INFECTION OF SOMATIC EMBRYOS OF TALL FESCUE WITH ACREMONIUM-COENOPHIALUM

The availability of a tall fescue (Festuca arundinacea Schreb.) clone whose members are infected with different strains of its endophytic fungus, Acremonium coenophialum Morgan-Jones & Gams, would help identify the individual contributions of the endophyte and tall fescue toward traits normally associated with endophyte-infected tall fescue plants. As embryos and very young seedlings are the only stages of the life cycle during which infection by the endophyte naturally occurs in planta, attempts were made to duplicate this phenomenon in vitro. Callus cultures of tall fescue were induced using leaf basal tissue as the explant source, and Murashige and Skoog (MS) basal medium supplemented with the synthetic auxins 2,4-D [(2,4-dichlorophenoxy)acetic acid], 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid], or picloram (4-amino-3,5,6-trichloropicolinic acid). Two tall fescue genotypes were exposed to four concentrations (15, 30, 45, or 60-mu-M) of each auxin. Calli were transferred to hormone-free basal medium to permit the development of somatic embryos. Elongating shoots were counted to determine the effectiveness of each treatment and transferred to fresh medium to permit their development into plants. The most effective and consistent auxin concentration to stimulate somatic embryogenesis was 2,4-D at 30-mu-M, producing almost 14 embryos per callus. Next, 60 explants of one fescue genotype were placed on MS medium supplemented with 30-mu-M 2,4-D. Two isolates of the endophytic fungus were used to infect the explants and developing calli by inoculating at weekly intervals. The highest infection rate was obtained by inoculating somatic embryos with the fungus at the time the callus was transferred to hormone-free medium, the stage at which somatic embryos begin to elongate and germinate. This stage corresponds with the stage at which infection is thought to occur in planta. This method will permit further studies of the interactions between tall fescue and its endophyte.