No abstract

SummaryConfocal laser scanning microecopy (CLSM) has been used to Image phloem transport and unloading in the root tip of Atwbidopmis. The fluorescent probe 5(6) carboxyfluorascein (CF) was ester loaded into a single cotyledon end the entire seedling placed within an observation chamber under the micrcmcope. Translocation of CF to the root tip was rapid, followed by unloading into discrete concentric flies of ceils. The position of the prominent unloading 'zone' corresponded precilmly with that of the two protophloem files of sieve elements, demonstrating s functional role of these cells in symplsatic sieve-element unloading. Symplsatic transport following unloading was confined to the elongating zone of the root with little besipetal transport to more mature cells. Following photobieaching of the unloading zone, phloem transport was restored immediately into the protophloem sieve elements, followed rapidly by lateral, symplastic sieve-element unloading. The results demonstrate that phloem transport processes can now be Imaged in real time, and non-invasively, within an intact plant system.

Potato virus X (PVX) is a filamentous plant virus infecting many members of the family Solanaceae. A modified form of PVX, PVX.GFP-CP which expressed a chimeric gene encoding a fusion between the 27-kDa Aequorea victoria green fluorescent protein and the amino terminus of the 25-kDa PVX coat protein, assembled into virions and moved both locally and systemically. The PVX.GFP-CP yrions were over twice the diameter of wild-type PVX virions.

Using noninvasive imaging techniques, we compared phloem unloading of the membrane-impermeant, fluorescent solute carboxyfluorescein (CF) with that of potato virus X expressing the gene for the green fluorescent protein. Although systemic virus transport took considerably longer to occur than did CF transport, unloading of both solute and virus occurred predominantly from the class 111 vein network, a highly branched veinal system found between class II veins. The minor veins (classes IV and V) played no role in solute or virus import but were shown to be functional in xylem transport at the time of import by labeling with Texas Red dextran. After virus exit from the class 111 phloem, the minor veins eventually became infected by cell-to-cell virus movement from the mesophyll. During the sink/source transition, phloem unloading of CF was inhibited from class 111 veins before the cessation of phloem import through them, suggesting a symplastic isolation of the phloem in class 111 veins before its involvement in export. The progression of the sink/source transition for carbon was unaffected by the presence of the virus in the sink leaf. However, the virus was unable to cross the sink/source boundary for carbon that was present at the time of vira1 entry, suggesting a limited capacity for cell-to-cell virus movement into the apical (source) region of the leaf. A functional model of the sink/source transition in Nicotiana benthamiana is presented. This model provides a framework for the analysis of solute and virus movement in leaves.

Using noninvasive imaging techniques, we compared phloem unloading of the membrane-impermeant, fluorescent solute carboxyfluorescein (CF) with that of potato virus X expressing the gene for the green fluorescent protein. Although systemic virus transport took considerably longer to occur than did CF transport, unloading of both solute and virus occurred predominantly from the class 111 vein network, a highly branched veinal system found between class II veins. The minor veins (classes IV and V) played no role in solute or virus import but were shown to be functional in xylem transport at the time of import by labeling with Texas Red dextran. After virus exit from the class 111 phloem, the minor veins eventually became infected by cell-to-cell virus movement from the mesophyll. During the sink/source transition, phloem unloading of CF was inhibited from class 111 veins before the cessation of phloem import through them, suggesting a symplastic isolation of the phloem in class 111 veins before its involvement in export. The progression of the sink/source transition for carbon was unaffected by the presence of the virus in the sink leaf. However, the virus was unable to cross the sink/source boundary for carbon that was present at the time of vira1 entry, suggesting a limited capacity for cell-to-cell virus movement into the apical (source) region of the leaf. A functional model of the sink/source transition in Nicotiana benthamiana is presented. This model provides a framework for the analysis of solute and virus movement in leaves.