Gene expression analysis and halting of ethylene receptors signaling pinpoint ethylene as a positive regulator of direct somatic embryogenesis in Coffea arabica.

Main conclusion: Ethylene acts as a positive regulator of Coffea arabica direct somatic embryogenesis when using leaves as explants. Ethylene is a gaseous growth regulator synthesized by plants, acting on senescence and biotic and abiotic stress responses. It has also been associated with fruit ripening, carotenoid and anthocyanin accumulation, and embryogenesis. However, the role of ethylene during zygotic and somatic embryogenesis is not entirely elucidated. In this work, we aimed to investigate the role of early ethylene signaling during embryogenesis in Coffea arabica, one of the most important commercial plants. Thorough morphological and gene expression analyses were performed during fruit development and direct somatic embryogenesis (DSE). In addition, tissue culture explants in sealed flasks were used to evaluate the ethylene production during DSE. The genes encoding the ethylene receptors Ethylene Receptor 1 (ETR1), Ethylene Receptor 2 (ETR2), Ethylene Response Sensor 1 (ERS1), and Ethylene Insensitive 4 (EIN4) and one gene encoding the enzyme responsible for ethylene synthesis 1-aminocyclopropane-1-carboxylic oxidase (ACO) are expressed during direct somatic embryogenesis (DSE) and in zygotic embryogenesis during fruit development, each having a different gene expression profile. The addition of silver nitrate, an inhibitor of ethylene signaling, to tissue culture medium, positively influenced on pro-embryogenic mass formation but harmed embryo formation, suggesting that ethylene is involved in DSE in C. arabica. In addition, we confirmed ethylene production by leaf explants during DSE. Overall, we suggest that ethylene is an important positive regulator of direct somatic embryogenesis in C. arabica. We discuss the participation of ethylene perception in embryonic development and its connection to cytokinin and explant wounding.