STUDIES ON THE LIFE CYCLE AND TAXONOMY OF LIGNIERA VERRUCOSA

MILLER, CHARLES E. (A. and M. College of Texas, College Station.) Studies on the life cycle and taxonomy of Ligniera verrucosa. Amer. Jour. Bot. 46(10): 725-729. Illus. 1959.-A study of the roots of Veronica persica Poir. and V. hederaefolia L. plants infected with Sorosphaera veronicae Schroeter revealed intracellular cystosori and zoosporangial sori of Ligniera verrucosa. The zoosporangial phase of this species has been heretofore unknown. The plasmodia of L. verrucosa occur in root hairs, and other epidermal and sub-epidermal cells of the roots. Zoosporangial and cystosoral plasmodia are indistinguishable until cleavage has started. It is thought that plasmodia produced during early infection develop into zoosporangia, while those produced later develop into resting spores. Zoospores discharged from zoosporangia may reinfect host cells developing there into zoosporangial or cystosoral plasmodia. No evidence for any sexual process was observed. The spherical zoosporangia making up a single zoosporangial sorus may be interconnected; a single discharge pore may serve to liberate zoospores from different zoosporangia. In the Plasmodiophorales the classical basis for generic distinction has been the arrangement of the resting spores in the sorus. Ligniera, because of the supposedly uncharacteristic nature of its cystosori, has been suggested as a host-variety of Sorosphaera. A comparative study of the cystosori and zoosporangia of Ligniera and Sorosphaera growing in a single host has led to the conclusion that these genera should be considered distinct. WHILE SURVEYING the roots of Veronica persica Poir. and V. hederaefolia L., host plants of Sorosphaera veronicae Schroeter heavily infected with cystosori which commonly occur in enlargements of theshoot system,the writer (1958b) foundcystosori and zoosporangial sori of the genus Ligniera Maire and Tison (1911). Further studies indicated that it was L. verrucosa Maire and Tison, a species in which the zoosporangial phase has not previously been found. Because of this, and also since the validity of the species and the genus has been questioned (Palm and Burk, 1933), it seemed opportune to make a study of this parasite. DEVELOPMENT OF THE PARASITE.-The following description of L. verrucosa is made from living and preserved material of the parasite in the roots of Veronica persica and V. hederaefolia. Prior to this report, the species was described as a parasite of the root hairs and roots of V. arvensis L., Beta vulgaris L., Chenopodium album L., Bromus L., and Festuca L.3 The parasite apparently completes its life cycle in the root hairs, other epidermal cells and sub-epidermal cells of the roots of the host (fig. 1, 3, 8). As is typical of Ligniera, no hypertrophy of the host cells occurs. The zoospore after penetrating the root cell develops into a multinucleate plasmodium which may fill the host cell (fig. 2, 21). No empty cysts are found after the zoospores penetrate the host cells. Zoosporangial and cystosoral plasmodia are in1 Received for publication May 2, 1959. 2 Initial studies forming a basis for this paper comprised a part of a Ph.D. dissertation submitted to the Faculty of the University of North Carolina. The author is grateful to Dr. John N. Couch for his guidance. The completion of these studies was supported by a grant from the McCandless Fund of Emory University. 3See J. S. Karling, 1942, The Plasmodiophorales, p. 62. distinguishable until sometime after cleavage has started. It is possible that a situation similar to that described by Couch et al. (1939) for Octomyxa achlyae Couch, Leitner, Whiffen occurs. These workers found that the early-formed plasmodia may develop into zoosporangia, while those formed later become cystosori. The protoplasm of the zoosporangial plasmodium after cleavage into the spherical zoosporangia, which are surrounded by a delicate cell wall, has a lumpy aspect (fig. 7, 11). The protoplasm gradually becomes more granular, and a central vacuole forms which separates the protoplasm into the zoospore initials (fig. 10, 18). The zoosporangia in a sorus may be interconnected (fig. 5, 17, 19). Consequently a single discharge pore may serve to liberate the zoospores from several zoosporangia. A similar condition has been described for the zoosporangia of other genera of the Plasmodiophorales (Couch et al., 1939; Goldie-Smith, 1951; Miller, 1958a,b, 1959). The walls of the zoosporangia are very thin, and they may collapse soon after the zoospores discharge (fig. 3, 5, 8, 17, 20). Figure 20 illustrates zoosporangia which have discharged their zoospores within the host cells. The zoospores lost their flagella and became slightly amoeboid after 2 hr. They were not seen to fuse or to develop further. Perhaps this was because of the poor environmental conditions which necessarily existed. Zoosporangia measured 4-9M (most 6.6,u) in diameter. Zoospores from zoosporangia of Ligniera junci (Schwartz) Maire and Tison have been reported as being uniflagellate by Cook (1928, 1933). However, in the light of Ledingham's (1934, 1935, 1939) discoveries of biflagellate heterokontean zoospores in Plasmodiophora Woronin, Spongospora Brunchorst and Polymyxa Ledingham and the subsequent dis-