A polygalacturonase gene clustered into clade E involved in lychee fruitlet abscission

Polygalacturonase (PG), which degrades pectin in cell wall, was proposed to participate in the process of plant organ abscission. In an effort to understand the molecular mechanisms of lychee fruitlet abscission, a full-length cDNA encoding a polygalacturonase, termed as LcPG1, was isolated from lychee by using RT-PCR and RACEs. The LcPG1 cDNA was 1666 bp in length with an open reading frame (ORF) of 1443 bp, encoding a polypeptide of 481 amino acid residues. Homologous and phylogenetic analysis showed that the LcPG1 included three of the four typical domains conserved in PG proteins, and was clustered into the clade E of PG genes. Expression analysis in the fruitlet abscission zone (AZ) indicated that the expression of LcPG1 was positively correlated with the lychee fruitlet abscission, either induced by ethephon or inhibited by 2,4-dichlorophenoxyacetic acid (2,4-D). Moreover, the carbohydrate stress caused by the girdling plus defoliation was found to stimulate the fruitlet abscission, which was paralleled with an increase in LcPG1 expression level. All these results suggested that the LcPG1 played an essential role in the process of lychee fruitlet abscission. To our knowledge, it is the first case indicating that a PG gene, belonging to the clade E, is involved in organ shedding. (C) 2012 Elsevier B.V. All rights reserved.