The dynamics of sugars, hormones, and cell wall epitopes dictate early somatic embryogenesis in passion fruit (Passiflora edulis Sims)

Sugars, hormones and plant cell wall components regulate many aspects of somatic embryogenesis which requires detailed investigations of the process. In the present study, we investigated the histology, chemical composition of the cell wall, sugar and nitrogen metabolism, and hormonal profile associated with the induction and formation of embryogenic calluses in Passiflora edulis Sims. Zygotic embryos were cultured in induction medium supplemented with 72.4 µM dichlorophenoxyacetic acid and 4.4 µM 6-benzyladenine for 30 days. The zygotic embryo consisted of a uniseriate protoderm and an undifferentiated mesophyll. After 20 days, globular-stage somatic embryos were observed at the periphery of embryogenic calluses. During callus formation, the cell wall of dividing cells showed high levels of methyl-esterified homogalacturonan (HGs) epitopes, recognized for JIM7 antibody. Epitopes of heteromannans (LM21) were identified only in the initial explants, whereas epitopes of xyloglucans (LM15) appeared throughout the morphogenetic process. Cells undergoing transdifferentiation in embryogenic calluses exhibited a drop in epitopes recognized for JIM7 and LM15 and absence of JIM5 labeling. Lipids and proteins were metabolized during early somatic embryogenesis. In contrast, hexose, starch, and amino acid concentrations increased. Hormonal dynamics were also altered: while indole-3-acetic acid and cytokinins decreased during the embryogenic process, jasmonic acid and ethylene showed the opposite trend, indicating a possible role of these stress hormones in the early somatic embryogenesis of passion fruit. These results shed light on the structural and physiological aspects of passion fruit somatic embryogenesis which may help optimize regeneration via somatic embryogenesis of this important commercial passion fruit species.