The role of stress on unravelling of somatic embryogenesis competence

Global climatic changes and growing demographic pressure have increased demand for agronomic resources, leading to an increasing lack of land suitable for agriculture; they have moreover provoked several abiotic stresses which, added to the biotic ones, result in physiological and metabolic disorders that ultimately impact on yield when it most needs to be improved. Understanding and resolving the impact of stress on yield is a major scientific and agronomic challenge and biotechnological breeding would be an efficient alternative. However, to reduce risks of somaclonal variations among regenerants and transformants, it is better to produce them through somatic embryogenesis. This may occur directly from the explant or gametes or indirectly from callus or cell suspensions, leading to a regeneration of globular somatic embryos that will follow a common developmental path, heart, torpedo and cotyledonary stages, and finally reach the phase of mature embryos that subsequently develop into viable plantlets provided they have accumulated enough storage products. However, blockages may occur i) during early development of embryos that are arrested at the globular stage, mitoses stop and embryos become brown or are covered by de novo callus overgrowth, ii) during the transition from torpedo to cotyledonary phase embryos display formation of fused cotyledons and/or embryos become pale due to a lack of accumulation of storage compounds, and iii) at the latest stages leading to rooting, when only the aerial part of somatic embryos develops and they have to be transferred for rooting where, in unsuitable media, callus proliferates instead and plantlets lack vascular connection between the root and shoot portions and die upon transfer to in vivo. Somatic embryogenesis can be triggered by growth regulators in the medium (auxin or sometimes cytokinin alone, or a combination of both), but it has been more and more frequently shown that stress agents (ethylene, active charcoal, cold or heat shocks, osmotic, electricity, sonication, centrifugation, etc.) can also favour the induction and development of somatic embryos in many species. This contribution addresses the relationships between embryogenesis and stress and their impact on the development of novel genotypes more apt for a sustainable agriculture.