RNA expression, protein activity, and interactions in the ACC synthase gene family in cucumber (Cucumis sativus L.)

Ethylene is an important regulator of developmental and stress responses in plants, and 1-aminocyclopropane-1-carboxylic acid synthase (ACS) proteins catalyze one of the rate-limiting steps in endogenous ethylene biosynthesis. Accordingly, the function of ACS family genes has been extensively studied in plants including Arabidopsis and tomato; however, little is known about the systemic function of ACS genes in cucumber (Cucumis sativus L.). Here, we investigated the expression patterns of eight cucumber ACS family genes (CsACSs) in a variety of tissues and sex types, as well as in response to exogenous ethylene application, and their ACS activities. Tissue-specific expression profiling in monoecious and gynoecious cucumber plants revealed that some CsACSs were differentially expressed. In particular, the five genes CsACS1, CsACS1-2, CsACS2, CsACS6, and CsACS11 were highly expressed in the shoot apex regions of gynoecious and hermaphroditic cucumber plants. The expression of most CsACSs was also induced by exogenous ethylene application. Furthermore, three CsACS isoforms (CsACS9, CsACS10, and CsACS12) showed no ACS enzymatic activity, which was associated with the amino acid variations in the conserved active residues of CsACS proteins. However, an in vitro pull-down assay revealed that two enzymatically inactive isoforms (CsACS9 and CsACS10) did not significantly interact with four active isoforms (CsACS1, CsACS1-2, CsACS2, and CsACS6). Taken together, our findings will be valuable for elucidating the relationship between RNA expression, ACS activity, protein–protein interactions between CsACSs and cucumber sex types.