THE ROLE OF AUXIN IN THE DIFFERENTIATION OF ROOT AND SHOOT PRIMORDIA FROM ROOT CUTTINGS OF TARAXACUM AND CICHORIUM

THE PROBLEM of differentiation-of how the mature organism, with its scores of specialized organs, tissues, and cells, develops from a single fertilized egg-has long been a nettling perplexity to the biologist. The geneticist, particularly, has been challenged; on the basis of Mendelian laws and genes, he has been confronted with the paradox of trying to account for the differences in cells of an organism, all of whose nuclei have identical2 complements of chromosomes and genes. Certain of the milky-juiced composites of the genera Taraxacum and Cichorium have provided material for such important early researches on regeneration and differentiation as those of Wiesner (1892), Goebel (1902, 1905, 1908), Kiister (1903), Nemec (1908, 1924, 1928), and many others. The fleshy tap-roots of these plants possess remarkable powers of regeneration. These may be cut into segments a centimeter or more in length and, within a period of 1-2 weeks, shoot meristems with their rosettes of leaves will differentiate from callus tissue formed on the proximal cut surfaces; callus, and sometimes roots, will develop on the surfaces of the lower, or distal, ends. At the moment any single cut is made, the distal end of one root segment and the proximal end of the next below-being the two faces of a single incision-must be very similar in all respects. Yet, when allowed to lie on moist filter paper in a covered vessel for several days, the behavior of these two surfaces is strikingly different; the cells of one produce a shoot primordium and leaflets, while those of the other produce callus tissue and roots (fig. la,b). It would appear that cutting disturbs the physiological unity of the root and causes a redistribution of some substance or substances, which in turn influences the direction of differentiation of the callus tissues formed on the two cut surfaces. The important earlier works of Czaja (1931, 1935), Fischnich (1938, 1939), Plant (1940, 1941), Howard (1938, 1940), Prokofiev (1944), and the recent work of Gautheret and his co-workers (1946) suggest strongly that the inherent polarity of the root is responsible for the redistribution of the substance, and most likely, that this substance is auxin. It was the purpose of the present work (1) to experimentally alter the concentration of auxin at the two ends of root segments, to determine if the 1 Received for publication August 16, 1949. The authors were members of the staff of the Institute of Tropical Agriculture of the University of Puerto Rico during the major portion of this investigation. 2 Some notable exceptions have been pointed out recently (Huskins, 1947, 1948; Huskins and Steinitz, 1948). direction of differentiation could thus be altered, and (2) to assay the proximal and distal cut surfaces of untreated segments for auxin during normal regeneration to determine if differences in auxin concentration at the two ends could be detected. A preliminary report of this work has already been made (Warmke and Warmke, 1946, 1947). EFFECT OF ALTERING AUXIN CONCENTRATIONS.Materials and methods.-Roots of a single plant of dandelion, Taraxacum oflicinale Weber (selected for strong regenerative capacity), and of one of chicory, Cichorium Intybus L. (similarly selected), were increased vegetatively until approximately