Is the redox state of the PQ pool involved in regulating the ET biosynthesis pathway of CAM facultative semi-halophytes?

The mechanisms and factors regulating ethylene (ET) biosynthesis and its action remain largely unexplored, particularly in CAM-facultative and (semi)halophytic plants. The use of Mesembryanthemum crystallinum L. (ice plant) provides a unique opportunity to examine plastoquinone (PQ) - ET interactions in semi-halophytes and CAM-facultative plants simultaneously. Here, we present the results of an analysis of the common ice plant's response to prolonged (14-day) salinity stress and DCMU, which maintains the PQ pool in a more oxidised state, thereby mimicking darkness conditions. Differentially expressed gene (DEG) analysis showed that the expression of genes involved in ET regulation was not significantly altered after DCMU application. However, in C₃ plants not affected by salinity stress, the expression of genes related to both photosystems, photosynthesis, and the photosynthetic electron transport chain was significantly affected by DCMU. We propose that sustained salinity stress and the occurrence of CAM photosynthesis render physiological processes insensitive to disruptions caused by a modified PQ pool redox state. The UPLC-MS analysis of the ET biosynthesis pathway central intermediate – 1-aminocyclopropane-1-carboxylic acid (ACC) – confirmed the molecular analysis results, as ACC content was similarly affected in salinity untreated and treated plants. Moreover, the analysis of key antioxidative system components, namely catalase and superoxide dismutases, suggests that PQ pool redox state does not directly regulate them. Instead, an alternative regulation mechanism involving reactive oxygen species (ROS) accumulation and a ROS-induced signalling cascade has been proposed.