IS THE PREDOMINANT PERIOD OF CELL ARREST AND PRESENCE OF ENDOREDUPLICATED CELLS COINCIDENT WITH PRODUCTION OF SECONDARY VASCULAR TISSUES IN INTACT AND CULTURED ROOTS OF RAPHANUS SATIVUS L.

Experimental results show that predominant cell arrest in G2 and the presence of endoreduplicated cells are coincident with presence of secondary vascular tissues while preponderant cell arrest in GI and absence of polyploid cells are coincident with an absence of secondary vascular tissues in mature root tissues of intact and cultured roots of Raphanus sativus L. In mature tissues of intact seedling roots, most cells arrest in G2, and both polyploid cells and secondary vascular tissues are present. If excised roots are grown on simple medium, most mature cells arrest in G 1, none undergo endoreduplication, and only primary vascular tissues are present. When bases of these excised roots are later placed in a medium with auxin, cytokinin, and myo-inositol that produces secondary vascular tissues in vitro, preponderant cell arrest occurred in G2 with some polyploid cells. The general relationship of predominant period of cell arrest, presence of polyploid cells, and presence of secondary vascular tissues in mature roots among plants of various taxa is surveyed. ROOT MORPHOGENESIS and maturation at the root meristem have presented researchers with a proliferative cell population in which cells differentiate to produce widely divergent cell types both structurally and functionally. Patterns of cell differentiation are observed in transverse tissue sections at various distances from the proliferative cell population (Williams, 1947; Heimsch, 1951; Popham, 1955; Jensen and Kavaljian, 1958). Many cell types in roots retain a potential to proliferate some distance from the meristem and to produce new tissues. For example, pericycle cells proliferate to produce secondary roots, vascular cambium cells proliferate to produce secondary vascular tissues; adventitious shoots may develop from root tissues. These examples illustrate the potential of specific root cell types to proliferate in response to intracellular chemical and/or physical influences. Specific targets for initiation of cell cycle activity remain obscure, but evidence is available to suggest that cycle controls for continued proliferation are located in GI and G2 [See Prescott (1976) for review]. Loomis and Torrey (1964) demonstrated that particular chemicals in appropriate concentrations could stimulate cell proliferation in the vas1 Received for publication 1 April 1977; revision accepted 10 June 1977. Research supported in part by Grant #1-ROl-GM23232-01 from the National Institutes of General Medical Sciences. The authors gratefully acknowledge the use of a computerized microspectrophotometer at Brookhaven National Laboratory by Dr. J. Van't Hof, and the excellent technical assistance of Sophia Almeida. cular cambium of excised roots of radish to produce secondary vascular tissues in vitro. This developmental system has been described in great detail (Torrey and Loomis, 1967a, b; Radin and Loomis, 1969; Webster and Radin, 1972). The applied constituents initiate cell proliferation in the vascular cambium, specifically, and the latter subsequently produces both secondary xylem and phloem in a manner similar to intact roots. In another experimental technique, Van't Hof, Hoppin, and Yagi (1973) and Evans and Van't Hof (1974) demonstrated that cells in intact roots of Vicia faba and Pisum sativum normally arrest in G2 (with a 4C amount of DNA/nucleus), but when roots are excised and grown in aseptic culture, cells arrest predominantly in GI. Moreover, excised roots of V. faba and P. sativum produce only primary vascular tissues in vitro (Torrey, 1963; personal observation), while intact roots produce secondary vascular tissues. The experiments described in this paper integrate elements of the cell cycle with proliferation of the vascular cambium and subsequent morphogenesis in excised roots of R. sativus. Two questions are asked. First, is there a correlation of predominant cell arrest in G1 and G2 and presence of polyploid cells with stimulation of the vascular cambium in excised roots of R. sativus? Second, is the nuclear DNA content of mature root cells prescribed in meristems of experimentally manipulated roots of R. sativus? A general relationship between predominant period of cell arrest, presence of polyploid cells, and pres-