CHARACTERIZATION OF DEVELOPMENT IN MAIZE THROUGH THE USE OF MUTANTS. II. THE ABNORMAL GROWTH CONDITIONED BY THE KNOTTED MUTANT

The maize mutant Knotted (Kn) is characterized by hollow, finger-like outgrowths (knots) occurring mainly in the leaf blade. Portions of the ligule are displaced from the normal position to more distal locations within the blade. Knots apparently result from continued meristematic activity of isolated patches of cells surrounded by maturing tissue. Small knots appear to be centers of cell division. Epidermal cells overlying a small knot have been observed to undergo periclinal divisions. In addition to cell division, a reorientation of the axis of cell elongation is associated with knot formation. The pattern of knot distribution varies at different levels on the plant axis and within a leaf blade. From leaf 4 to leaf 10 or 11 the number of knots per leaf increases progressively, then declines in leaves initiated later. Knots always occur in association with lateral veins. The greatest number per vein occurs on the 3rd or 4th vein from the midrib. One plant developing from an X-rayed heterozygous seed possessed a sector of normal tissue bisecting the plant in a vertical plane passing through the midrib of each leaf except the top two. The normal sector was knot-free and had the ligule restored to the normal position. These observations suggest that cells with the characteristics of those from intercalary meristems occur throughout the blade in Knotted plants. DEVELOPMENT can be profitably studied using genetic variants from the normal course. Analyses of such mutants, although providing no information concerning the time of gene action, reveal those developmental stages at which specific gene expression is essential for continued normal development (Postlethwait and Nelson, 1964). The Kn mutation is a dominant allele located on chromosome 1 of maize (Bryan and Sass, 1941). The leaves of Kn/Kn or Kn/+ plants have numerous protuberances called knots extending mainly from the abaxial surfaces (Fig. 1). Superficially, the appearance suggests a number of tumors distributed over the leaf. However, the knots are hollow folds of tissue resembling the fingers of a glove (Fig. 2, 3). In mutant plants centers of intercalary mernstematic activity occur along the veins of the lamina and apparently remain active throughout the growth of the blade. The continued meristematic activity and the physical restraint of the I Received for publication 3 January 1969. This work was supported in part by a National Science Foundation grant (G-16036, S. N. Postlethwait), an institutional grant from the American Cancer Society (O. E. Nelson), and an N. D. E. A. Fellowship (D. Gelinas). It is based in part on portions of a thesis submitted to the Department of Biology, Purdue University, by the senior author in partial fulfillment of the requirements for the Ph.D. degree. The authors gratefully acknowledge the photographic assistance of Evart Cable. 2 Present address: Department of Botany and Plant Pathology, University of Maine, Orono, Maine 04473. surrounding non-meristematic tissue give rise to the protuberances of tissue forming the knots. Analyses of the distribution of knots, the displacement of the ligule, and the frequent absence of the intercalary meristem from its normal position suggest an homology between the intercalary meristem and the knots. This apparent homology is consistent with the patterns of development seen in an X-ray-induced mosaic of normal tissue in a Knotted plant. MATERIALS AND METHODS-The Kn allele was introduced into the inbred strain HY, then maintained by sib-crossing. Material was collected from plants grown under greenhouse or field conditions. Samples for histological study were collected at various intervalsfrom leaves in all stages of development and at various levels on the plant. Data on leaf size and distribution of knots on the leaf blade were collected from 10 plants in each of three groups identified on the basis of phenotype as Kn/Kn, Kn/+, and +/+. Heterozygous seed selected for X-irradiation was exposed prior to imbibition to 2,000 R, then grown in pots in a greenhouse. Material for histological study was either mounted whole in a small amount of immersion oil or killed and fixed in formalin-acetic acidalcohol, dehydrated by a tertiarv butyl alcohol series, embedded in Tissuemat, sectioned, and stained with safranin and fast green (Johansen, 1940).