Seeley, K.A., Cheng, J-.C. and Sung, Z.R.
Plant Biology Department, 111 Koshland Hall, University of California, Berkeley, CA 94720
The root meristemless mutant, rml1, has identified a gene required for cell proliferation at the root tip after the root primordia are fully formed (Figure 1). A root meristem is defined as the root apical region containing isodiametrically-shaped, cytoplasmically dense, and actively dividing cells (Cheng et al., 1995). Without cell division a root meristem is absent in rml1 mutants. The formation of a normal root primordium in rml1 mutants suggests that root organogenesis does not begin with the meristem.
In order to fully characterize Arabidopsis root growth in terms of cell division, we analyzed longitudinal seedling sections and found the root-hypocotyl boundary and determined the size of the embryonic root (Cheng et al., 1995). Our measurements indicate that the embryonic root primordium is 17 epidermal and cortical cells in length. When the rml1 mutants were analyzed we determined that the rml1 primary root did not increase the number of cells beyond the 17 cell length. We found that the root apical cells, when arrested in cell division, undergo differentiation as evidenced by the formation of vascular tissue, root hairs and vacuolated cortical cells in the root tip (Cheng et al., 1995).
The arrested mutant root cells can carry out normal root functions, such as, the production of lateral roots from the pericycle. Of interest, these new roots grow to a size of about 17 cells in length, arrest cell division, and differentiate (Figure 1). Similarly, callus initiated from rml1 mutant plants regenerate roots which also arrest cell division at about the 17 cell length (about 1 mm). The fact that both the embryonic root, adventitious root, and lateral roots arrest at this same size suggests that the Arabidopsis root development occurs in two stages, first, the development of the root primordium and, second, the formation of an active meristem allowing growth in an indeterminate fashion.
The unique properties of the rml1 mutant phenotype imply that there are at least two cell proliferation programs necessary at each root development stage. The first program would allow cell proliferation during pattern formation resulting in the 17-cell root primordium. Once this root primordium is formed a second program would activate apical cell division forming an active meristem. This first program would be reactivated each time the lateral roots are initiated. This reactivation would allow the reiteration of the root pattern giving rise to a new root primordia which resembles the structure of the primary root. The activation of the second program, characterized by the expression of the rml1 gene, would give rise to indeterminate growth and result in the activation of the classical meristem only after the root primordium is formed.
The idea of determinate and indeterminate root growth is consistent with the evolution of roots from lower to higher vascular plants. In a variety of lower vascular plants the root (rhizoid) system is more or less determinate (Taylor, 1988) and could correlate to the first stage of higher plant root development. Perhaps the rml1 gene was acquired to allow the indeterminate growth in the rhizoid. In summary, we conclude that the unique properties of the rml1 phenotype point to the possibility that the development of the Arabidopsis root occurs in two stages; the formation of the 17 cell root primordium followed by the activation of a root meristem.
Dolan, L, Janmaat, K, Willemsen, V, Linstead, P, Poethig, S, Roberts, K, Scheres, B. (1993) Cellular organization of the Arabidopsis thaliana root. Development 119, 71-84.
Cheng, J-C, Seeley, KS, Sung, ZR. (1995) rml1 and RML2, Arabidopsis genes required for cell proliferation at the root tip. Plant Physiol. 107, 365-376.
Galway, ME, Masucci, JD, Lloyd, AM, Walbot, V, Davis, RW, Schiefelbein, JW. (1994) The TTG gene is required to specify epidermal cell fate and cell patterning in the Arabidopsis root. Dev. Biol. 166,470-754.
Scheres, B, Wolkenfelt, H, Willemsen, V, Terlouw, M, Lawson, E, Dean, C, Weisbeek, P. (1994) Embryonic origin of the Arabidopsis primary root and root meristem initials. Development 120, 2475-2487.
Taylor, TN. (1988) The origin of land plants: some answers, more questions. Taxon 37, 805-833.