Two cellulases involved in litchi fruit abscission are directly activated by an HD-Zip transcription factor LcHB2.

Cellulases play important roles in the shedding of plant organs. However, little is yet known about the functions of cellulase genes during the process of organ abscission. Abnormal fruitlet abscission is a severe problem in the production of litchi, an economically important fruit widely grown in South Asia. Here, two abscission-accelerating treatments (carbohydrate stress and ethephon application) were evaluated in litchi fruitlets. Cell wall degradation and cell separation were clearly observed in treated fruitlet abscission zones, consistent with enhanced cellulase activities and reduced cellulose contents. The expression of two cellulase genes (LcCEL2 and LcCEL8) was strongly associated with litchi fruitlet abscission. Floral organs of transgenic Arabidopsis thaliana overexpressing LcCEL2 or LcCEL8 showed remarkably precocious abscission. Further, electrophoretic mobility shift assays and transient expression experiments demonstrated that a novel Homeodomain-leucine Zipper transcription factor (LcHB2) could directly bind to and activate HD-binding cis-elements in the LcCEL2 and LcCEL8 promoters. Our results provide new information regarding the transcriptional regulation of cellulase genes responsible for cell wall degradation and cell separation during plant organ shedding. In the future, the kinetics of litchi fruitlet abscission could be manipulable by modulation of these two cellulase activities.