engleski

Mechanisms of thylakoid redifferentiation in cucumber fruit

In plants which grow in the light, chloroplasts commonly develop from undifferentiated proplastids. However, in some cases, chloroplasts may be formed from fully differentiated plastids (amyloplasts, leucoplasts, chromoplasts or even senescent plastids). In the cucumber fruit, the plastids which develop from chloroplasts during fruit ripening are able to revert to chloroplasts, spontaneously. In the present work, the mechanisms of redifferentiation of the photosynthetic apparatus during cucumber fruit regreening are described. For ultrastructural studies, pieces of fruit pericarp tissue were fixed for 20 min in 2% glutaraldehyde in 0.05 M cacodylate buffer (pH 7.2), and postfixed for 1 h in 1% OsO4. After dehydration in a graded series of ethanol, the tissue was embedded in Spurr's resin. Ultrathin sections were stained with uranyl acetate and lead citrate. For the localization of the activity of Photosystem I, the tissue was fixed in 2% formaldehyde in 0.05 M phosphate buffer. After diaminobenzidine treatment (Wrischer 1978), the tissue was postfixed for 1 h in 1% OsO4, dehydrated in ethanol and embedded in Spurr's resin. Tissue sections were examined using a Zeiss EM10A electron microscope. In the pericarp of mature cucumber fruit, plastids were roundish in shape and rather small (1, 5-3 μ m in diameter). They usually contained a few single thylakoids which, in most cases, showed a negative DAB reaction. Some plastids possessed vacuole-like structures, which probably formed by extensive dilatation of thylakoids. Large osmiophilic plastoglobules (100-300 nm in diameter), phytoferritin inclusions and roundish granular structures were commonly observed in the plastid stroma. In fruit which started to reegreen, progressive redifferentiation of the thylakoid system took place. During the early stages of regreening, morphologically distinct types of membranous structures were found: tubulous structures with lacunar spaces, sinuous thylakoidal structures, membranous structures which resembled thylakoids and typical thylakoids. In many plastids, a vacuole-like structure was also observed. These plastid "vacuoles" varied in size from small (150 nm in diameter) to very large (several μ m in diameter) which thus occupied most of the plastid volume. During further plastid development, membranous structures appeared on the membranes of large "vacuoles", suggesting that they probably form by growth and differentiation of vacuolar membranes. Some of these membranous structures had typical thylakoid morphology (flattened membranous sacks), while others appeared as swollen thylakoids. Flattened thylakoidal structures immediately adjacent to the "vacuoles" contained deposits of photooxidised DAB indicating the presence of components of photosystem I. In the course of thylakoid system redifferentiation, vesicular, tubular and thylakoid-like membranous structures were observed in the peripheral stroma. Direct connection of these structures with the plastid envelope was commonly found, indicating that they form by invagination of the inner envelope membrane. In the latter stages of regreening, the number of thylakoids increased and they arranged into an organised membranous system, showing positive DAB reaction. Finally, fully regreened tissue contained roundish or lens-shaped chloroplasts (3-6 μ m in length) with a well developed thylakoid system. Ultrastructural studies indicated that the biogenesis of the thylakoid system during cucumber fruit regreening probably proceeds in several different ways: 1. by invagination of the inner membrane of the plastid envelope, 2. by differentiation via tubular or lamellar structures, 3. by differentiation via vacuole-like structures.

thylakoids; cucumber; fruit

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