Hybrid embryos of Vicia faba develop enhanced sink strength, which is established during early development

Selfed and crossed seeds of two homozygous Vicia faba lines served as models for the analysis of the physiological and molecular mechanisms underlying embryo heterosis. Profiles of transcripts, metabolites and seed contents of developing embryos were analysed to compare the means of reciprocally crossed and selfed seeds growing on the same mother plants. The mean weight of mature hybrid seeds was demonstrably higher, revealing mid-parent heterosis. Hybrid embryos exhibited a prolonged early phase of development and delayed onset of storage activity. Accordingly, transcript profiling indicates stimulation of cell proliferation, an effect, which is potentially mediated by activation of auxin functions within a framework of growth-related transcription factors. At the transcript level, activated cell proliferation increased assimilate uptake activity and thereby seed sink strength. This situation might finally lead to the increased size of the hybrid seeds. We conclude that hybrid seeds are characterised by accelerated growth during early development, which increases storage capacity and leads to higher metabolic fluxes. These needs are, at least partially, met by increased assimilate uptake capacity. The stimulated growth of hybrid seeds shifted metabolite profiles and potentially depleted available pools. Such metabolic shifts are most likely secondary effects resulting from the higher storage capacity of hybrid seeds, a heterotic feature, which is itself established very early in seed development.