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

Reproductive Structures and Seed Formation 13homotypic division. The simultaneous type of wall formation is common in dicotyledonsand the successive type is common in monocotyledons.Recent studies using TEM and fluorescence microscopy have shown that duringearly stages of prophase of heterotypic division of meiosis, plasmodesmatal connectionsoccur between the tapetum and sporogenous cells as well as among thesporogenous cells, forming a syncytium. Subsequently, from pachytene I to telophaseI, the sporogenous cells gradually develop refractive walls of callose. The synthesisof callose is retarded during homotypic division. The callose isolates the microsporocytesand microspore tetrads from the tapetum and also from one another (Westerkeyn,1962). The autoradiographic study of Heslop-Harrison and Mackenzie (1967)has shown that the labeled thymidine derivatives do not penetrate microspore mothercells or microspores when they are surrounded by callose. Any irregularity in calloseformation results in male sterility (Frankel, Izhar, and Nitsen, 1969).2.3.1.2 Development and Structure of Male GametophyteWith the formation of microspore tetrads, the callose starts to dissolve due to theaction of b-1,3-glucanase (Stiegelitz and Stern, 1973). The microspores of a tetradusually separate, and the uninucleate microspores become more or less spherical(Figure 2.3G, H). Further development of uninucleate microspores follows a uniformpattern in angiosperms (Figure 2.4A to F). The microspore nucleus divides mitoticallyforming a generative cell close to the wall and the vegetative cell. Electronmicroscope studies have revealed that the formation of a wall around the generativecell corresponds to that in other cells (Karas and Cass, 1976). The generative cellacquires a spindle shape in the pollen grain. The generative cell divides in situ(Figure 2.4G, H) or after pollination in the pollen tube to form two sperms(Figure 2.4I, J).Mature pollen grains have a double wall, the exine and the intine (Figure 2.3I,J). The intine is thin and made up of pectocellulose while the principal componentof the exine is sporopollenin, derived from tapetum. The exine is thick andremarkably durable. The intine and the exine also contain enzymes (proteins) thatare released at the time of fertilization and degrade the cuticle of stigma papillae(Tsinger and Petrovskaya-Baranova, 1961). The intine proteins are the secretoryproducts of pollen protoplast whereas the proteins of the exine originate from thetapetum. The exine proteins are involved in the penetration of the pollen tube intothe stigma and also in pollen–stigma interaction that determines the incompatibilityrelationship (Heslop-Harrison, 1975).2.3.2 CARPELOne of the distinctive features of the angiosperms is the carpel, which bears theovules. A typical carpel consists of three parts, the basal fertile region — the ovary— and two sterile parts, the style and the stigma. The gynoecium is termedapocarpous when carpels in a flower are free, or syncarpous when carpels becomefused. In some angiosperms, the carpels are not completely closed, e.g., Degenaria,Drimys, and Reseda. Butomaceae, Hydrocharitaceae, and intraovarian pollen grainshave been reported in Butomopsis and Reseda.