Histopathology of Seed-Borne Infections - Applied Research Center ...

Location of Fungal Hyphae in Seeds 151mycelium. At maturity such panicles bear seeds of all kinds from completely abortedto apparently sound seeds that germinate normally (Western and Cavett, 1959).Cyperus virens plants infected by the endophyte, Balansia cyperi, produceaborted inflorescence covered with fungal stroma. The stromatic tissue bears abundantconidial fructifications and, more rarely, ascostromata. Viviparous plantlets areoccasionally produced on the aborted panicles of infected plants. The plantlets arealso infected by B. cyperi (Clay, 1986). Sampson and Western (1954) have alsoreported that when Poa bulbosa, a viviparous grass, is infected by E. typhina, thebulbils are also infected. Clay (1986) considers the induced vivipary in C. virens torepresent a mechanism of vegetative reproduction wherein host and fungus aredispersed simultaneously by the same propagule.5.6.2 NONSTROMATIC INFECTIONSSymbiotic or mutualistic associations between the endophyte and the host usuallydo not affect the sexual reproduction, and viable seeds are produced. Although fungalsexual stages occur in many of these endophytes, it is their anamorph stage that isusually encountered in host tissues. Sampson (1935) traced Epichloe typhina in allparts of flowers — palea, lemma, lodicules, stamens, and young ovaries — of Festucarubra. The fungus invades the ovule and embryo sac (Figure 5.19A), and in seed itoccurs in the pericarp, endosperm including aleurone layer, and, rarely, in embryo(Figure 5.19 B to F). Thus in F. rubra (red fescue) viable, but often infected, seedsare produced. On germination these seeds produce seedlings with mycelium in theplumule (Figure 5.19G, H). The occurrence of endophyte mycelium in seeds ofseveral other grasses has been demonstrated (Table 5.7).Philipson and Christey (1986) have provided an elegant account of the relationshipof host and endophyte mycelium during flowering, fruiting, and seed germinationin Lolium perenne and Festuca arundinacea. The endophyte progressesintercellularly from the vegetative apex into the inflorescence primordium, floralapices, ovary, and ovule. The mycelium aggregates outside the embryo sac wall(Figure 5.20A) and subsequently penetrates it. It has been observed in antipodalcells. During early embryogenesis, mycelium occurs on the surface of the embryoand penetrates it at the notched stage. At seed maturity, hyphae are widespread withinthe embryo, including the plumule apex (Figure 5.20C, D) and in the pericarp,aleurone layer, and the space between them (Figure 5.20B).5.6.3 VIABILITY OF MYCELIUM IN SEEDEndophyte viability in infected seed declines as the seed ages (Latch and Christensen,1982; Siegel et al., 1984b). Siegel, Latch, and Johnson (1985) believe that mostendophyte-infected seeds, which have been stored in warehouses for 2 years, containlittle or no viable endophyte. Endophyte viability in infected seeds of tall fescue islost after 7 to 11 months of storage at 21°C (Siegel et al., 1984b), but at lowtemperatures and low humidity it is retarded. The perennial rye grass seed infectedby the endophyte and stored at 0 to 5°C and near-zero humidity contained livingendophyte mycelium for 15 years.