Figure 14.

Hypothetical model about the involvement of the AtCRK5 protein kinase in the coordination of embryo morphogenesis in Arabidopsis thaliana. The auxin transport proteins having central role in the establishment of auxin gradients controlling embryo morphogenesis are potential targets of AtCRK5. The phosphorylation of these transporters by AtCRK5 increases their stability. In the absence of AtCRK5 their level is decreased without affecting their distribution pattern. The auxin gradients therefore are properly established in the Atcrk5-1 mutant but at lower auxin maxima in the corresponding regions. The lower auxin concentration in these regions limits gibberellin synthesis, via the decreased expression of key genes of gibberellin metabolisms. The lower GA level results in lower expression of the genes coding for auxin transport proteins strengthening the phenotype. An auxin-transport-independent effect of the AtCRK5 kinase on gibberellin metabolism cannot be excluded. In summary, in the absence of AtCRK5, embryogenesis progresses at correctly organised but lower auxin and gibberellin levels delaying but not preventing the processes of morphogenesis.