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

Location of Fungal Hyphae in Seeds 149Singh, 1998). In sunflower, infection is common in the pericarp and endosperm, butrare in the embryo (Raut, 1983; Godika, Agarwal, and Singh, 1999).Singh and Singh (1979) observed that M. phaseolina causes usually symptomaticand rarely symptomless infection in sesame seeds. In symptomatic seeds, profusethick, knotty, dark brown to black, and septate mycelium and also microsclerotiawere seen in all components and also between the spaces among the components(Figure 5.16C to F). Weak infection induced cell divisions in the endosperm andembryo, quite characteristic in the palisade cell region of cotyledons (Figure 5.3A,B). Heavy infection resulted in abundant aggregation of mycelium and microsclerotiain seed components and depletion of food contents (Figure 5.16F).Hedgecock (1904) first reported Rhizoctonia solani to be internally seed-bornein beans. Mycelium and sclerotia were present in the seed coat of seeds obtainedfrom pods in contact with soil. Baker (1947), who examined seeds of Capsicumfrutescens from rotted fruits that were in contact with soil, found R. solani asmycelium and sclerotia on the surface of the seed coat and as mycelium in theremnants of the funiculus, seed coat, endosperm, and embryo, particularly at the tipof the radicle. Chitkara, Singh, and Singh (1986b) observed R. solani mycelium andmicrosclerotia in the seed coat and endosperm in seeds of C. annuum obtained fromfruits borne on plants above the soil surface.5.6 ENDOPHYTESEndophytic fungi constitute an interesting group present in conifers to grasses(Alexopoulos, Mims, and Blackwell, 1996). Taxonomically they belong to Hypocreales(Clavicipitaceae), Loculoascomycetes, and Inoperculate Discomycetes(Rhytismatales). The term endophyte has been defined as an organism contained orgrowing entirely within a plant, parasitically or symbiotically (Snell and Dick, 1971;Siegel, Latch, and Johnson, 1987). The endophytic fungi, which are seed-borne andbelong to the tribe Balansiae of the Clavicipitaceae (Diehl, 1950; Siegel, Latch, andJohnson, 1987), are considered here. The infection is systemic, and nonhaustorialand intercellular mycelium occur in all parts of the affected plant except roots.The presence of endophytic mycelium in seed of a grass (Lolium temulentum)was first reported by Vogl (1898). Sampson (1933, 1935, 1937) showed the maternaltransmission of the endophyte. This has been confirmed on E – ¥ E + pair crosses byNeill (1940, 1941), Siegel et al. (1984a, 1985), and Manuel and Do Valle (1993).The effects of the fungal endophytes on the reproductive cycle of their hosts vary,ranging from complete sterility to complete fertility. There is an inverse correlationof host and fungus sexuality. In parasitic associations, the fungus is sexual (producesboth ascospores and conidia) and the host is asexual or with serious derangementsin the reproductive cycle. In symbiotic or mutualistic associations, the host is sexual(produces viable seeds) and the fungus is asexual (Clay, 1986). The behavior of thesame endophyte may also vary in different hosts (Sampson, 1935).Table 5.7 gives information on infection of endophytes in flowers and seeds ofgrasses. The taxonomic identity of the endophyte in many of these cases has notbeen determined. Latch, Christensen, and Samuels (1984) isolated five endophytesof Lolium and Festuca in New Zealand and discussed their taxonomy and taxonomic