We characterized SCARECROW-LIKE genes induced by auxin in rooting-competent cuttings of two distantly related forest species (Pinus radiata D. Don and Castanea sativa Mill.) before the activation of cell division that results in adventitious root formation. The predicted protein sequences contain domains characteristic of the GRAS protein family and show a strong similarity to the SCARECROW-LIKE proteins, indicating conserved functions of these proteins. Quantitative RT-PCR analysis showed that these genes are expressed at relatively high levels in roots. Induction of increased mRNA levels in rooting-competent cuttings of both species in response to exogenous auxin was observed within the first 24 h of the root induction process, a time when cell reorganization takes place, but before the resumption of cell division and the appearance of adventitious root primordia. These results suggest that SCARECROW-LIKE genes play a role during the earliest stages of adventitious root formation.
The possibility of regenerating whole plants from somatic differentiated cells emphasizes the plasticity of plant development. Cell-type respecification during regeneration can be induced in adult tissues as a consequence of injuries, changes in external or internal stimuli or changes in positional information. However, in many plant species, switching the developmental program of adult cells prior to organ regeneration is difficult, especially in forest species. Besides its impact on forest productivity, basic information on the flexibility of cell differentiation is necessary for a comprehensive understanding of the epigenetic control of cell differentiation and plant development. Studies of reprogramming adult cells in terms of regulative expression changes of selected genes will be of great interest to unveil basic mechanisms regulating cellular plasticity.
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