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KMID : 0903619940350020139
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Journal of the Korean Society for Horticultural Science
1994 Volume.35 No. 2 p.139 ~ p.147
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Comparison of Plastid Ultrastructures between the Fruits of Two Isogenic Lines of Cucurbita pepo at Different Developmental Stages
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Boyer, Charles D.
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Abstract
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Plastid ultrastructures of two isogenic lines of squash (Cucurbita pepo L. cv. Early Prolific), affecting fruit pigmentation patterns (YY B^+B^+: fruits is initially green, then yellow, & YY BB: precociously yellow) were compared at different stages of fruit development. The sampling stages for two isogenic lines were based on fruit maturity; 2 days before pollination, at pollination, 3 days after pollination, 10 days after pollination, and 20 days after pollination. Plastids with few internal membranes were observed in the ovaries of YY B^+B^+ plant at prepollination stage. Chloroplasts in the ovaries of YY B^+B^+ plants at pollination contained smaller stacked gram with a few extended thylakoids compared with typical chloroplasts of green leaves or persistently green fruits. Appearance of plastoglobuli and internal membrane structures in early stage of chromoplast ($quot;prochromoplasts$quot;) was in parallel with the disappearance of thylakoid membranes in fruit chloroplasts of YY B^+B^+ plants. Plastids of YY B^+B^+ plants 10 days after pollination revealed typical membranous chromoplast structures with several well-developed internal membranes. Chromoplasts from firm fruits of YY B^+B^+ plants 20 days after pollination lost their internal membrane structures, and chromoplasts underwent changes in their fine structures from the membranous chromoplasts to the globular chromoplasts. Chromoplasts in precocious (YY BB) fruits, however, seemed to be derived directly from proplastids without an intermediate chloroplast stage prior to chromoplast formation. Membranous chromoplasts observed in the fruits of YY BB plants 3 days after pollination were almost equivalent to those of YY B^+B^+ plants 10 days after pollination. Neither the mature fruit color nor the ultrastructures were different between the fruits of YY B^+B^+ and YY BB plants 20 days after pollination. The plastids in mature fruits of both genotypes were devoid of internal membranes and contained several large plastoglobuli. Based on previous hypothesis and results from this study, B and Y genes may be involved in controling the timing of chromoplast or chloroplast appearance rather than changes in plastid ultrastructure and carotenoid content. These isogenic lines of squash can be good candidates for plastid gene regulation studies in the future, providing a logical model system.
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