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