ACC synthase is the rate-limiting enzyme during ethylene biosynthetic pathway. Using PCR amplification,
a gene encoding an ACC synthase was isolated from pear (Pyrus pyrifolia) and consequently designated as
PpACS1b. This gene was shown to contain three introns with typical GT/AG boundaries defining the splice
junctions. The deduced PpACS1b protein contains the conserved features of ACSs: Aminotransferases
class-I pyridoxal-phosphate attachment site and aminotransferase class I and II domain. Phylogenetic
analyses clearly demonstrated that PpACS1b has the highest homology with pear PbACS1b and PcACS1b.
Moreover, the PpACS1b was grouped into type-2 plant ACS subfamily based on its conserved domain and
phylogenetic status. PpACS1b transcripts were mainly detected in young leaves, with low signal being
detected in anthers and fruit. In particular, expression of PpACS1b was developmentally regulated during
fruit ripening and senescence. Further study demonstrated that PpACS1b expression in pear fruit was
regulated by salicylic acid and ethylene. Given all the above results together, these data suggested that
PpACS1b might be involved in the response to SA and ethylene during pear fruit late development.