Molecular And Biochemical Role Of Auxin And Cytokinin In Dedifferentiation And Organogenesis Of Arabidopsis
Boyle, John A.
Cell dedifferentiation is a cell fate regression process in which the cell fate memory of a differentiated cell is erased, leading to regain stem cell characteristics. Auxin regulates both cell dedifferentiation and differentiation in plants. It is unknown how auxin controls the two opposite processes. Here the minimal auxin requirements for cell dedifferentiation were found, molecular markers associated with the cell dedifferentiation event were identified. When cellular auxin concentration exceeds the level of meristem cell, most differentiated cells undergo dedifferentiation. In differentiated cells, the polar auxin efflux system prevents cell dedifferentiation by reducing auxin accumulation, particularly in the presence of exogenous auxin. Classic plant tissue culture experiments have shown that exposure of cell culture to a high auxin to cytokinin ratio promotes root formation and a low auxin to cytokinin ratio leads to shoot regeneration. Since the auxin level is highly elevated in the shoot meristem tissues, it is unclear how a low auxin to cytokinin ratio promotes the regeneration of shoots. To identify genes mediating the cytokinin and auxin interaction during organogenesis in vitro, three allelic mutants that display root instead of shoot regeneration in response to a low auxin to cytokinin ratio are identified using a forward genetic approach in Arabidopsis. Molecular characterization shows that the mutations disrupt the AUX1 gene, which has been reported to regulate auxin influx in plants. Meanwhile, it was found that cytokinin substantially stimulates auxin accumulation and redistribution in calli and some specific tissues of Arabidopsis seedlings. In the aux1 mutants, the cytokinin regulated auxin accumulation and redistribution is substantially reduced. These results suggest that auxin elevation and other changes stimulated by cytokinin, instead of low auxin or exogenous auxin directly applied, is essential for shoot regeneration. In this study, as a part of interaction between auxin and cytokinin it was identified that the induction of ARR5 and ARR6 expression by cytokinin is subjected to the regulation of auxin. The expression of ARR5 and ARR6 follows a mutual exclusive pattern in response to the induction of exogenous auxin in Arabidopsis seedlings and calli. The results suggest that auxin interacts with the cytokinin via a gene and tissue specific induction of the negative regulators in the cytokinin signaling pathway.