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

16 Histopathology of Seed-Borne Infectionsnutrients to the pollen tube and permits it to reach the destination. Arber (1937)called this tissue the transmitting tissue. Based on its distribution, the styles areclassified as hollow and solid. The hollow style, which is common in monocotyledons,is characterized by the presence of a stylar canal, lined entirely or in longitudinalstrips by glandular transmitting tissue. In Lilium longiflorum, the cells of thetransmitting tissue are rich in organelles and contain abundant multivesicular bodies(Rosen and Thomas, 1970; Dashek, Thomas, and Rosen, 1971).The solid styles lack stylar canal and show one or more strands of transmittingtissue, leading to the placentae of ovary. The cells of the transmitting tissue areelongated, rich in cytoplasm, and possess intercellular spaces. The pollen tubes passthrough the intercellular spaces, which, as reported in Lycopersicon, possess aviscous fluid (Cresti et al., 1976). A solid style is common in dicotyledons.A third type of style, half-closed, has been reported in some members of Cactaceae(Hanf, 1935) and Artabotrys (Rao and Gupta, 1951). In these plants the styleis hollow, and the transmitting tissue develops only on one side of the stylar canal.In addition to the transmitting tissue, the style consists of parenchyma withvascular supply. The epidermis may have stomata and is covered by the cuticle.2.3.2.3 StigmaThe distal part of the carpel, having special features to facilitate pollen reception,germination, and penetration of the pollen tube into the closed carpel, is called thestigma. The stigmatic surface is generally papillate or hairy and rarely smooth. InPoaceae and other wind-pollinated plants, the stigmatic surface develops into longbranched hairs. According to Konar and Linskens (1966a), the stigma in Petuniacomprises two zones — an upper secretory zone of epidermis and the lower storagezone. The epidermal cells are covered with the cuticle. Mattson et al. (1974) observedthe presence of an extracuticular proteinaceous layer (pellicle) on the stigma papillaeof many angiosperms. The pellicle remains intact in fresh stigma, but forms cracksand fissures on older stigma through which the cuticle can be seen after stainingwith benzpyrene and observing with a fluorescent microscope.The stigma secretes the stigmatic fluid, which in Petunia consists of oil, sugars,and amino acids (Konar and Linskens, 1966b). The protein contents of the stigmaticexudate are specific recognition factors that interact with proteins released from thepollen. In case of acceptance or compatible reaction, the pollen germinates, forminga tube, which penetrates the cuticle of the stigma, whereas rejection is indicated bythe failure of pollen to germinate. Heslop-Harrison et al. (1975) using fluorochromaticand immunofluorescence methods have concluded that the stigma pellicle isthe site of recognition responses. In compatible reactions, the ejected pollen proteinsbind very quickly to the pellicle, and it is in the binding zone that erosion of thecuticle takes place to facilitate pollen tube entry. In Raphanus (Dickinson and Lewis,1973) and Iberis (Heslop-Harrison, Knox, and Heslop-Harrison, 1974), the rejectionreaction is signified by the deposition of callose in both the pollen tube and thestigma papillae within 4 to 6 hours of pollination. Dickinson and Lewis (1975)observed that in Raphanus the incompatible pollen tubes may penetrate the papillarwall through enzymatic action, but soon after penetration a lenticular callose reaction