of GAD activity and GABA content in the watercore fruit. Above all, these findings suggested that the hypoxic response was marked by a significant increase of the hypoxia-inducible metabolites succinic acid and GABA and that PpGAD1 may play a key role in response to watercore by controlling the GABA synthesis.

The γ-Aminobutyric Acid (GABA) Synthesis Gene Regulates the Resistance to Water Core-Induced Hypoxia Stress for Pear Fruits

“…In our previous research, the expression of three PpGADs was significantly increased, which correlated with the GABA content in watercore fruit [2]. To further evaluate the transcript abundance of PpGADs, we used PpGAD1 in healthy fruit as a control (CK) and compared the PpGADs expression level again.…”

“…The symptoms of watercore are translucent fruit flesh and watery tissue. Our previous study suggested that watery tissue increased anaerobic respiration and caused the fruit to suffer from hypoxia stress [2]. Under hypoxia stress, plants experience a derangement of the cellular energy metabolism, causing a decrease in the pH in the cytoplasm, the inhibition of the ion transport pathway or nutrient acquisition, and the accumulation of toxic products from anaerobic respiration and reactive oxygen species (ROS) [3].…”

“…CaM itself has no catalytic activity; however, after binding with Ca 2+ , it can interact with a downstream CaM-binding protein (CaMBP) to activate the function of these interacting proteins, and GAD is the CaMBP [12]. Our previous study reported that in watercore fruit, several GAD genes were significantly induced to up-regulated [2]. These PpGADs may play a key role in regulating fruit to resist hypoxia stress.…”

Recent advances of γ-aminobutyric acid: Physiological and immunity function, enrichment, and metabolic pathway

“…A hypoxic environment will cause the up-regulated expression of gad , a key gene of the GABA synthesis pathway in plants, which promote the accumulation of GABA ( 37 ). Under anoxic stress conditions, Pyrus pyrifolia showed watercore symptoms where the metabolites succinic acid and GABA were significantly accumulated, accompanied with the up-regulated expression of PpGAD gene which indicates the enrichment of GABA ( 38 ). Nitrogen deficiency has adverse effects on tree growth.…”