William A. Russin

Modification of a Specific Class of Plasmodesmata and Loss of Sucrose Export Ability in the sucrose export defective1 Maize Mutant.

We report on the export capability and structural and ultrastructural characteristics of leaves of the sucrose export defective1 (sed1; formerly called sut1) maize mutant. Whole-leaf autoradiography was combined with light and transmission electron microscopy to correlate leaf structure with differences in export capacity in both wild-type and sed1 plants. Tips of sed1 blades had abnormal accumulations of starch and anthocyanin and distorted vascular tissues in the minor veins, and they did not export sucrose. Bases of sed1 blades were structurally identical to those of the wild type and did export sucrose. Electron microscopy revealed that only the plasmodesmata at the bundle sheath-vascular parenchyma cell interface in sed1 minor veins were structurally modified. Aberrant plasmodesmal structure at this critical interface results in a symplastic interruption and a lack of phloem-loading capability. These results clarify the pathway followed by photosynthates, the pivotal role of the plasmodesmata at the bundle sheath-vascular parenchyma cell interface, and the role of the vascular parenchyma cells in phloem loading.

Anatomical and Ultrastructural Changes Associated with Sink-to-Source Transition in Developing Maize Leaves

We studied the anatomical and ultrastructural changes accompanying sink-to-source transition in developing maize (Zea mays L. cv. W273) leaves, in which sink, transition, and source regions had been identified by whole-leaf autoradiography. In the leaves examined, a complete structural gradient existed from nonimporting to importing regions of the blade. Although all components, except metaxylem elements. of the large (transport) bundles reach maturity before their counterparts in intermediate and small (loading) bundles, the final events in structural maturation are uniform for all bundle types across the blade. Among the very last structures to mature are the plasmodesmata at the interfaces between mesophyll cells and between mesophyll cells and bundle sheath cells. Maturation of the plasmodesmata coincides with maturation of the thick-walled sieve tubes, the last components of the vascular bundles to mature. Significantly, the vasculature reaches structural maturity in advance of cessation of import, and maturation of bundles involved with phloem loading is not closely correlated with initiation of export from the blade. Deposition of suberin lamellae in the walls of the bundle sheath cells coincides with the deposition of secondary walls in the metaxylem vessels. It is suggested that a primary role of the suberin lamellae may be to prevent leakage of sucrose from the bundles.

Distribution and frequency of plasmodesmata in relation to photoassimilate pathways and phloem loading in the barley leaf

Large, intermediate, and small bundles and contiguous tissues of the leaf blade of Hordeum tvulgare L. ‘Morex’ were examined with the transmission electron microscope to determine their cellular composition and the distribution and frequency of the plasmodesmata between the various cell combinations. Plasmodesmata are abundant at the mesophyll/parenchymatous bundle sheath, parenchymatous bundle sheath/mestome sheath, and mestome sheath/vascular parenchyma cell interfaces. Within the bundles, plasmodesmata are also abundant between vascular parenchyma cells, which occupy most of the interface between the sieve tube-companion cell complexes and the mestome sheath. Other vascular parenchyma cells commonly separate the thick-walled sieve tubes from the sieve tube-companion cell complexes. Plasmodesmatal frequencies between all remaining cell combinations of the vascular tissues are very low, even between the thin-walled sieve tubes and their associated companion cells. Both the sieve tube-companion cell complexes and the thick-walled sieve tubes, which lack companion cells, are virtually isolated symplastically from the rest of the leaf. Data on plamodesmatal frequency between protophloem sieve tubes and other cell types in intermediate and large bundles indicate that they (and their associated companion cells, when present) are also isolated symplastically from the rest of the leaf. Collectively, these data indicate that both phloem loading and unloading in the barley leaf involve apoplastic mechanisms.

Immunocytochemical localization of taxol in Taxus cuspidata

The supply of taxol, a valuable anticancer compound, depends completely on the extraction of taxol from Taxus brevifolia (Pacific yew) plants. Although taxol is found in virtually all parts of the plant, nothing is known about the localization of taxol within the cells or tissues of the plant. Portions of T. cuspidata stems were chemically fixed, dehydrated in ethanol, and then embedded in standard resins. In these tissues, immunolocalization using polyclonal antitaxol antiserum indicates that taxol is associated with vacuolar tannin inclusions in axial phloem parenchyma cells. Little or no label is bound over the cell walls of any cell type. However, chemical preparation causes diffusion of taxol, which results in an erroneous localization of taxol. In contrast, using cryotechniques and a water-soluble melamine resin (Nanoplast), taxol is localized almost exclusively in the cell walls of phloem, vascular cambium, and xylem. Our method yields insight into the problems associated with the intra- and extracellular localization of lipophilic plant secondary compounds. It also offers an alternative tissue-preparation protocol that could be useful for the localization of other plant metabolites.

Carbonic Anhydrase Localization in Charophycean Green Algae: Ecological and Evolutionary Significance

Two carbonic anhydrase (CA) antibodies, one to the primarily periplasmic alpha‐type enzyme of the chlorophycean Chlamydomonas and another to pea (Pisum sativum) beta‐type CA, which is primarily located in the chloroplast stroma, were applied to the charophycean green alga Mougeotia sp., shoot cells of the later divergent charophycean, Chara zeylanica, and control Chlamydomonas cells and pea leaf tissues. Immunolocalizations were quantitatively assessed at the ultrastructural level; nonimmune labeling was negligible in all cases. The Chlamydomonas antibody intensely labeled inner cell wall, periplasmic region, and peripheral cytoplasm of Chlamydomonas, Mougeotia, and Chara. The concentration of periplasmic label did not significantly differ among these green algae. This is the first immunolocalization evidence for occurrence of a periplasmic CA in Chara. Chloroplasts of Mougeotia, Chara, and pea were also labeled by the Chlamydomonas antibody, suggesting that plants inherited a plastid thylakoid‐based, alpha‐type CA from charophycean ancestors. The pea antibody labeled Chara chloroplasts but not those of Chlamydomonas or Mougeotia. The concentration of pea antibody stromal label did not significantly differ between pea and Chara. This is the first immunolocalization evidence for occurrence of a higher‐plant, beta‐type stromal CA in green algal plastids.