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Genome-Wide Identification of the 1-Aminocyclopropane-1-carboxylic Acid Synthase (ACS) Genes and Their Possible Role in Sand Pear (Pyrus pyrifolia) Fruit Ripening

Ethylene production is negatively associated with storage life in sand pear (Pyrus pyrifoliaNakai), particularly at the time of fruit harvest. 1-Aminocyclopropane-1-carboxylic acid synthase (ACS) is the rate-limiting enzyme in ethylene biosynthesisand is considered to be important for fruit storage life. However, the candidate ACS genes and their roles in sand pear remain unclear. The present study identified 13ACS genes from the sand pear genome.Phylogenetic analysiscategorizedthese ACS genesinto four subgroups (type Ⅰ, type Ⅱ, type Ⅲ and putative AAT), and indicated a close relationship between sand pear and Chinese white pear (P. bretschneideri). According to the RNA-seq data and qRT-PCR analysis, PpyACS1, PpyACS2, PpyACS3, PpyACS8, PpyACS9, PpyACS12 and PpyACS13 were differently expressed in climacteric and non-climacteric-typepear fruits,‘Ninomiyahakuri’ and ‘Eli No.2′, respectively, during fruit ripening. In addition, the expressions of PpyACS2, PpyACS8, PpyACS12 and PpyACS13 werefound to be associated with system 1 of ethylene production, while PpyACS1, PpyACS3, and PpyACS9 werefound to be associated with system 2, indicating that these ACS genes have different roles in ethylenebiosynthesis during fruit development. Overall, our study provides fundamental knowledge onthe characteristics of the ACS gene family in sand pear, in addition to their possible roles infruit ripening.