Figure 9.

A hypothetical model for preharvest fruit drop triggered by a series of molecular events in litchi. In phase 1, fruits sense the internal developmental signals prior to maturity. In phase 2, the synthesis of hormones and their signal transduction pathways within the seed are altered. This alteration results in the enhancement of abscission signals mediated by abscisic acid (ABA) and ethylene (ETH), while auxin-related signals promoting fruit retention are suppressed. The disruption of hormone balance in the fruit leads to reduced levels of indole-3-acetic acid (IAA), increased levels of ABA and ETH, and variations in other hormonal concentrations. These abscission cues are integrated to inhibit the polar auxin transport (PAT) from the fruit to the abscission zone (AZ), and enhance the sensitivity of AZ to ETH, which in turn activates the abscission process. This activation perturbs hormonal balance further, causing a burst of reactive oxygen species (ROS). The abscission cues exert a “remotely control” by fruit over the AZ, which is involved in cell turgor modulation, programmed cell death, and cell wall degradation. Finally, in phase 3, the cells within the AZ separate, leading to the completion of preharvest fruit drop. The direction of the arrows indicates whether a process is enhanced and decreased, respectively.