Comparison of Promeristem Structure and Ontogeny of Procambium in Primary Roots of Zea mays ssp. Mexicana and Z. mays "Honey Bantam' with Emphasis on Metaxylem Vessel Histogenesis

Classical histology describes the histological organization in Zea mays as having a closed organization that differs from Arabidopsis with the development of xylem conforming to predictable rules. We speculated that root apical meristem organization in a wild subspecies of Z. mays (a teosinte) would differ from a domestic sweetcorn cultivar (Honey Bantam'). Careful comparison could contribute to understanding how evolutionary processes and the domestication of maize have affected root development. Root tips of seedlings were prepared and sectioned for light microscopy. Most sections were treated with RNase before staining to increase contrast between the walls and cytoplasm. Longitudinal and serial transverse sections were analyzed using computer imaging to determine the position and timing of key xylem developmental events. Metaxylem development in mexicana teosinte differed from sweetcorn only in that the numbers of late-maturing metaxylem vessels in the latter are typically two-fold greater and the number of cells in the transverse section of procambium were greater in the latter, but parenchymatous cell sizes were not statistically different. Promeristems of both were nearly identical in size and organization, but did not operate quite as previously described. Mitotic activity was rare in the quiescent centers, but occasionally a synchronized pulse of mitoses was observed there. Our reinterpretation of histogen theory and procambium development should be useful for future detailed studies of regulation of development, and perhaps its evolution, in this species.