Germination Program in Non-Dormant Seeds: Programming, Saving and Implementation

Review emphasizes the central principle of seed germination program in terms of programming, saving and implementation employing embryo axes of orthodox non-dormant seeds to avoid the complications imposed by dormancy. Program frames are extended from post-fertilization to primary root development. These boundaries are illustrated by origination of root quiescent center after zygotic division and root growth from it during program completion. Programming is controlled by subsequently synthesized cytokinins, auxin and gibberellins, its readiness manifests itself as precocious germination, which is prevented by ABA. ABA terminates proliferation also in embryos, promoting maturation, formation of ABA-responsive stored mRNAs and reserve deposition. Thus its function is over. Program saving is provided by binding mRNA transcripts to nucleoproteins to be then liberated, translated and degraded. Implementation of program is achieved by imbibition, being triggered by inflowing water that results in successive activation of main metabolic systems after overcoming specific hydration thresholds. Hydration-driven translation of stored mRNAs provides wide spectrum of proteins. For safe, partial amplification of germination program concerning ribosomal proteins and initial metabolites is possible, as well as de novo transcription of some metabolites by passing mRNA storage. Preparation for radicle protrusion includes vacuole restoration and tonoplast aquaporin synthesis. For cell-wall polymer modification, hydration-driven activation of plasmalemma H+-ATPase induces acidification of polymers and their extensibility, resulting in initiation of cell elongation and radicle protrusion, in its turn ending with root growth. Germination program does not include shoot growth, but provides competent, safe, reliable and rapid germination, adequate to environmental conditions.