Initiation and Development of Root Nodules of Casuarina (Casuarinaceae)

Roots of seedlings of the "beefwood" tree, Casuarina cunninghamiana Miq. grown in nitrogenfree nutrient solution were inoculated with a suspension prepared from crushed root nodules taken from mature plants. Marked deformation of root hairs was evident but no infection threads were observed in root hairs. The mode of infection remains undetermined. Root nodules were initiated within three weeks and thereafter numerous upward-growing nodule roots developed from each nodule. Nodules in this symbiotic nitrogen-fixing plant resulted from an infection caused by an unidentified actinomycete-like soil microorganism. Anatomical analysis of nodule formation showed that nodules are the result of repeated endogenous lateral root initiations, one placed upon another in a complexly branched and truncated root system. The endophyte-infected cortical tissues derived from successive root primordia form the swollen nodular mass. Nodule roots develop from nodule lobes after escaping from the initial inhibitory effects of the endophyte. Included is a discussion of the anatomical similarities between nodules of Casuarina which produce nodule roots and those of Alnus which form coralloid nodules usually lacking nodule roots. CASUARINA (Casuarinaceae) is one of the genera of non-leguminous angiosperms whose species are capable of forming root nodules in response to invasion from the soil by an actinomycete-like organism. The genus comprises some 45 species, most of them trees inhabiting the Old World tropics and Australia, but present as introductions in many other areas, including Florida and California. Evidence that the nodules are nitrogenfixing has been provided by Aldrich-Blake (1932), Mowry (1933) and Bond (1957a, b). The root nodules of Casuarina are of complex morphology, comparable to those of Alnus, Myrica and several other genera. From a single infection point in a root there can be formed eventually a spherical mass several cm in diameter composed of close-packed, radiating nodule lobes. In Casuarina, as well as in Myrica and Comptonia, fairly normal, uninfected roots ("noduleroots") emerge from the tips of the swollen noidule lobes. The initiation, development and structure of nodules on roots of nitrogen-fixing non-leguminous angiosperms have been reviewed by Bond 1 Received for publication 24 February 1975. This study was initiated in the laboratory of Professor G. Bond, Department of Botany, University of Glasgow, Scotland where the author was an Honorary Senior Research Fellow. Thanks are due to Professor M. B. Wilkins for providing research space and facilities and to Professor Bond for providing plant materials, cultural expertise, enthusiastic and continued interest in the work and careful and critical review and editing of the manuscript. The research has been supported by research grant BMS74-20563 from the National Science Foundation and by the Maria Moors Cabot Foundation for Botanical Research, Harvard University. (1963, 1967) and Becking (1968, 1970a). Nodule formation among these plants has been studied most carefully in species of Alnus. Relatively little detailed information is available concerning the process of nodulation in the genus Casuarina. Miehe (1918) described the general structure of the nodules of Casuarina equisetifolia, and noted its resemblance to that of the nodules of Alnus. Microscopic study of the branching process in the nodule lobes led him to conclude that branching was initiated in the apex by a segmentation of the meristem, though he included no illustrations to support this view. Miehe failed to observe nodule roots in his material. McLuckie (1923), unaware of Miehe's work, made a similar general study of the nodules of C. cunninghamiana. Although he observed thread-like ndophytic structures in newly infected cells, he believed that they were bacteria present in an infection thread. McLuckie paid particular attention to the method of branching of the nodule lobe, and to the manner in which the nodule-roots arose. He reported that after the apical meristem of the nodule lobe had produced a certain amount of elongation, it divided into several distinct groups of cells, commonly four in his material. Of these, a central group produced the nodule-root, whereas the others gave rise to new nodule lobes. He described the branching as due to meristem bifurcation. As will be mentioned later, the branching of the nodule lobes in other genera, e.g., Alnus, has been interpreted by various authors as the result of an apical process of dior polychotomy, though not on the basis of any rigorous study of the phenomenon.