LcVHA2 and LcVHA3 positively modulate energy metabolism and reduce pericarp browning in litchi fruit after application of Bacillus siamensis

Abstract

Pericarp browning, postharvest disease, and a short storage life are the leading causes of economic losses in the litchi industry globally. Energy metabolism plays an integral role in regulating browning, disease resistance, and the shelf-life of litchi fruit. The ability of Bacillus siamensis (N-1) to control postharvest diseases and maintain the quality of a variety of tropical fruits has previously been reported, however its effect on litchi fruit has not been explored. In the present study, we investigated the effect of Bacillus siamensis (N-1) on the postharvest energy metabolism of litchi fruit and its relation to pericarp browning, as well as the expression of energy-related genes. Results indicated that litchi fruit treated with Bacillus siamensis (N-1) exhibited a significant improvement in storability as evidenced by a decrease in the rate of the browning index (BI), color change, rate of weight loss, and laccase (LAC) and polyphenol oxidase (PPO) activity. There was also an increase in the energy status of litchi fruit due to the enhanced activity of Ca2+-ATPase, H+-ATPase, oxidase (CCO), cytochrome C, and succinic dehydrogenase (SDH), as well as the expression of their corresponding genes. LcVHA2 and LcVHA3, two major vacuolar-type H+-ATPase genes, were cloned and characterized. Phylogenetic analysis revealed that these genes are members of the V-ATPase subunit D and E gene family, respectively. Both heterologous (in tomato) and homologous (in litchi) transient overexpression (OE) assays indicated that both LcVHA2 and LcVHA3 have the potential to delay changes in fruit coloration and pericarp browning, but also increase ADP, ATP, and V-H+-ATPase activity, as well as the energy charge (EC) of fruit. Collectively, results indicated that the LcVHA2 and LcVHA3 play a critical role in preventing the browning and senescence of litchi fruit. This study provides new information on the ability of Bacillus siamensis (N-1), which has been previously demonstrated to be an effective postharvest biocontrol agent, to also improve the storage life of litchi fruit.