Phenylalanine ammonia lyase gene expression during abscission in citrus

Phenylalanine ammonia lyase (PAL) gene expression was examined in fruit and leaf abscission zones of Valencia orange for periods up to 72 h after induction of abscission with Ethrel(R) (CEPA) or the citrus mature fruit-specific abscission material 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMN-P). PAL gene expression was detected in mature fruit abscission zones beginning 6 and 24 h after CEPA and CMN-P application, respectively, and continued up to 72 h. PAL gene expression was detected in leaf abscission zones 6 h after CEPA application and continued throughout the remainder of the study. In contrast, PAL gene expression was not detected in leaf abscission zones treated with CMN-P. 2-aminoindan-2-phosphonic acid (AIP), a specific inhibitor of PAL enzyme activity, decreased CMN-P-induced PAL expression in fruit abscission zones, and this was accompanied by a lack of fruit drop. PAL transcripts were low in abscission zones of immature fruit; however, the transcript was induced by CMN-P but less by CEPA application. The data suggest that downstream products of PAL activity may be important not only for wound healing and defense reactions that occur at the abscission zone fracture plane, but also for regulation of PAL gene expression during abscission.