A Transformant with Reduced Arabinan in Its Pectin

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Pectins are complex cell wall polysaccharides that cement neighboring plant cells together and embed the load-bearing structures of the cell wall (cellulose microfibrils and hemicelluloses). Assigning specific functions to particular pectin types is in its infancy, in part, because of the limited number of transformants and mutants available with modified pectic polymers in their walls. In this report, Skjøt et al. (pp. 95-102) describe the generation of potato (Solanum tuberosum) tuber transformants that produce pectic rhamnogalacturonan I (RGI) with a low level of arabinosylation. This feat was accomplished by targeting a rat -2,6 sialyl transferase-endo--1,5-arabinanase fusion protein to the Golgi compartment of potato tuber cells, with the effect that the arabinan side-chains on RGI are hydrolyzed at the site of pectin biosynthesis. Sugar composition analyses of RGI isolated from transformed and wild-type (WT) tubers showed that the Ara content was decreased by approximately 70% in transformed cell walls compared with wild type. This transformant should be useful in settling fundamental debates concerning the role of arabinans in cell wall structure: For example, are arabinans rigid, structural components of the cell wall or mobile, fluidizing agents?

	Gibberellin and Cortical Wound Repair

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In Japan, cucumber (Cucumis sativus) is often grafted onto squash (Cucurbita pepo) stock to prevent damage from soil-borne diseases. In this procedure, the apical tip and first leaf of the squash stock are removed, but the cotyledons of the scion and stock are preferentially left on the hypocotyl to improve grafting efficiency. Although the exact role of the cotyledon in the formation of the graft union is not understood, it is possible that the cotyledon produces compounds required for the formation of the graft union. In this issue, Asahina et al. (pp. 201-210) cut cucumber hypocotyls to one-half of their diameter transversely and performed morphological and histochemical analyses of the process of tissue reunion in the cortex. Cell division in the cortex commenced 3 d after cutting, and the cortex was nearly fully united within 7 d. Cell division during tissue reunion was strongly inhibited when the cotyledon was removed. The application of gibberellin (GA) to the apical tip of the cotyledon-less plant reversed this inhibition. Moreover, cell division in the cortex was inhibited by treatment of the cotyledon with uniconazole-P (an inhibitor of GA biosynthesis). The requirement of GA for tissue reunion in cut hypocotyls was also evident from studies of the GA-deficient gib-1 mutant of tomato (Lycopersicon esculentum). These novel results suggest that GA, possibly produced in the cotyledons, is essential for cell division during the cortical repair of cut hypocotyls.

	The Advantage of Small Chloroplasts

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Why do the photosynthetic cells of higher plants contain so many small chloroplasts rather than a few large ones? In this issue, Jeong et al. (pp. 112-121) adopt a novel approach to this question. The authors use tobacco (Nicotiana tabacum) transformants that have been modified, by the overexpression of NtFtsZ1-2 (a gene involved in plastid division), to contain only one to three enlarged chloroplasts per mesophyll cell (Fig. 1). Despite the similarities in photosynthetic components and ultrastructure of photosynthetic machinery between WT and transgenic plants, the overall growth of transgenic plants under low- and high-light conditions was retarded. In WT plants, the chloroplasts moved toward the face position under low-light conditions, and toward the profile position under high-light conditions. In contrast, chloroplast rearrangement in transgenic plants in response to light conditions was not evident. The defective positive phototaxis of the enlarged chloroplasts under low-light conditions may decrease light absorption and, hence, growth. Under high-light conditions, defective negative phototaxis may cause the amount of absorbed light to exceed the photosynthetic utilization capacity, resulting in photodamage to the photosynthetic machinery and decreased growth. The evidence presented suggests that the presence of a large number of small and/or rapidly moving chloroplasts in the cells of higher land plants permits more effective chloroplast phototaxis.

View larger version (104K): [in this window] [in a new window]   Figure 1.   Light (A and B) and confocal (C and D) photomicrographs of protoplasts from wild type (A and C) and transformed cells containing a few, giant chloroplasts (B and D).

	Are Bundle Sheath Extensions Light Guides?

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Heterobaric leaves are characterized by the occurrence of transparent regions in the leaf blade that are easily seen as a network of bright lines on a dark green background under low magnification with transmitted light. These transparent areas are created because the bundle sheaths of these leaves extend to the epidermises on both sides of the leaf, forming bundle sheath extensions (BSEs), which project as ribs on both surfaces of the lamina. Many plant tissues or single cells can behave as light guides or transparent windows, transferring light to the neighboring cells. For example, the leaves of some underground growing desert plants possess areas from which photosynthetic parenchyma layers are absent. The epidermis and the underlying water storage tissue in these window-leaved plants are transparent to allow light penetration to the internal chlorenchyma layers. This anatomical adaptation presumably developed to allow photosynthesis to occur underground so as to reduce water losses and heat load of the leaves. It has been hypothesized that BSEs, apart from their water-conducting, mechanical, and protective functions, may also behave as "transparent windows," transferring visible light to internal layers of mesophyll. In this issue, Nikolopoulos et al. (pp. 235-243) report on their attempts to test this hypothesis. Image analysis showed that the percentage of photosynthetically active leaf area (PALA) of the heterobaric leaves of 31 plant species ranged from 91% in Malva sylvestris to only 48% in Gynerium sp. Although a significant portion of the leaf surface does not correspond to photosynthetic tissue, the photosynthetic capacity of these leaves, expressed per unit of area, was not considerably affected by the size of their transparent leaf area. The results suggest that although the PALA of heterobaric leaves is reduced, the photosynthetic performance of each transparent area is increased, possibly due to the light-transferring capacity of BSEs. This morphological adaptation may have allowed for increases in leaf thickness without reductions in photosynthesis, an advantageous adaptation in xerothermic environments.

	Why Miniature's Endosperm Is Miniature

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The "miniature" endosperm of the mn1 mutant of maize (Zea mays) is drastically reduced in size, its weight being only 20% that of the WT. The cause of the mn1 seed phenotype is the loss of cell wall invertase. Conceivably, the reduced size of the mn1 mutant endosperm could result from impairments in cell mitosis, cell expansion, or in the endoreduplication process that commonly occurs in developing endosperm. During endoreduplication, multiple rounds of DNA replication take place, but this is not followed by chromosome condensation, segregation, or cytokinesis: The result is enlarged, highly polyploid cells. To distinguish between these three possibilities, Vilhar et al. (pp. 23-30) made detailed comparisons of various cytological parameters of developing WT and mn1 maize kernels. They analyzed the spatial distribution of endosperm cells by sizes and endopolyploidy levels (C-values) using image cytometry, and on the basis of longitudinal sections, constructed a three-dimensional model of the endosperm. Compared to WT, the number of cells in the miniature endosperm was 55%, while the endosperm volume was only 25%, indicating that in addition to impaired cell proliferation there is also a reduction in the cell size. However, they detected no alterations in the progress of endoreduplication in the mutant as compared to the WT. These results are consistent with the hypothesis that the cleavage of Suc cell wall invertase during the early stages of seed development plays a critical role in providing hexose sugars for the maintenance of cell division and cell expansion.