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

2Reproductive Structuresand Seed FormationSeed is the end product of sexual reproduction that takes place through sequentialchanges in the reproductive shoot, the flower in angiosperms. The ovules are bornein a closed structure, the ovary. The ovules and seeds may get infected at any stageduring their development either directly through the mother plant or through theinfected floral parts, including bracts and nectaries. The contacts between the plantand flower and in turn with the ovule and seed are important in understanding thecourse of such infections. The interface of pathogen and host may cause morphologicaland anatomical changes in tissues of the flower and may also affect thereproductive cycle, including seed formation. Green ear disease of pearl millet,caused by Sclerospora graminicola, results in the transformation of florets into leafystructures. Inflorescence or flower infection by Protomyces macrosporus in corianderand Albugo candida in Brassicaceae cause hypertrophy in the latter, and fruit gallformation in the former. Claviceps purpurea and other species, when infecting cerealflorets, cause the ovary to form fungal sclerotia instead of producing a kernel. Thecontents in the ovary are replaced by smut chlamydospores due to the infection ofSphacelotheca, Tilletia, Tolyposporium, and Ustilago in various cereal crops.The structure of flowers, the reproductive cycle, and seed formation are describedin this chapter. The aspects that may be relevant to pathogenesis are highlighted. Anoutline of the reproductive cycle is given in Figure 2.1.2.1 FLOWERThe flowers may occur singly (cotton, kenaf, cucumber, pumpkin, and melon) oraggregated in inflorescence — small (tomato, potato, and brinjal) to massive (rice,wheat, pearl millet, sunflower, and coriander). Stalked or sessile flowers are borneloosely (coriander, carrot, and tomato) or compactly (wheat, pearl millet, maize, andsunflower), have an orderly arrangement, and depending on the order of developmentof flowers, the inflorescence is classified into indeterminate (racemose) or determinate(cymose) type. In the former, the order of development of flowers is acropetal,with the youngest bud and flower near the growing tip (Brassica, Raphanus, andLinum), whereas in the latter, it is basipetal, with the growing point ending in aflower and subsequent ones produced in the axil(s) of bracts below the terminalflower (Lycopersicon, Sesamum, and Solanum). The presence of accessory structures,e.g., spathe in spadix (maize); involucre of bracts in capitulum (sunflower) andumbel (coriander and carrot); and glumes, palea, and lemma with florets in simple7