Experimental Induction of a Normal Morphological Phenocopy of an Inflorescence in a Zea. Mays L. Mutant

FASCICLED EAR (Fa}, a dominant ZEA MAYS L. Mutant effecting the form of the inflorescence, was treated with aqueous solutions of a-naphthaleneacetic acid, indoleacetic acid, gibberellic acid, maleic hydrazide and triiodobenzoic acid every three days from the seedling stage until tassel emergence. The mutant responded to a-naphthaleneacetic acid and indoleactic acid in such a manner that certain treated plants developed a normal-appearing ear and tassel. The idea that an ear and tassel shoot meristem is a "plastic" system genetically programmd at specific points in development through a hormonal balance is discussed. Understanding the development of a multicellular organism is a major challenge to biologists today (Stebbins, 1964). The sequence of morphological changes that occur as a zygote is transformed into a complete multicellular organism appears to be an ordered pattern of events. To understand the processes underlying these morphological changes we must be prepared to investigate the relationships between form, metabolism and hereditary information (Stebbins, 1965). Apical meristems in plants are generally in a sustained embryonic condition. Morphogenesis continues in an active shoot apex thus providing an excellent experimental system for the study of plant development. A special problem which has attra:cted a number of investigators is the transformation of a vegetative shoot apex to a reproductive shoot apex. Various experimental techniques may disturb the normal metabolic activities of apical shoot meristems. For example, surgical operations (Snow and Snow, 1947; Wardlaw, 1959, 1963; Ball, 1960, 1963), x-ray treatments (Stein and Steffensen, 1959), tissue culture (Petru and Retovsky, 19 5 7), and application of natural and synthetic growth substances (Phinney, 1956, 1961; Nickerson, 1960; Anderson, 1963), indicate that many cells in an embryonic condition can be deflected from their normal destiny. Another useful technique has been to substitute a mutant allele for its normal counterpart. Suitability of mutant genes for expc"mental studies of develop1nent was aptly explained by Phinney and West ( 1960). Experimental alteration of events leading to normal development of Zea mays L. inflorescence by gene substitution has lead to the idea that an indispensable relay system of gene action must occur at specific stages in maize inflorescence development (See Postlethwait and Nelson's Figure 1, 1964). A similar hypothesis was also p-oposed by Heslop-Harrison ( 1961) on the basis of studies involving inflorescense development of Zea mays. lUniversity of Northern Iowa, Cedar Falls, Iowa 50613. 69 1 Orr: Experimental Induction of a Normal Morphological Phenocopy of an Published by UNI ScholarWorks, 1968 70 IOWA ACADEMY OF SCIENCE [Vol. 75 It seems possible that if a relay system of gene action does underly morphological events in the development of a maize inflorescence it might, in part, function through a hormonal system. The substitution of a mutant gene may conceivably alter the hormonal balance to the extent that a mutant inflorescence would arise. Therefore, sustained treatment with certain growth substances might modify the expression of mutant genes so that treated plants would develop morphologically normal inflorescences .. MATERIALS AND :METHODS Mutant (Fascicled Ear) ZEA mays L. seeds were planted and grown under a 16-hour photoperiod during 1962-1964 in the greenhouse of the Department of Biological Sciences, Purdue University. Treatments of various growth substances (Table 1) were initiated to 13-day-old seedlings and maintained throughout the early development of both the tassel and the ear. Growth substances were applied every three days above the growing point by placing a 0.2 ml. molar solution in the central leaf whorl. Control seedlings were simultaneously treated with 0.2 ml. of an aqueous solution lacking the growth substances. Fifteen tassels per treatment were collected and the data summarized by the Duncan analysis of variance (Steel and Torrie, 1960). Appropriate samples of both tassels and ears were photographed to illustrate effects of the growth substances on the phenotypic expression of the Fascicled Ear gene.