GROOT, E.P.1*, S.A. NICHOL1, J.A. DOYLE2, and T.L. ROST1. 1Section of Plant Biology, University of California, Davis, CA 95616; 2Section of Evolution and Ecology, University of California, Davis, CA 95616. - Phylogenetic relationships and root apical meristem organization in the dicots.
There are two basic types of root apical meristem (RAM) organization
in dicotyledonous angiosperms: open and closed. In taxa with closed
organization, longitudinal cell files terminate apically into distinct
tiers of initials sometimes called histogens. In RAMs with open
organization, the cell files terminate in a zone of initials with
uncertain cell lineage. An intermediate type of open organization has
specific initials for the epidermis and peripheral root cap cell
lineages, but the other histogens do not appear to be clonally
distinct. In a survey of one-week-old seedlings grown in sand, about
half of the dicot families represented had closed organization.
One-third of the families had the intermediate type of open
organization, and about 10% of the families were mixed. Only
Cucurbitaceae and Fabaceae had the true open type. Mapping RAM
organization on recent phylogenies based on DNA sequences of several
genes indicated that the ancestral state in angiosperms was the
intermediate type of open organization, and the closed type originated
in the eudicots, above Ranunculales. Several clades in the eudicots
showed reversals to the more primitive, intermediate open type,
resulting in some taxa containing a mix of closed and intermediate
open members. What particular ecological adaptations these RAM
organizations represent remains to be investigated.
Key words: angiosperms, eudicots, phylogeny, plant anatomy, root apical meristem