Somatic embryogenesis and plant regeneration in avocado (Persea americana Mill.): influence of embryogenic culture type

Embryogenic cell lines of Persea americana were classified as SE- or PEM-type based on their morphology in maintenance medium. PEM-type cultures consisted of proembryonic masses (PEMs) with occasional development of proembryos and early-stage somatic embryos, while SE-type cultures consisted of somatic embryos from globular to cotyledonary stages with only a low frequency of proembryos and PEMs. Moreover, histological analysis revealed signs of cellular organisation, with a higher proportion of small cells in peripheral regions of the callus mass in the SE-type, but not in PEM-type cell lines. The effects of time in suspension, inoculum density and maintenance medium on the ability of avocado cell lines to produce mature somatic embryos were evaluated. Morphological differences observed during proliferation were correlated to the subsequent capacity of cultures to develop mature somatic embryos. The period of time in suspension and inoculum density were critical factors that influenced the ability of cultures to undergo maturation. Optimal conditions differed between SE-type cultures and PEM-type cultures. In SE-type cultures, the highest number of somatic embryos was observed in cultures grown in suspension for 9?days and 0.4?g inoculum; in PEM-type cultures a slight increase in mature somatic embryos production could be observed with 4?g inoculum size and 14?days in suspension. The presence of agar in the maintenance medium of SE-type cultures was essential for the maturation of somatic embryos.