Deciphering the regulatory network underlying auxin depletion-induced fruitlet abscission in litchi ( Litchi chinensis Sonn.)

Auxin transport through the abscission zone (AZ) is crucial for preventing organ abscission in plants; however, the regulatory mechanisms involved remain poorly understood. Here, we demonstrate that exogenous auxin application alone fully inhibits abscission triggered by litchi (Litchi chinensis Sonn.) fruitlet removal, which depletes auxin in the AZ. Following fruitlet removal, we observed sequential and significant alterations in 7 biological processes within the AZ, including the transient suppression of auxin signaling and activation of ethylene and abscisic acid signaling, followed by the increased production of reactive oxygen species, reduced carbohydrate content, and ultimately, the induction of programmed cell death and cell wall remodeling (CWR). Moreover, we identified 34 transcription factors as potential key regulators and constructed transcriptional regulatory networks involved in auxin depletion-induced abscission. Notably, we characterized LcMYB62 as a positive regulator of abscission, likely functioning by transactivating genes associated with CWR. We further showed that the transcription factor AUXIN RESPONSE FACTOR 5 (LcARF5) binds to and activates LcMYB62, suggesting a LcARF5-LcMYB62-CWR transcriptional regulatory cascade in litchi fruitlet abscission. Overall, our findings provide a comprehensive overview of the gene regulatory network governing auxin-mediated fruitlet abscission in litchi, providing insights into the mechanisms by which auxin depletion in the AZ triggers this process.