In situ histochemical localisation of alkaloids and acetogenins in the endosperm and embryonic axis of Annona macroprophyllata Donn. Sm. seeds during germination.

Currently, the Annonaceae family is characterised by the production of acetogenins (ACGs), and also by the biosynthesis of alkaloids, primarily benzylisoquinolines derived from tyrosine. The objective of this study was to confirm the presence of alkaloids and acetogenins in the idioblasts of the endosperm and the embryonic axis of A. macroprophyllata seeds in germination. The Dragendorff, Dittmar, Ellram, and Lugol reagents were used to test for alkaloids, and Kedde's reagent was used to determine the presence of acetogenins in fresh sections of the endosperm and embryonic axis of seeds after twelve days of germination. A positive reaction was observed for all the reagents, and the presence of alkaloids and acetogenins was confirmed in the idioblasts of the endosperm and those involved in the differentiation of the embryonic axis of the developing seedling. We concluded that the idioblasts store both metabolites, acetogenins and alkaloids. Beginning at differentiation, the idioblasts of the embryonic axis simultaneously biosynthesise acetogenins and alkaloids that are characteristic of the species during the development of the seedling. The method used here can be applied to histochemically confirm the presence of acetogenins and alkaloids in tissues and structures of the plant in different stages of its life cycle.

Histochemical detection of acetogenins and storage molecules in the endosperm of Annona macroprophyllata Donn Sm. seeds.

Acetogenins (ACGs) are bioactive compounds with cytotoxic properties in different cell lines. They are antitumoural, antiparasitic, antimalarial, insecticidal, antimicrobial, antifungal and antibacterial. These secondary metabolites function in plant defence and are found in specific organelles and specific cells, thereby preventing toxicity to the plant itself and permitting site-specific defence. The aim of this work was to histochemically determine the in situ localisation of ACGs in the endosperm of Annona macroprophyllata seeds using Kedde's reagent. Additionally, the colocalisation of ACGs with other storage molecules was analysed. The seeds were analysed after 6 and 10 days of imbibition, when 1 or 2 cm of the radicle had emerged and metabolism was fully established. The seeds were then transversally cut in half at the midline and processed using different histological and histochemical techniques. Positive reactions with Kedde's reagent were only observed in fresh, unfixed sections that were preserved in water, and staining was found only in the large cells (the idioblasts) at the periphery of the endosperm. The ACGs' positive reaction with Sudan III corroborated their lipid nature. Paraffin sections stained with Naphthol Blue Black showed reactions in the endosperm parenchyma cells and stained the proteoplasts blue, indicating that they might correspond to storage sites for albumin-like proteins. Lugol's iodine, which is similar in chemical composition to Wagner's reagent, caused a golden brown reaction product in the cytoplasm of the idioblasts, which may indicate the presence of alkaloids. Based on these results, we propose that Kedde's reagent is an appropriate histochemical stain for detecting ACGs in situ in idioblasts and that idioblasts store ACGs and probably alkaloids. ACGs that are located in idioblasts found in restricted, peripheral areas of the endosperm could serve as a barrier that protects the seeds against insects and pathogen attack.

Liriodenine alkaloid in Annona diversifolia during early development.

Plants of the Annonaceae family produce a series of alkaloids, including liriodenine oxoaporphine. Its distribution in these primitive angiosperms suggests that it plays an important role, but very little is known about which plant organs it accumulates in, or in which developmental stages it is synthesised. Accordingly, liriodenine production was studied during the early stages of germination and seedling development in Annona diversifolia Saff. Liriodenine samples were obtained from the roots and were characterised on the basis of spectroscopic data. Quantification was done by HPLC in the organs and tissues of newly collected seeds, seeds following 1-, 2-, 5- and 10-day imbibitions, upon emergence of the radicle and at the seedling stage. According to our results, liriodenine could not have originated from the parent plant, nor during embryogenesis because it appears for the first time in the endosperm approximately 5 days after the start of imbibition. Therefore, its synthesis does not depend directly on photosynthesis. During the seedling stage it is found in the root and the stem but it is absent from the cotyledonary leaves and the first true leaves. Liriodenine biosynthesis begins during the early stages of development in the endosperm and seed radicles.