Exine development in Encephalartos altensteinii (Cycadaceae): ultrastructure, substructure and the modes of sporopollenin accumulation

It has been found that in Encephalartos altensteinii all the main characters of the exine appear within the tetrad period. The framework of the exine—the plasma membrane glycocalyx—appears first and consists of close packed cylinder-like units 120–160 nm in diameter, oriented perpendicular to the microspore plasma membrane. The main character of the glycocalyx framework is its dynamism: the units are capable of building up throughout the period of exine development. Sporopollenin acceptor particles (SAPs) occur distributed alongside the walls of the cylinder-like units, bringing about a sporopollenin accumulation and the appearance of cylindrical alveolae of the exine pattern. The location of SAPs, which are probably enzyme molecules, is carried out mainly under control of genome, and only the final step occurs by self-orientation, as a result of a combination of both strong and weak interactions between glycocalyx macromolecules and enzyme molecules. The initial accumulation of sporopollenin apparently occurs in the form of microglobules around SAPs with diameter of 30–50 nm. Subsequent sporopollenin accumulation, which results in the appearance of the mature alongate alveolate pattern of the exine, is independent of SAPs and can be regarded as secondarily accumulated sporopollenin. We suggest two stages in the exine development which are realized by self-assembly: (i) the formation of the glycocalyx and (ii) the accumulation of receptor-independent sporopollenin. The intermediate stage—the initial accumulation of receptor-dependent sporopollenin—is carried out under the control of SAPs.