Substance metabolism, IAA and CTK signaling pathways regulating the origin of embryogenic callus during dedifferentiation and redifferentiation of cucumber cotyledon nodes

Somatic embryogenesis system was an ideal one for the genetic transformation and propagation in vitro because it propagates efficiently, quickly and stably. The mechanism of the origin of somatic embryogenesis especially in cucumber has not been well illustrated since it is explored. The embryogenic callus was observed to be induced from the adaxial side and the non-embryogenic callus were induced from the abaxial side of the cucumber cotyledon nodes, and then the embryos generated from the embryogenic callus but not from the non-embryogenic callus. The histology and transcriptomic analysis were performed to probe the dedifferentiation and redifferentiation difference between the adaxial side and the abaxial side of the cucumber cotyledon nodes. It showed that embryogenic callus having dense cytoplasm, clear nucleus and regular shaped cells was induced from the adaxial side of the cotyledon node and the non-embryogenic callus with unclear border of nucleus and irregular shaped cells were generated from the abaxial side. A total of 5838 differentially expressed genes (DEGs) were screened between embryogenic and non-embryogenic callus, which were highly enriched in auxin and CTK synthesis and signaling pathways and substance metabolism pathway such as starch and amino acid in embryogenic callus. The IPA (Indole-3-pyruvic acid) genes, AUX1 (Auxin 1) genes and IPT (Isopentenyl transferase) genes involved in auxin and CTK biosynthesis and signaling pathways were up-regulated in embryogenic callus comparing with that in non-embryogenic callus. SS1 (starch synthase) genes related with starch synthesis, OATase (Ornithine acetyltransferase) genes related with arginine and proline synthesis were more highly induced in embryogenic callus than that in non-embryogenic callus. It will provide a reliable foundation for elucidating the difference between the origin of somatic embryogenesis during dedifferentiation and redifferentiation of cucumber cotyledon nodes.