Microsporogenesis in Pinus sylvestris. VI. Exine and tapetal development during the tetrad period

Abstract

Coverage is of microspore tetrad period from end of cytokinesis to introduction of endexine in Pinus sylvestris. The ectexine of aperture, cap zone and sacci and the endexine are initiated while microspores are in the tetrad condition and enveloped in callose. Ectexine patterning including considerable expansion of sacci develops prior to the initiation of the endexine. Alveoli, sacci and alveoli within sacci are initiated by cytoplasmic invaginations which are sites of uptake of cell surface coat (glycocalyx) along with nutrients bound to the glycocalyx. Applications of tracers show that glycocalyx elements bind to cations and transport them to the cytoplasm. From the beginning of exine formation these invaginations are largest in the regions of future sacci and very small in the aperture. As growth progresses cytoplasm surrounding invaginations partially retracts, but callose contact is retained. Thus, these invaginations become callose covered hemispheroids (alveoli) that are “open” to the cell surface proximally and covered by callose distally but only partially so at the sides of the “cup-shaped” alveoli. Until introduction of the endexine part of the alveolar-sides are made up of cytoplasmic protrusions which contact the callose protrusions, even across sacci expanded more than 3 μm. Glycocalyx elements become aligned on the inner surface of the callosic alveoli and are sites for sporopollenin accumulation. The template for endexine components consists of glycocalyx elements that become aligned near the plasma membrane. Our observations indicate that uptake from the loculus to the microspore cytoplasm changes after introduction of the endexine. Henceforth, uptake is assisted by the endexine, as shown by tracers. Tapetal cells undergo two periods of hyperactivity during the period covered. Hyperactivity took place at the beginning of uptake by microspores and during endexine formation. The extra tapetal lamellation and its tapetal markers begin to exhibit the intense staining, after endexine initiation.