In Arabidopsis, the root meristem originates from the
hypophyseal cell and from an adjoining cell tier that is
distinct at the heart stage of embryogenesis. We have
analysed mutations in the HOBBIT (HBT) gene that is
essential for root meristem formation. hbt embryos display
incorrect hypophyseal cell development from the quadrant
stage onward. At the heart stage, the adjoining cell tier of
hbt embryos develops abnormally, 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
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differentiated columella root cap cells, the cell types
derived from the wild-type hypophysis. Furthermore, they
have no mitotically active root meristem and lack a
differentiated lateral root cap. Secondary roots of hbt
mutants and roots obtained from cultured cells of hbt
mutants have similar defects. Therefore the HBT gene is
required for root meristem formation in different
developmental contexts
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