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

4Penetration andEstablishment of Fungiin SeedFungal infection of seed-borne pathogens may reach the ovule and seed at any stagefrom the initiation of ovule to mature seed. The previous chapters provided anaccount of the stages in ovule development, ovule contacts with the mother plant,probable passages and barriers to infection, and the structure of the mature seed.The developing ovule and seed are enclosed in the ovary and in contact with themother plant while the disseminated or threshed seed is an independent unit. Differentfactors determine the entry of the pathogen under these two conditions.The major pathogen groups, namely fungi, bacteria, and viruses, differ in theirmodes of multiplication and attack on the host. Fungal propagules germinate, andthe hyphae grow. Germination of propagule and initiation of hyphal growth areimportant factors that determine the entry of fungal pathogens in any tissue, includingthe fruit and seed. Phytopathogenic bacteria multiply while the viruses replicateintracellularly, and both lack the phenomenon of growth. This chapter discussespenetration and establishment of fungal pathogens in the ovule and seed alone.4.1 ENVIRONMENT OF OVULE AND SEEDThe ovule and seed develop in the pistil, which is enclosed by other floral appendagesin the hypogynous flower, but the pistil is exposed to the environment in the epigynousflowers. Bracts, bracteoles, and other accessory structures may also limit directexposure of pistil in the hypogynous as well as epigynous flowers. These constitutethe immediate environment for the ovule and seed. The physical environment, whichis also important, is greatly affected by the immediately surrounding protectivetissues. The developing ovules and seeds are actively growing structures, and waterand humidity inside the ovary and developing fruit cannot be the limiting factors.The microclimate in enclosures formed by the fruit wall and enveloping accessorystructures, if present, has been little studied. Apparently, these may provide favorableconditions for infection, establishment, and spread of fungi. However, physiologicaland biochemical factors inside the fruits in general and the fleshy fruit in particularmay further control the establishment of infection. This is borne out by the fact thatseeds of fleshy fruits such as cucumber, squash, melon, and tomato are usuallyremarkably free from fungi.81