Wheat Kernel Hormone Levels During Development and Their Relevance to Zygotic and Somatic Embryogenesis

Wheat (Triticum aestivum L.) zygotic embryogenesis occurs in a dynamically regulated ovular environment, and in ovulo hormones regulate embryogenic processes. Levels of ABA, IAA, and the cytokinins Z, ZR, DHZ, DHZR, iP, and iPA were studied in developing wheat kernels from anthesis to maturity. High cytokinin and low IAA and ABA levels were associated with the early stage of embryo formation and active tissue histodifferentiation. Following histodifferentiation, cytokinin levels declined while IAA accumulated throughout the stage of active grain growth and then declined with grain maturity. ABA levels increased at the soft-dough developmental stage and through to grain maturity. Endogenous +ABA levels in developing wheat grains treated with fluridone, which indirectly blocks ABA synthesis, declined at the soft-dough stage. As a result, mature desiccated fluridone-treated kernels exhibited little dormancy. However, fluridone-treated kernels were not viviparous, suggesting a strong caryopsis-embryo interaction in maintaining embryogenically competent tissues. Induction of embryogenically competent wheat callus cultures was highly variable between genotypes and pre-initiation environments. Genotypic and environmental influences altered endogenous hormone levels and affected embryogenic competence. Establishment of competent embryo explants for somatic embryo induction was favored by a high cytokinin-to-auxin ratio and very low ABA levels throughout histodifferentiation (around 4 to 8 DPA). Similar to embryos forming within the caryopses, competent callus cultures had a high cytokinin (Z)-to-auxin ratio at 7 DPI. Increased frequency of embryogenic cultures was achieved when embryo explants were excised during a narrow window of low hormone levels. Wheat line and pre-initiation environment affected this window. Simulation in vitro of the in ovulo wheat kernel environment improved zygotic embryogenesis in vitro . Embryos exposed to physiologically normal ABA levels and low 0 2 tensions of 2.5 mM (7%) most closely approached morphological and physiological normalcy. The culmination of these studies clearly defines windows of embryo development for explant excision, associated roles of plant hormones in embryogenesis, and in ovulo hormone levels that vastly improve the frequency of successful embryogenesis when simulated in in vitro culture systems