THE HOBBIT GENE IS REQUIRED FOR FORMATION OF THE ROOT-MERISTEM IN THEARABIDOPSIS EMBRYO

In Arabidopsis, the root meristem originates from the hypophyseal cell and from an adjoining cell tier that is distinct at the heart stage o f embryogenesis. We have analysed mutations in the HOBBIT (HBT) gene t hat is essential for root meristem formation, hbt embryos display inco rrect hypophyseal cell development from the quadrant stage onward. At the heart stage, the adjoining cell tier of hbt embryos develops abnor mally, in that the activation of cell division and the formation of a lateral root cap layer are disturbed. Strong hbt mutants give rise to seedlings that lack an anatomically recognisable quiescent centre and differentiated columella root cap cells, the cell types derived from t he wild-type hypophysis. Furthermore, they have no mitotically active root meristem and lack a differentiated lateral root cap. Secondary ro ots of hbt mutants and roots obtained from cultured cells of hbt mutan ts have similar defects. Therefore the HBT gene is required for root m eristem formation in different developmental contexts.