Carotenoid and ultrastructure variations in plastids of Arum italicum Miller fruit during maturation and ripening

The changes in the pigment pattern and composition occurring in the Arum it alicum berry during the various steps of maturation (ivory to deep-green st ages) and ripening (yellow and red-orange stages) were studied and correlat ed to the ultrastructural modifications of plastids, Transmission electron microscopy showed that each stage was characterized by a specific plastidia l type following the unusual sequence amyloplast-->chloroplast-->chromoplas t. Plastidial transitions were accompanied by profound modifications in the pigmental composition, in particular, in the pattern of carotenoids and th eir precursors. The HPLC analysis of the carotenoids showed that, besides t he two usual all-trans metabolic pathways leading to lutein through a-carot ene and to auroxanthin through beta-carotene, an additional cis-isomeric bi osynthetic pathway leading to cis-neoxanthin through cis-beta-carotene exis ts. During the pre-ripening stages, the three pathways were present even if with qualitative and quantitative variations. When complete ripening was r eached, a block occurred at the cyclization level causing the accumulation of both all-trans (i.e, gamma-carotene and neurosporene) and cis-isomer (i. e. lycopene and zeta-carotene) carotene precursors. Because of the occurren ce of unusual pigments and the presence of the three main plastidial types, the fruit of A. italicum may constitute a most instructive model for the s tudy of carotenogenesis.