HighlightPhotosynthesis by cucumber fruits, through direct fixation of atmospheric CO2 and recapture of respired CO2, makes an important contribution to fruit growth.
A genetic map was developed with microsatellite (simple sequence repeat, SSR) markers and 148 recombinant inbred lines (RILs) derived from a cross between two cultivated cucumber (Cucumis sativus L.) inbred lines 9110Gt and 9930, which was also segregating for seven horticulturally important traits including bitterfree foliage (bi)
Sucrose synthase (SUS; EC 2.4.1.13) plays important roles in sugar metabolism and abiotic stress response. But the genes encoding SUS in cucumber (Cucumis sativus L.) have not been well studied. Here, we isolated four cucumber sucrose synthase genes (CsSUS). Among them, CsSUS3, which highly expressed in the roots, was chosen for further study. Immunolocalization and subcellular localization analysis indicated that CsSUS3 localized in the cytosol and the plasma membrane, and mainly existed in the companion cells of phloem in the roots. When suffering hypoxia stress from flooding, CsSUS3 expression and SUS activity in roots increased, especially in the lateral roots; moreover, the soluble SUS activity increased clearly, but the membrane fraction hardly changed. Compared with the wild-type cucumbers, the transgenic lines with antisense expression of CsSUS3 were more sensitive to flooding. After 6 d of flooding, the SUS activity, soluble sugar and uridine 5′-diphosphate glucose (UDPG) content and the ratio of ATP/ADP in the roots of transgenic plants were significantly lower than that in wildtype plants. Moreover, the transgenic lines grew more slowly with more yellow necrosis in the leaves. These findings suggested CsSUS3 participated in resisting hypoxic stress. Furthermore, the mechanism of CsSUS3 in resisting hypoxic stress was also discussed.
Phloem unloading plays a pivotal part in photoassimilate transport and partitioning into sink organs. However, it remains unclear whether the unloading pathway alters to adapt to developmental transitions in sinks, especially in fleshy fruits accumulating a high level of soluble sugars. Using a combination of electron microscopy, transport of the phloem-mobile symplasmic tracer carboxyfluorescein and assays of acid invertase, the pathway of unloading was investigated in different varieties of Chinese jujube fruit (Zizyphus jujuba Mill. cv Dongzao and Lizao). Structural investigation showed that the sieve element-companion-cell complex of bundles feeding the fruit flesh is symplasmically connected with surrounding parenchyma cells at the middle stage, and isolated during the early and late stages. Numerous plasmodesmata are present between phloem parenchyma cells and flesh cells throughout fruit development. Confocal laser scanning images of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands during the early and late stages of fruit development, whereas it was released from the functional phloem at the middle stage. The levels of both the expression and activities of cell wall acid invertase and soluble acid invertase varied in an inverse pattern relative to fruit development. These results provided clear evidence for the predominance of an apoplasmic phloem unloading pathway interrupted with a symplasmic pathway and simultaneous symplasmic and apoplasmic unloading pathways in post-phloem transport during fruit development. Similar unloading pathways were obtained in different varieties of jujube fruit. The mechanisms and significance of the adaptive switch in the phloem-unloading pathway during fruit development were discussed.
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